# The wide-field, multiplexed, spectroscopic facility WEAVE: Survey design, overview, and simulated implementation

Shoko Jin, Scott C. Trager, Gavin B. Dalton, J. Alfonso L. Aguerri, J. E.

Drew, Jesús Falcón-Barroso, Boris T. Gänsicke, Vanessa Hill, Angela Iovino, Matthew M. Pieri, et al.

► To cite this version:

Shoko Jin, Scott C. Trager, Gavin B. Dalton, J. Alfonso L. Aguerri, J. E. Drew, et al.. The wide-field, multiplexed, spectroscopic facility WEAVE: Survey design, overview, and simulated implementation. Monthly Notices of the Royal Astronomical Society, 2024, pp.2688-2730. 10.1093/mnras/stad557. insu-04040492

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Submitted on 24 May 2024

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# The wide-field, multiplexed, spectroscopic facility WEAVE: Survey design, overview, and simulated implementation

Shoko Jin<sup>1,2,3,4*</sup> Scott C. Trager,<sup>2*</sup> Gavin B. Dalton,<sup>1,3*</sup> J. Alfonso L. Aguerri,<sup>5,6</sup> J. E. Drew<sup>7,8</sup>

Jesús Falcón-Barroso,<sup>5,6</sup> Boris T. Gänsicke<sup>9</sup>, Vanessa Hill,<sup>10</sup> Angela Iovino,<sup>11</sup> Matthew M. Pieri,<sup>12</sup>

Bianca M. Poggianti,<sup>13</sup> D. J. B. Smith<sup>7</sup>, Antonella Vallenari,<sup>13</sup> Don Carlos Abrams,<sup>14</sup>

David S. Aguado,<sup>15,16,17</sup> Teresa Antoja<sup>18,19,20</sup> Alfonso Aragón-Salamanca<sup>21</sup> Yago Ascasibar,<sup>22</sup>

Carine Babusiaux,<sup>23,24</sup> Marc Balcells,<sup>5,6,14</sup> R. Barrena<sup>5,6</sup>, Giuseppina Battaglia,<sup>5,6</sup> Vasily Belokurov<sup>17</sup>

Thomas Bensby,<sup>25</sup> Piercarlo Bonifacio<sup>24</sup>, Angela Bragaglia,<sup>26</sup> Esperanza Carrasco<sup>27</sup>

Ricardo Carrera<sup>13</sup>, Daniel J. Cornwell<sup>21</sup>, Lilian Domínguez-Palmero,<sup>5,6,14</sup> Kenneth J. Duncan<sup>28</sup>

Benoit Famaey<sup>29</sup>, Cecilia Fariña,<sup>5,6,14</sup> Oscar A. Gonzalez,<sup>30</sup> Steve Guest,<sup>3</sup> Nina A. Hatch<sup>21</sup>

Kelley M. Hess,<sup>2,31,32</sup> Matthew J. Hoskin<sup>9</sup>, Mike Irwin,<sup>17</sup> Johan H. Knapen,<sup>5,6,33</sup>

Sergey E. Koposov<sup>17,18,28,34</sup>, Ulrike Kuchner<sup>21</sup>, Clotilde Laigle,<sup>35</sup> Jim Lewis,<sup>17†</sup> Marcella Longhetti,<sup>11</sup>

Sara Lucatello,<sup>13,36‡</sup> Jairo Méndez-Abreu<sup>5,6</sup>, Amata Mercurio<sup>37,89</sup>, Alireza Molaeinezhad,<sup>1,5,6,17</sup>

Maria Monguió<sup>7,18,19,20</sup> Sean Morrison<sup>12,38</sup>, David N. A. Murphy,<sup>17</sup> Luis Peralta de Arriba<sup>17</sup>

Isabel Pérez,<sup>40,61</sup> Ignasi Pérez-Ràfols<sup>12,18,41</sup> Sergio Picó,<sup>14</sup> Roberto Raddi<sup>42</sup>

Mercè Romero-Gómez,<sup>18,19,20</sup> Frédéric Royer,<sup>24</sup> Arnaud Siebert,<sup>29</sup> George M. Seabroke,<sup>43</sup>

Debopam Som<sup>44</sup>, David Terrett,<sup>3</sup> Guillaume Thomas,<sup>5,6</sup> Roger Wesson<sup>8</sup>, C. Clare Worley,<sup>17</sup>

Emilio J. Alfaro,<sup>31</sup> Carlos Allende Prieto,<sup>5,6</sup> Javier Alonso-Santiago,<sup>45</sup> Nicholas J. Amos<sup>14,46</sup>

Richard P. Ashley,<sup>14</sup> Lola Balaguer-Núñez<sup>18,19,20</sup> Eduardo Balbinot<sup>2</sup>, Michele Bellazzini<sup>26</sup>

Chris R. Benn,<sup>14</sup> Sara R. Berlanas<sup>47,48</sup> Edouard J. Bernard<sup>10</sup>, Philip Best,<sup>28</sup> Daniela Bettoni,<sup>13</sup>

Andrea Bianco,<sup>49</sup> Georgia Bishop,<sup>3</sup> Michael Blomqvist,<sup>12</sup> Corrado Boeche,<sup>13</sup> Micol Bolzonella<sup>26</sup>

Silvia Bonoli,<sup>50,51</sup> Albert Bosma<sup>12</sup>, Nikolay Britavskiy,<sup>5,6,97</sup> Gianni Busarello,<sup>37</sup> Elisabetta Caffau,<sup>24</sup>

Tristan Cantat-Gaudin,<sup>18,19,20</sup> Alfred Castro-Ginard,<sup>18,19,20,52</sup> Guilherme Couto,<sup>53</sup>

Juan Carbajo-Hijarrubia,<sup>18,19,20</sup> David Carter,<sup>33†</sup> Laia Casamiquela,<sup>24,54</sup> Ana M. Conrado,<sup>31</sup>

Pablo Corcho-Caballero<sup>55,56,57</sup>, Luca Costantin,<sup>58</sup> Alis Deason<sup>59,60</sup>, Abel de Burgos,<sup>5</sup>

Sabrina De Grandi,<sup>49</sup> Paola Di Matteo,<sup>24</sup> Jesús Domínguez-Gómez<sup>61</sup>, Ricardo Dorda,<sup>96</sup> Alyssa Drake,<sup>7</sup>

Rajeshwari Dutta<sup>49,63</sup>, Denis Erkal<sup>64</sup>, Sofia Feltzing<sup>25</sup>, Anna Ferré-Mateu<sup>5</sup>, Diane Feuillet<sup>25</sup>

Francesca Figueras,<sup>18,19,20</sup> Matteo Fossati<sup>11,63</sup>, Elena Franciosini<sup>16</sup>, Antonio Frasca<sup>45</sup>

Michele Fumagalli<sup>63,65</sup>, Anna Gallazzi<sup>16</sup>, Rubén García-Benito<sup>31</sup>, Nicola Gentile Fusillo<sup>9</sup>

Marwan Gebran,<sup>66</sup> James Gilbert,<sup>1,67</sup> T. M. Gledhill<sup>7</sup>, Rosa M. González Delgado,<sup>31</sup> Robert Greimel,<sup>68</sup>

Mario Giuseppe Guarcello,<sup>69</sup> Jose Guerra,<sup>70</sup> Marco Gullieuszik,<sup>13</sup> Christopher P. Haines<sup>11,39</sup>

Martin J. Hardcastle<sup>7</sup>, Amy Harris<sup>7</sup>, Misha Haywood,<sup>24</sup> Amina Helmi,<sup>2</sup> Nauzet Hernandez,<sup>70</sup>

Artemio Herrero<sup>5,6</sup>, Sarah Hughes,<sup>1</sup> Vid Iršič,<sup>71</sup> Pascale Jablonka,<sup>24</sup> Matt J. Jarvis<sup>1,72</sup>

Carme Jordi,<sup>18,19,20</sup> Rohit Kondapally,<sup>28</sup> Georges Kordopatis,<sup>10</sup> Jens-Kristian Krogager<sup>73</sup>

Francesco La Barbera,<sup>37</sup> Man I Lam,<sup>53</sup> Søren S. Larsen<sup>74</sup>, Bertrand Lemasle,<sup>75</sup> Ian J. Lewis,<sup>1</sup>

Emilie Lhomé,<sup>14,76</sup> Karin Lind,<sup>36,77,79</sup> Marcello Lodi,<sup>70</sup> Alessia Longobardi<sup>49,63</sup>, Ilaria Lonoce<sup>11,80</sup>

Laura Magrini<sup>16</sup>, Jesús Maíz Apellániz<sup>81</sup>, Olivier Marchal,<sup>29</sup> Amparo Marco,<sup>62</sup>

Nicolas F. Martin<sup>29,78</sup>, Tadafumi Matsuno,<sup>2</sup> Sophie Maurogordato,<sup>10</sup> Paola Merluzzi,<sup>37</sup>

<footnote>* E-mail: jin@astro.rug.nl (SJ); sctrager@astro.rug.nl (SCT); gavin.dalton@physics.ox.ac.uk (GBD)</footnote>

<footnote>† Deceased.</footnote>

<footnote>‡ Anna Boyksen fellow.</footnote>
# The wide-field, multiplexed, spectroscopic facility WEAVE: Survey design, overview, and simulated implementation

Shoko Jin<sup>1,2,3,4*</sup> Scott C. Trager,<sup>2*</sup> Gavin B. Dalton,<sup>1,3*</sup> J. Alfonso L. Aguerri,<sup>5,6</sup> J. E. Drew<sup>7,8</sup>

Jesús Falcón-Barroso,<sup>5,6</sup> Boris T. Gänsicke<sup>9</sup>, Vanessa Hill,<sup>10</sup> Angela Iovino,<sup>11</sup> Matthew M. Pieri,<sup>12</sup>

Bianca M. Poggianti,<sup>13</sup> D. J. B. Smith<sup>7</sup>, Antonella Vallenari,<sup>13</sup> Don Carlos Abrams,<sup>14</sup>

David S. Aguado,<sup>15,16,17</sup> Teresa Antoja<sup>18,19,20</sup> Alfonso Aragón-Salamanca<sup>21</sup> Yago Ascasibar,<sup>22</sup>

Carine Babusiaux,<sup>23,24</sup> Marc Balcells,<sup>5,6,14</sup> R. Barrena<sup>5,6</sup>, Giuseppina Battaglia,<sup>5,6</sup> Vasily Belokurov<sup>17</sup>

Thomas Bensby,<sup>25</sup> Piercarlo Bonifacio<sup>24</sup>, Angela Bragaglia,<sup>26</sup> Esperanza Carrasco<sup>27</sup>

Ricardo Carrera<sup>13</sup>, Daniel J. Cornwell<sup>21</sup>, Lilian Domínguez-Palmero,<sup>5,6,14</sup> Kenneth J. Duncan<sup>28</sup>

Benoit Famaey<sup>29</sup>, Cecilia Fariña,<sup>5,6,14</sup> Oscar A. Gonzalez,<sup>30</sup> Steve Guest,<sup>3</sup> Nina A. Hatch<sup>21</sup>

Kelley M. Hess,<sup>2,31,32</sup> Matthew J. Hoskin<sup>9</sup>, Mike Irwin,<sup>17</sup> Johan H. Knapen,<sup>5,6,33</sup>

Sergey E. Koposov<sup>17,18,28,34</sup>, Ulrike Kuchner<sup>21</sup>, Clotilde Laigle,<sup>35</sup> Jim Lewis,<sup>17†</sup> Marcella Longhetti,<sup>11</sup>

Sara Lucatello,<sup>13,36‡</sup> Jairo Méndez-Abreu<sup>5,6</sup>, Amata Mercurio<sup>37,89</sup>, Alireza Molaeinezhad,<sup>1,5,6,17</sup>

Maria Monguió<sup>7,18,19,20</sup> Sean Morrison<sup>12,38</sup>, David N. A. Murphy,<sup>17</sup> Luis Peralta de Arriba<sup>17</sup>

Isabel Pérez,<sup>40,61</sup> Ignasi Pérez-Ràfols<sup>12,18,41</sup> Sergio Picó,<sup>14</sup> Roberto Raddi<sup>42</sup>

Mercè Romero-Gómez,<sup>18,19,20</sup> Frédéric Royer,<sup>24</sup> Arnaud Siebert,<sup>29</sup> George M. Seabroke,<sup>43</sup>

Debopam Som<sup>44</sup>, David Terrett,<sup>3</sup> Guillaume Thomas,<sup>5,6</sup> Roger Wesson<sup>8</sup>, C. Clare Worley,<sup>17</sup>

Emilio J. Alfaro,<sup>31</sup> Carlos Allende Prieto,<sup>5,6</sup> Javier Alonso-Santiago,<sup>45</sup> Nicholas J. Amos<sup>14,46</sup>

Richard P. Ashley,<sup>14</sup> Lola Balaguer-Núñez<sup>18,19,20</sup> Eduardo Balbinot<sup>2</sup>, Michele Bellazzini<sup>26</sup>

Chris R. Benn,<sup>14</sup> Sara R. Berlanas<sup>47,48</sup> Edouard J. Bernard<sup>10</sup>, Philip Best,<sup>28</sup> Daniela Bettoni,<sup>13</sup>

Andrea Bianco,<sup>49</sup> Georgia Bishop,<sup>3</sup> Michael Blomqvist,<sup>12</sup> Corrado Boeche,<sup>13</sup> Micol Bolzonella<sup>26</sup>

Silvia Bonoli,<sup>50,51</sup> Albert Bosma<sup>12</sup>, Nikolay Britavskiy,<sup>5,6,97</sup> Gianni Busarello,<sup>37</sup> Elisabetta Caffau,<sup>24</sup>

Tristan Cantat-Gaudin,<sup>18,19,20</sup> Alfred Castro-Ginard,<sup>18,19,20,52</sup> Guilherme Couto,<sup>53</sup>

Juan Carbajo-Hijarrubia,<sup>18,19,20</sup> David Carter,<sup>33†</sup> Laia Casamiquela,<sup>24,54</sup> Ana M. Conrado,<sup>31</sup>

Pablo Corcho-Caballero<sup>55,56,57</sup>, Luca Costantin,<sup>58</sup> Alis Deason<sup>59,60</sup>, Abel de Burgos,<sup>5</sup>

Sabrina De Grandi,<sup>49</sup> Paola Di Matteo,<sup>24</sup> Jesús Domínguez-Gómez<sup>61</sup>, Ricardo Dorda,<sup>96</sup> Alyssa Drake,<sup>7</sup>

Rajeshwari Dutta<sup>49,63</sup>, Denis Erkal<sup>64</sup>, Sofia Feltzing<sup>25</sup>, Anna Ferré-Mateu<sup>5</sup>, Diane Feuillet<sup>25</sup>

Francesca Figueras,<sup>18,19,20</sup> Matteo Fossati<sup>11,63</sup>, Elena Franciosini<sup>16</sup>, Antonio Frasca<sup>45</sup>

Michele Fumagalli<sup>63,65</sup>, Anna Gallazzi<sup>16</sup>, Rubén García-Benito<sup>31</sup>, Nicola Gentile Fusillo<sup>9</sup>

Marwan Gebran,<sup>66</sup> James Gilbert,<sup>1,67</sup> T. M. Gledhill<sup>7</sup>, Rosa M. González Delgado,<sup>31</sup> Robert Greimel,<sup>68</sup>

Mario Giuseppe Guarcello,<sup>69</sup> Jose Guerra,<sup>70</sup> Marco Gullieuszik,<sup>13</sup> Christopher P. Haines<sup>11,39</sup>

Martin J. Hardcastle<sup>7</sup>, Amy Harris<sup>7</sup>, Misha Haywood,<sup>24</sup> Amina Helmi,<sup>2</sup> Nauzet Hernandez,<sup>70</sup>

Artemio Herrero<sup>5,6</sup>, Sarah Hughes,<sup>1</sup> Vid Iršič,<sup>71</sup> Pascale Jablonka,<sup>24</sup> Matt J. Jarvis<sup>1,72</sup>

Carme Jordi,<sup>18,19,20</sup> Rohit Kondapally,<sup>28</sup> Georges Kordopatis,<sup>10</sup> Jens-Kristian Krogager<sup>73</sup>

Francesco La Barbera,<sup>37</sup> Man I Lam,<sup>53</sup> Søren S. Larsen<sup>74</sup>, Bertrand Lemasle,<sup>75</sup> Ian J. Lewis,<sup>1</sup>

Emilie Lhomé,<sup>14,76</sup> Karin Lind,<sup>36,77,79</sup> Marcello Lodi,<sup>70</sup> Alessia Longobardi<sup>49,63</sup>, Ilaria Lonoce<sup>11,80</sup>

Laura Magrini<sup>16</sup>, Jesús Maíz Apellániz<sup>81</sup>, Olivier Marchal,<sup>29</sup> Amparo Marco,<sup>62</sup>

Nicolas F. Martin<sup>29,78</sup>, Tadafumi Matsuno,<sup>2</sup> Sophie Maurogordato,<sup>10</sup> Paola Merluzzi,<sup>37</sup>

<footnote>* E-mail: jin@astro.rug.nl (SJ); sctrager@astro.rug.nl (SCT); gavin.dalton@physics.ox.ac.uk (GBD)</footnote>

<footnote>† Deceased.</footnote>

<footnote>‡ Anna Boyksen fellow.</footnote>
Jordi Miralda-Escudé,¹⁸,¹⁹,²⁰,⁸² Emilio Molinari,⁸³ Giacomo Monari,²⁹ Lorenzo Morelli,¹¹,³⁹
Christopher J. Mottram,³³ Tim Naylor,⁸⁴ Ignacio Negueruela,⁴⁷ Jose Oñorbe,⁸⁵ Elena Pancino,¹⁶
Sébastien Peirani,¹⁰,³⁵ Reynier F. Peletier,² Lucia Pozzetti,²⁶ Monica Rainer,⁴⁹ Pau Ramos,¹⁸,¹⁹,²⁰,⁸⁶
Shaun C. Read,²¹ Elena Maria Rossi,⁵² Huub J. A. Röttgering,⁵² Jose Alberto Rubiño-Martín,⁵,⁶
Jose Sabater,²⁸ José San Juan,⁷⁰ Nicoletta Sanna,¹⁶ Ellen Schallig,¹,⁸⁷ Ricardo P. Schiavon,³³
Mathias Schultheis,¹⁰ Paolo Serra,⁸³ Timothy W. Shimwell,³² Sergio Simón-Díaz,⁵,⁶ Russell J. Smith,⁶⁰
Rosanna Sordo,¹³ Daniele Sorini,²⁸,⁵⁹,⁸⁸ Caroline Soubiran,⁵⁴ Else Starkenburg,² Iain A. Steele,³³
John Stott,⁴⁶ Remko Stuik,⁵²,⁸⁷ Eline Tolstoy,² Crescenzo Tortora,³⁷ Maria Tsantaki,¹⁶
Mathieu Van der Swaelmen,¹⁶ Reinout J. van Weeren,⁵² Daniela Vergani,²⁶ Marc A. W. Verheijen,²
Kristiina Verro,² Jorick S. Vink,⁹⁰ Miguel Vioque,⁹¹,⁹²,⁹³ C. Jakob Walcher,⁵³ Nicholas A. Walton,¹⁷
Christopher Wegg,¹⁰ Anne-Marie Weijmans,⁹⁴ Wendy L. Williams,⁵² Andrew J. Wilson,⁸⁴
Nicholas J. Wright,⁴⁸ Theodora Xylakis-Dornbusch,⁷⁹,⁹⁵ Kris Youakim,⁷⁷ Stefano Zibetti,¹⁶
and Cristina Zurita⁵,⁶

*Affiliations are listed at the end of the paper*

Accepted 2022 December 4. Received 2022 December 4; in original form 2022 September 10

## ABSTRACT

WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William Herschel Telescope, saw first light in late 2022. WEAVE comprises a new 2-deg field-of-view prime-focus corrector system, a nearly 1000-multiplex fibre positioner, 20 individually deployable ‘mini’ integral field units (IFUs), and a single large IFU. These fibre systems feed a dual-beam spectrograph covering the wavelength range 366–959 nm at *R* ~ 5000, or two shorter ranges at *R* ~ 20 000. After summarizing the design and implementation of WEAVE and its data systems, we present the organization, science drivers, and design of a five- to seven-year programme of eight individual surveys to: (i) study our Galaxy’s origins by completing *Gaia*’s phase-space information, providing metallicities to its limiting magnitude for ~3 million stars and detailed abundances for ~1.5 million brighter field and open-cluster stars; (ii) survey ~0.4 million Galactic-plane OBA stars, young stellar objects, and nearby gas to understand the evolution of young stars and their environments; (iii) perform an extensive spectral survey of white dwarfs; (iv) survey ~400 neutral-hydrogen-selected galaxies with the IFUs; (v) study properties and kinematics of stellar populations and ionized gas in *z* < 0.5 cluster galaxies; (vi) survey stellar populations and kinematics in ~25 000 field galaxies at 0.3 ≲ *z* ≲ 0.7; (vii) study the cosmic evolution of accretion and star formation using >1 million spectra of LOFAR-selected radio sources; and (viii) trace structures using intergalactic/circumgalactic gas at *z* > 2. Finally, we describe the WEAVE Operational Rehearsals using the WEAVE Simulator.

**Key words:** instrumentation: spectrographs – surveys – stars: general – Galaxy: general – galaxies: general – cosmology: observations.

## 1 INTRODUCTION

The last decade has seen the opening of new windows on the Universe, made possible by the development of new instrumentation. Two of these new windows are ultra-high-precision astrometry, enabled by the Gaia mission (Gaia Collaboration 2016) and providing us with radically new views on the structure, dynamics, and evolution of our own Milky Way system, and low-frequency radio astronomy, enabled by Low Frequency Array Telescope (LOFAR; van Haarlem et al. 2013). The latter is already revealing millions of active galactic nuclei (AGNs) and star-forming galaxies (SFGs) across cosmic time, and their interactions with their local circumgalactic and intergalactic media (e.g. Duncan et al. 2019, 2021; Shimwell et al. 2019; Williams et al. 2019; Kondapally et al. 2021; Sabater et al. 2021; Tasse et al. 2021), while the former has provided unprecedented insights into – or constraints on – subject matters such as star formation in the Galactic disc (e.g. Ruiz-Lara et al. 2020), the dynamics of the

Milky Way disc (e.g. Antoja et al. 2018; Bennett & Bovy 2019; Laporte et al. 2019), the mass of the Milky Way (e.g. Posti & Helmi 2019; Wegg, Gerhard & Bieth 2019; Cautun et al. 2020), the dynamics of satellite galaxies (e.g. Battaglia et al. 2022), the effect of the Large Magellanic Cloud on the dynamics of the Milky Way and stellar streams (e.g. Vasiliev, Belokurov & Erkal 2021), stellar streams in the Galactic halo (e.g. Malhan, Ibata & Martin 2018), a major accretion event (Gaia Enceladus Sausage; e.g. Belokurov et al. 2018; Helmi et al. 2018), the chemo-dynamics of stellar streams (Mackereth et al. 2019) and determination of stellar ages in the disc (Bland-Hawthorn et al. 2019) through combining Gaia data with those from complementary surveys, and the Galactic disc as seen from its component stellar clusters (Cantat-Gaudin et al. 2020).

At the start of the last decade, the European astronomical community, through the European Commission-funded ASTRONET programme, laid out a set of recommendations for solving pressing
Jordi Miralda-Escudé,¹⁸,¹⁹,²⁰,⁸² Emilio Molinari,⁸³ Giacomo Monari,²⁹ Lorenzo Morelli,¹¹,³⁹
Christopher J. Mottram,³³ Tim Naylor,⁸⁴ Ignacio Negueruela,⁴⁷ Jose Oñorbe,⁸⁵ Elena Pancino,¹⁶
Sébastien Peirani,¹⁰,³⁵ Reynier F. Peletier,² Lucia Pozzetti,²⁶ Monica Rainer,⁴⁹ Pau Ramos,¹⁸,¹⁹,²⁰,⁸⁶
Shaun C. Read,²¹ Elena Maria Rossi,⁵² Huub J. A. Röttgering,⁵² Jose Alberto Rubiño-Martín,⁵,⁶
Jose Sabater,²⁸ José San Juan,⁷⁰ Nicoletta Sanna,¹⁶ Ellen Schallig,¹,⁸⁷ Ricardo P. Schiavon,³³
Mathias Schultheis,¹⁰ Paolo Serra,⁸³ Timothy W. Shimwell,³² Sergio Simón-Díaz,⁵,⁶ Russell J. Smith,⁶⁰
Rosanna Sordo,¹³ Daniele Sorini,²⁸,⁵⁹,⁸⁸ Caroline Soubiran,⁵⁴ Else Starkenburg,² Iain A. Steele,³³
John Stott,⁴⁶ Remko Stuik,⁵²,⁸⁷ Eline Tolstoy,² Crescenzo Tortora,³⁷ Maria Tsantaki,¹⁶
Mathieu Van der Swaelmen,¹⁶ Reinout J. van Weeren,⁵² Daniela Vergani,²⁶ Marc A. W. Verheijen,²
Kristiina Verro,² Jorick S. Vink,⁹⁰ Miguel Vioque,⁹¹,⁹²,⁹³ C. Jakob Walcher,⁵³ Nicholas A. Walton,¹⁷
Christopher Wegg,¹⁰ Anne-Marie Weijmans,⁹⁴ Wendy L. Williams,⁵² Andrew J. Wilson,⁸⁴
Nicholas J. Wright,⁴⁸ Theodora Xylakis-Dornbusch,⁷⁹,⁹⁵ Kris Youakim,⁷⁷ Stefano Zibetti,¹⁶
and Cristina Zurita⁵,⁶

*Affiliations are listed at the end of the paper*

Accepted 2022 December 4. Received 2022 December 4; in original form 2022 September 10

## ABSTRACT

WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William Herschel Telescope, saw first light in late 2022. WEAVE comprises a new 2-deg field-of-view prime-focus corrector system, a nearly 1000-multiplex fibre positioner, 20 individually deployable ‘mini’ integral field units (IFUs), and a single large IFU. These fibre systems feed a dual-beam spectrograph covering the wavelength range 366–959 nm at R ~ 5000, or two shorter ranges at R ~ 20 000. After summarizing the design and implementation of WEAVE and its data systems, we present the organization, science drivers, and design of a five- to seven-year programme of eight individual surveys to: (i) study our Galaxy’s origins by completing *Gaia’s* phase-space information, providing metallicities to its limiting magnitude for ~3 million stars and detailed abundances for ~1.5 million brighter field and open-cluster stars; (ii) survey ~0.4 million Galactic-plane OBA stars, young stellar objects, and nearby gas to understand the evolution of young stars and their environments; (iii) perform an extensive spectral survey of white dwarfs; (iv) survey ~400 neutral-hydrogen-selected galaxies with the IFUs; (v) study properties and kinematics of stellar populations and ionized gas in z < 0.5 cluster galaxies; (vi) survey stellar populations and kinematics in ~25 000 field galaxies at 0.3 ≲ z ≲ 0.7; (vii) study the cosmic evolution of accretion and star formation using >1 million spectra of LOFAR-selected radio sources; and (viii) trace structures using intergalactic/circumgalactic gas at z > 2. Finally, we describe the WEAVE Operational Rehearsals using the WEAVE Simulator.

**Key words:** instrumentation: spectrographs – surveys – stars: general – Galaxy: general – galaxies: general – cosmology: observations.

## 1 INTRODUCTION

The last decade has seen the opening of new windows on the Universe, made possible by the development of new instrumentation. Two of these new windows are ultra-high-precision astrometry, enabled by the Gaia mission (Gaia Collaboration 2016) and providing us with radically new views on the structure, dynamics, and evolution of our own Milky Way system, and low-frequency radio astronomy, enabled by Low Frequency Array Telescope (LOFAR; van Haarlem et al. 2013). The latter is already revealing millions of active galactic nuclei (AGNs) and star-forming galaxies (SFGs) across cosmic time, and their interactions with their local circumgalactic and intergalactic media (e.g. Duncan et al. 2019, 2021; Shimwell et al. 2019; Williams et al. 2019; Kondapally et al. 2021; Sabater et al. 2021; Tasse et al. 2021), while the former has provided unprecedented insights into – or constraints on – subject matters such as star formation in the Galactic disc (e.g. Ruiz-Lara et al. 2020), the dynamics of the

Milky Way disc (e.g. Antoja et al. 2018; Bennett & Bovy 2019; Laporte et al. 2019), the mass of the Milky Way (e.g. Posti & Helmi 2019; Wegg, Gerhard & Bieth 2019; Cautun et al. 2020), the dynamics of satellite galaxies (e.g. Battaglia et al. 2022), the effect of the Large Magellanic Cloud on the dynamics of the Milky Way and stellar streams (e.g. Vasiliev, Belokurov & Erkal 2021), stellar streams in the Galactic halo (e.g. Malhan, Ibata & Martin 2018), a major accretion event (Gaia Enceladus Sausage; e.g. Belokurov et al. 2018; Helmi et al. 2018), the chemo-dynamics of stellar streams (Mackereth et al. 2019) and determination of stellar ages in the disc (Bland-Hawthorn et al. 2019) through combining Gaia data with those from complementary surveys, and the Galactic disc as seen from its component stellar clusters (Cantat-Gaudin et al. 2020).

At the start of the last decade, the European astronomical community, through the European Commission-funded ASTRONET programme, laid out a set of recommendations for solving pressing
astrophysical problems in the following decade and the infrastructure required for those solutions. A clear recommendation from both the ASTRONET Wide-Field Spectroscopy Working Group<fnref n="1" /> and the report on Europe's 2-4 m telescopes by the European Telescope Strategic Review Committee<fnref n="2" /> was for an intermediate-resolution (R ~ 5000), wide-field (≥1-deg diameter) multi-object spectrograph with a multiplex of >500 (and preferably >1000) to follow up the Gaia mission and to allow for precision-cosmology experiments; an extension to even higher resolution (R ≥ 20 000) with the same instrument was deemed desirable, as this extension would allow for chemo-dynamical labelling of structures in the Milky Way.

Roughly simultaneously, in 2010 January, the Isaac Newton Group of Telescopes (ING) held a workshop on 'Science with the William Herschel Telescope 2010-2020',<fnref n="3" /> where ideas for wide-field spectroscopy with the William Herschel Telescope<fnref n="4" /> (WHT) were discussed. Previous, ongoing, and then-planned surveys [e.g. Sloan Digital Sky Survey (SDSS): York et al. 2000; Sloan Extension for Galactic Understanding and Exploration (SEGUE)-1: Yanny et al. 2009; SEGUE-2: Eisenstein et al. 2011; Apache Point Observatory Galactic Evolution Experiment (APOGEE)-1: Blanton et al. 2017; APOGEE-2: Eisenstein et al. 2011; RAdial Velocity Experiment (RAVE): Steinmetz et al. 2006; Large sky Area Multi-Object fibre Spectroscopic Telescope (LAMOST): Cui et al. 2012; Luo et al. 2015; Gaia-ESO: Gilmore et al. 2012, 2022; Randich et al. 2022; GALactic Archaeology with HERMES (GALAH): De Silva et al. 2015)] have demonstrated the immense power of wide-field spectroscopy for answering important questions about the formation and evolution of galaxies and their stars and the structure of the Universe, leading to the conclusion – fully supported by the ASTRONET recommendations – that such a survey instrument was vital for the ING's long-term vision. During follow-up discussions, a small team developed a set of desirable requirements for an instrument capable of both providing the missing aspects of the (then) upcoming survey instruments Gaia, LOFAR, and Apertif (Verheijen et al. 2008; van Cappellen et al. 2022), and science goals extending beyond those three instruments: these are listed in Table 1. These requirements and a well-developed design for such an instrument were presented to the community in 2015, while members of the community presented their own aspirations for multi-object spectroscopy (MOS) for the coming decade at the same conference (Skillen, Balcells & Trager 2016), resulting in a much broader science case covering cosmology, galaxy evolution, and stellar evolution on scales from the local solar neighbourhood to redshifts in excess of 4.

The WHT Enhanced Area Velocity Explorer (WEAVE<fnref n="5" />; Dalton et al. 2012, 2016a) is the result of these discussions: a next-generation wide-field spectroscopic survey facility for the 4.2 m WHT at the Observatorio del Roque de los Muchachos on La Palma, Spain. It provides the spectroscopic follow-up required for full scientific exploitation of the Gaia, LOFAR, and Apertif surveys (for references, see Table 1) in the Northern hemisphere and, furthermore, to study advanced phases of stellar evolution, galaxy evolution in different environments over the last 5-8 Gyr, and the changing scale of the Universe and therefore its most basic parameters since near its

<table><caption>**Table 1.** Desirable requirements for follow-up of the Gaia mission (Gaia Collaboration 2016), and LOFAR (van Haarlem et al. 2013) and Apertif (Verheijen et al. 2008) surveys.</caption><thead><tr><th>Instrument</th><th>Desirable requirements</th></tr></thead><tbody><tr><td>Gaia<sup>a, b</sup></td><td>R = 5000 for radial velocities at 17 mag ≤ V ≤ 20 mag<br>R = 20 000 for stellar abundances at<br>12 mag ≤ V ≤ 17 mag<br>10<sup>7</sup> stars over 10<sup>4</sup> contiguous square degrees</td></tr><tr><td>LOFAR<sup>c, d</sup></td><td>Continuous wavelength coverage over 370–980 nm<br>V ≤ 21.5 mag at S/N = 5 in continuum for redshifts<br>10<sup>7</sup> galaxies over 10<sup>4</sup> square degrees</td></tr><tr><td>Apertif<sup>e, f</sup></td><td>Large IFU and mini-IFUs for spatially resolved spectra of<br>gas-rich galaxies<br>10<sup>4</sup> galaxies over 10<sup>4</sup> continuous square degrees</td></tr></tbody></table>

beginning. The facility comprises a new 2-deg field-of-view prime-focus corrector system with a 1000-multiplex fibre positioner, 20 individually deployable 'mini' integral field units (mIFUs), and a single 'large' integral field unit (LIFU). The fibres feed a dual-beam spectrograph that will provide full coverage of most of the visible spectrum in a single exposure at a spectral resolution of ~5000, or a more limited wavelength coverage in both arms at a resolution of ~20 000. The timely arrival of WEAVE at the precise moment when Gaia and LOFAR are producing their groundbreaking results guarantees its utility to the community in the coming half-decade and beyond.

The design of WEAVE was driven by the fact that neither Gaia nor LOFAR provide all of the information required to fully exploit the data they produce. Gaia produces radial velocities mostly for sources with magnitude G < 14.5 at present (Gaia DR3; Katz et al. 2023) and G ~ 16–17⁶ by the end of the Gaia mission. In both cases, these limiting magnitudes are where the Gaia completeness is reasonably high, while measurements are still available in a tail of fainter stars. These limits are also far brighter than the astrometric magnitude limit of G < 20–21 in both cases. Similarly, although the exquisite low-frequency continuum sensitivity of the LOFAR Surveys Key Science Project (Röttgering et al. 2011) observations in the LOFAR Two-metre Sky Survey (LoTSS; e.g. Shimwell et al. 2017, 2019, 2022; Tasse et al. 2021) enables the detection of extremely faint radio sources, the redshift (i.e. distance) information that is necessary to distinguish between their apparent and intrinsic properties must be obtained via other means. Extensive optical spectroscopy over a large contiguous wavelength range is the ideal way of achieving this.

Furthermore, other pathfinder telescopes for the Square Kilometre Array,<fnref n="7" /> such as the Apertif focal-plane array system (Verheijen et al. 2008; van Cappellen et al. 2022) on the Westerbork Synthesis Radio Telescope, will reveal the neutral hydrogen sky through wide-field surveys not previously possible. These surveys are also incomplete, as connecting the H I sky – inherently three-dimensional (positions plus radial velocity are simultaneously observed) – to the optical sky

<footnote>ahttps://www.esa.int/Science_Exploration/Space_Science/Gaia</footnote>

<footnote>bhttps://www.cosmos.esa.int/web/gaia/data-release-3</footnote>

<footnote>chttps://science.astron.nl/telescopes/lofar/</footnote>

<footnote>dhttps://lofar-surveys.org/dr2_release.html</footnote>

<footnote>ehttps://science.astron.nl/telescopes/wsrt-apertif/</footnote>

<footnote>fhttps://tinyurl.com/2p8d64av</footnote>

<footnote n="6">⁶This G magnitude range corresponds to the $G_{RVS} \le 16$ limit expected (Sartoreti et al. 2023).</footnote>

<footnote n="7">⁷http://www.skatelescope.org</footnote>

<footnote n="1">¹https://www.astronet-eu.org/sites/default/files/d26-vsdef-2.pdf</footnote>

<footnote n="2">²https://www.astronet-eu.org/sites/default/files/plaquettet2_4m-final-2.pdf</footnote>

<footnote n="3">³http://www.ing.iac.es/conferences/wht201020/</footnote>

<footnote n="4">⁴http://www.ing.iac.es/astronomy/telescopes/wht/</footnote>

<footnote n="5">⁵https://ingconfluence.ing.iac.es/confluence/display/WEAV</footnote>
astrophysical problems in the following decade and the infrastructure required for those solutions. A clear recommendation from both the ASTRONET Wide-Field Spectroscopy Working Group<fnref n="1" /> and the report on Europe's 2–4 m telescopes by the European Telescope Strategic Review Committee<fnref n="2" /> was for an intermediate-resolution (R ~ 5000), wide-field (≥1-deg diameter) multi-object spectrograph with a multiplex of >500 (and preferably >1000) to follow up the Gaia mission and to allow for precision-cosmology experiments; an extension to even higher resolution (R ≥ 20 000) with the same instrument was deemed desirable, as this extension would allow for chemo-dynamical labelling of structures in the Milky Way.

Roughly simultaneously, in 2010 January, the Isaac Newton Group of Telescopes (ING) held a workshop on 'Science with the William Herschel Telescope 2010–2020',<fnref n="3" /> where ideas for wide-field spectroscopy with the William Herschel Telescope<fnref n="4" /> (WHT) were discussed. Previous, ongoing, and then-planned surveys [e.g. Sloan Digital Sky Survey (SDSS): York et al. 2000; Sloan Extension for Galactic Understanding and Exploration (SEGUE)-1: Yanny et al. 2009; SEGUE-2: Eisenstein et al. 2011; Apache Point Observatory Galactic Evolution Experiment (APOGEE)-1: Blanton et al. 2017; APOGEE-2: Eisenstein et al. 2011; RAdial Velocity Experiment (RAVE): Steinmetz et al. 2006; Large sky Area Multi-Object fibre Spectroscopic Telescope (LAMOST): Cui et al. 2012; Luo et al. 2015; Gaia-ESO: Gilmore et al. 2012, 2022; Randich et al. 2022; GALactic Archaeology with HERMES (GALAH): De Silva et al. 2015)] have demonstrated the immense power of wide-field spectroscopy for answering important questions about the formation and evolution of galaxies and their stars and the structure of the Universe, leading to the conclusion – fully supported by the ASTRONET recommendations – that such a survey instrument was vital for the ING's long-term vision. During follow-up discussions, a small team developed a set of desirable requirements for an instrument capable of both providing the missing aspects of the (then) upcoming survey instruments Gaia, LOFAR, and Apertif (Verheijen et al. 2008; van Cappellen et al. 2022), and science goals extending beyond those three instruments: these are listed in Table 1. These requirements and a well-developed design for such an instrument were presented to the community in 2015, while members of the community presented their own aspirations for multi-object spectroscopy (MOS) for the coming decade at the same conference (Skillen, Balcells & Trager 2016), resulting in a much broader science case covering cosmology, galaxy evolution, and stellar evolution on scales from the local solar neighbourhood to redshifts in excess of 4.

The WHT Enhanced Area Velocity Explorer (WEAVE<fnref n="5" />; Dalton et al. 2012, 2016a) is the result of these discussions: a next-generation wide-field spectroscopic survey facility for the 4.2 m WHT at the Observatorio del Roque de los Muchachos on La Palma, Spain. It provides the spectroscopic follow-up required for full scientific exploitation of the Gaia, LOFAR, and Apertif surveys (for references, see Table 1) in the Northern hemisphere and, furthermore, to study advanced phases of stellar evolution, galaxy evolution in different environments over the last 5–8 Gyr, and the changing scale of the Universe and therefore its most basic parameters since near its

<table><caption>**Table 1.** Desirable requirements for follow-up of the Gaia mission (Gaia Collaboration 2016), and LOFAR (van Haarlem et al. 2013) and Apertif (Verheijen et al. 2008) surveys.</caption><thead><tr><th>Instrument</th><th>Desirable requirements</th></tr></thead><tbody><tr><td>Gaia<sup>a, b</sup></td><td>R = 5000 for radial velocities at 17 mag ≤ V ≤ 20 mag<br>R = 20 000 for stellar abundances at<br>12 mag ≤ V ≤ 17 mag<br>10<sup>7</sup> stars over 10<sup>4</sup> contiguous square degrees</td></tr><tr><td>LOFAR<sup>c, d</sup></td><td>Continuous wavelength coverage over 370–980 nm<br>V ≤ 21.5 mag at S/N = 5 in continuum for redshifts<br>10<sup>7</sup> galaxies over 10<sup>4</sup> square degrees</td></tr><tr><td>Apertif<sup>e, f</sup></td><td>Large IFU and mini-IFUs for spatially resolved spectra of<br>gas-rich galaxies<br>10<sup>4</sup> galaxies over 10<sup>4</sup> continuous square degrees</td></tr></tbody></table>

beginning. The facility comprises a new 2-deg field-of-view prime-focus corrector system with a 1000-multiplex fibre positioner, 20 individually deployable 'mini' integral field units (mIFUs), and a single 'large' integral field unit (LIFU). The fibres feed a dual-beam spectrograph that will provide full coverage of most of the visible spectrum in a single exposure at a spectral resolution of ~5000, or a more limited wavelength coverage in both arms at a resolution of ~20 000. The timely arrival of WEAVE at the precise moment when Gaia and LOFAR are producing their groundbreaking results guarantees its utility to the community in the coming half-decade and beyond.

The design of WEAVE was driven by the fact that neither Gaia nor LOFAR provide all of the information required to fully exploit the data they produce. Gaia produces radial velocities mostly for sources with magnitude G < 14.5 at present (Gaia DR3; Katz et al. 2023) and G ~ 16–17⁶ by the end of the Gaia mission. In both cases, these limiting magnitudes are where the Gaia completeness is reasonably high, while measurements are still available in a tail of fainter stars. These limits are also far brighter than the astrometric magnitude limit of G < 20–21 in both cases. Similarly, although the exquisite low-frequency continuum sensitivity of the LOFAR Surveys Key Science Project (Röttgering et al. 2011) observations in the LOFAR Two-metre Sky Survey (LoTSS; e.g. Shimwell et al. 2017, 2019, 2022; Tasse et al. 2021) enables the detection of extremely faint radio sources, the redshift (i.e. distance) information that is necessary to distinguish between their apparent and intrinsic properties must be obtained via other means. Extensive optical spectroscopy over a large contiguous wavelength range is the ideal way of achieving this.

Furthermore, other pathfinder telescopes for the Square Kilometre Array,<fnref n="7" /> such as the Apertif focal-plane array system (Verheijen et al. 2008; van Cappellen et al. 2022) on the Westerbork Synthesis Radio Telescope, will reveal the neutral hydrogen sky through wide-field surveys not previously possible. These surveys are also incomplete, as connecting the H I sky – inherently three-dimensional (positions plus radial velocity are simultaneously observed) – to the optical sky

<footnote>ahttps://www.esa.int/Science_Exploration/Space_Science/Gaia</footnote>

<footnote>bhttps://www.cosmos.esa.int/web/gaia/data-release-3</footnote>

<footnote>chttps://science.astron.nl/telescopes/lofar/</footnote>

<footnote>dhttps://lofar-surveys.org/dr2_release.html</footnote>

<footnote>ehttps://science.astron.nl/telescopes/wsrt-apertif/</footnote>

<footnote>fhttps://tinyurl.com/2p8d64av</footnote>

<footnote n="6">⁶This G magnitude range corresponds to the $G_{RVS} \le 16$ limit expected (Sartoreti et al. 2023).</footnote>

<footnote n="7">⁷http://www.skatelescope.org</footnote>

<footnote n="1">¹https://www.astronet-eu.org/sites/default/files/d26-vsdef-2.pdf</footnote>

<footnote n="2">²https://www.astronet-eu.org/sites/default/files/plaquettet2_4m-final-2.pdf</footnote>

<footnote n="3">³http://www.ing.iac.es/conferences/wht201020/</footnote>

<footnote n="4">⁴http://www.ing.iac.es/astronomy/telescopes/wht/</footnote>

<footnote n="5">⁵https://ingconfluence.ing.iac.es/confluence/display/WEAV</footnote>
requires three-dimensional optical data. Obtaining such data is now
possible through large-field and/or multiple IFUs (e.g. Kelz et al.
2006; Drory et al. 2015). An instrument with both kinds of IFU
capabilities, capable of targeting large and multiple small targets, is
necessary for the full exploitation of upcoming HI surveys.

The following eight independent surveys will be carried out with WEAVE over a period of (at least) five years, producing more than 30 million spectra of nearly 10 million objects:

(i) a survey of the Milky Way galaxy, providing radial velocities and stellar abundances for stars too faint for these quantities to be measured by Gaia [WEAVE Galactic Archaeology (GA); Section 4.1];

(ii) a survey characterizing young and massive stellar populations and the interstellar medium (ISM), and thus probing star formation and evolution, across the Galactic Plane [Stellar, Circumstellar, and Interstellar Physics (SCIP); Section 4.2];

(iii) a survey studying the death of stars and constraining the local star formation history of the Galaxy through its white dwarf population (WEAVE White Dwarfs; Section 4.3);

(iv) a survey of the stellar and gaseous kinematics and physical properties of gas-rich galaxies, providing a necessary optical complement to Apertif's neutral hydrogen surveys of the local Universe (WEAVE-Apertif; Section 4.4);

(v) a survey probing the evolution of galaxies as a function of environment, from the cores of rich clusters to their lively environs, going from their smallest members out to the field at cosmological distances (WEAVE Galaxy Clusters; Section 4.5);

(vi) a survey providing the first detailed view of the stellar population properties of galaxies at $z = 0.3–0.7$ as a function of galaxy mass and environment, yielding star formation histories, stellar ages, stellar and gas metallicities, dust attenuation, gas kinematics, and stellar velocity dispersions [Stellar Populations at intermediate redshifts Survey (StePS); Section 4.6];

(vii) a survey probing galaxy evolution over cosmic time, providing much-desired redshifts and galaxy properties of LOFAR's radio sources [WEAVE-LOFAR (WL); Section 4.7];

(viii) a survey of large-scale structure using quasar absorption lines as a cosmic ruler to probe the expansion of the Universe, which also extends the study of gaseous environments to larger scales and earlier epochs [WEAVE-QSO (WQ); Section 4.8].

The WEAVE Survey will provide data that will help answer the questions: How did our Galaxy form and the stars within it evolve? How were other galaxies assembled? What are dark matter and dark energy? On these topics, WEAVE will be complementary to the surveys carried out by the 4-metre Multi-Object Spectroscopic Telescope (4MOST; de Jong et al. 2019), which has a similar design, including spectral range and resolution, and will operate in the Southern hemisphere on a similarly sized telescope, the Dark Energy Spectroscopic Instrument (DESI; DESI Collaboration 2016a, b), which will focus largely on baryonic acoustic oscillations of galaxies, SDSS-V (Kollmeier et al. 2017), which will have complementary, lower resolution optical spectra and, additionally, infrared spectroscopy, and the Subaru Prime Focus Spectrograph (PFS; Tamura et al. 2018), which will cover a smaller field on a larger telescope that is not fully dedicated to survey operations.

This paper first presents the design of the WEAVE instrument – in terms of both hardware and software – as relevant to WEAVE Survey preparation and execution (Section 2), followed by the structure of the WEAVE work and workforce including an overview of the key cross-team working groups involved in and responsible for the preparation and execution of the Survey (Section 3). We then present

<figure><img src="image_1.png" /><figcaption>**Figure 1.** A computer-aided diagram (CAD) representation of the WEAVE top-end assembly, incorporating the prime-focus (field) corrector system, instrument rotator, and fibre positioner. The four mounting units on the outer ring of the telescope structure provide for focus and tilt correction of the whole system.</figcaption></figure>

overviews of each of the eight component surveys that comprise the WEAVE Survey (Section 4), with each subsection summarizing the particular survey's science case and survey plan, as well as providing the relevant introduction and background to the field. This is followed by a description of the WEAVE Simulator (Section 5) and a summary of the simulated implementation of the Survey through 'Operational Rehearsals' (Section 6). Section 7 concludes the paper.

## 2 THE WEAVE FACILITY

The WHT is an alt-azimuth telescope with a Cassegrain focus and two Nasmyth foci (for a description of the original design features of the WHT, see Boksenberg 1985). WEAVE's predecessor on the WHT was the prime-focus instrument Autofib-2 (AF2; Parry et al. 1994), which had 150 fibres deployable over a 1-deg-diameter field of view. WEAVE has nearly 1000 fibres deployable over a field of view four times that of AF2 and, with four times the number of resolution elements of AF2 and three to four times its throughput, dramatically increases the resolution and multiplex power of the telescope.

Full details of the WEAVE design and performance will be presented in a separate paper (Dalton et al., in preparation). Here, we summarize only the key parameters of the WEAVE facility as they relate to the detailed design of the component surveys making up the total WEAVE Survey (cf. Dalton et al. 2016a).

### 2.1 A new top-end for the WHT

As part of the new top-end for the WHT (Fig. 1), the WEAVE prime-focus corrector system delivers a 2-deg-diameter field of view, with a flat focal plane at f/2.78 with full correction for atmospheric dispersion from 370–1000 nm to 65-deg zenith distance (Agócs et al. 2014). The telecentricity of the input beam will deviate by no more than 4 deg at the edge of the field. For a point-source image in the absence of seeing, the corrector delivers 80 per cent encircled energy within 0.71 arcsec, corresponding to a full width at half-maximum (FWHM) of 0.41 arcsec. The effect of differential image distortion with changing zenith distance is 0.13 arcsec across the field between 0- and 65-deg zenith distance.
requires three-dimensional optical data. Obtaining such data is now
possible through large-field and/or multiple IFUs (e.g. Kelz et al.
2006; Drory et al. 2015). An instrument with both kinds of IFU
capabilities, capable of targeting large and multiple small targets, is
necessary for the full exploitation of upcoming HI surveys.

The following eight independent surveys will be carried out with WEAVE over a period of (at least) five years, producing more than 30 million spectra of nearly 10 million objects:

(i) a survey of the Milky Way galaxy, providing radial velocities and stellar abundances for stars too faint for these quantities to be measured by Gaia [WEAVE Galactic Archaeology (GA); Section 4.1];

(ii) a survey characterizing young and massive stellar populations and the interstellar medium (ISM), and thus probing star formation and evolution, across the Galactic Plane [Stellar, Circumstellar, and Interstellar Physics (SCIP); Section 4.2];

(iii) a survey studying the death of stars and constraining the local star formation history of the Galaxy through its white dwarf population (WEAVE White Dwarfs; Section 4.3);

(iv) a survey of the stellar and gaseous kinematics and physical properties of gas-rich galaxies, providing a necessary optical complement to Apertif's neutral hydrogen surveys of the local Universe (WEAVE-Apertif; Section 4.4);

(v) a survey probing the evolution of galaxies as a function of environment, from the cores of rich clusters to their lively environs, going from their smallest members out to the field at cosmological distances (WEAVE Galaxy Clusters; Section 4.5);

(vi) a survey providing the first detailed view of the stellar population properties of galaxies at $z = 0.3-0.7$ as a function of galaxy mass and environment, yielding star formation histories, stellar ages, stellar and gas metallicities, dust attenuation, gas kinematics, and stellar velocity dispersions [Stellar Populations at intermediate redshifts Survey (StePS); Section 4.6];

(vii) a survey probing galaxy evolution over cosmic time, providing much-desired redshifts and galaxy properties of LOFAR's radio sources [WEAVE-LOFAR (WL); Section 4.7];

(viii) a survey of large-scale structure using quasar absorption lines as a cosmic ruler to probe the expansion of the Universe, which also extends the study of gaseous environments to larger scales and earlier epochs [WEAVE-QSO (WQ); Section 4.8].

The WEAVE Survey will provide data that will help answer the questions: How did our Galaxy form and the stars within it evolve? How were other galaxies assembled? What are dark matter and dark energy? On these topics, WEAVE will be complementary to the surveys carried out by the 4-metre Multi-Object Spectroscopic Telescope (4MOST; de Jong et al. 2019), which has a similar design, including spectral range and resolution, and will operate in the Southern hemisphere on a similarly sized telescope, the Dark Energy Spectroscopic Instrument (DESI; DESI Collaboration 2016a, b), which will focus largely on baryonic acoustic oscillations of galaxies, SDSS-V (Kollmeier et al. 2017), which will have complementary, lower resolution optical spectra and, additionally, infrared spectroscopy, and the Subaru Prime Focus Spectrograph (PFS; Tamura et al. 2018), which will cover a smaller field on a larger telescope that is not fully dedicated to survey operations.

This paper first presents the design of the WEAVE instrument – in terms of both hardware and software – as relevant to WEAVE Survey preparation and execution (Section 2), followed by the structure of the WEAVE work and workforce including an overview of the key cross-team working groups involved in and responsible for the preparation and execution of the Survey (Section 3). We then present

<figure><img src="image_2.png" /><figcaption>**Figure 1.** A computer-aided diagram (CAD) representation of the WEAVE top-end assembly, incorporating the prime-focus (field) corrector system, instrument rotator, and fibre positioner. The four mounting units on the outer ring of the telescope structure provide for focus and tilt correction of the whole system.</figcaption></figure>

overviews of each of the eight component surveys that comprise the WEAVE Survey (Section 4), with each subsection summarizing the particular survey's science case and survey plan, as well as providing the relevant introduction and background to the field. This is followed by a description of the WEAVE Simulator (Section 5) and a summary of the simulated implementation of the Survey through 'Operational Rehearsals' (Section 6). Section 7 concludes the paper.

## 2 THE WEAVE FACILITY

The WHT is an alt-azimuth telescope with a Cassegrain focus and two Nasmyth foci (for a description of the original design features of the WHT, see Boksenberg 1985). WEAVE's predecessor on the WHT was the prime-focus instrument Autofib-2 (AF2; Parry et al. 1994), which had 150 fibres deployable over a 1-deg-diameter field of view. WEAVE has nearly 1000 fibres deployable over a field of view four times that of AF2 and, with four times the number of resolution elements of AF2 and three to four times its throughput, dramatically increases the resolution and multiplex power of the telescope.

Full details of the WEAVE design and performance will be presented in a separate paper (Dalton et al., in preparation). Here, we summarize only the key parameters of the WEAVE facility as they relate to the detailed design of the component surveys making up the total WEAVE Survey (cf. Dalton et al. 2016a).

### 2.1 A new top-end for the WHT

As part of the new top-end for the WHT (Fig. 1), the WEAVE prime-focus corrector system delivers a 2-deg-diameter field of view, with a flat focal plane at f/2.78 with full correction for atmospheric dispersion from 370–1000 nm to 65-deg zenith distance (Agócs et al. 2014). The telecentricity of the input beam will deviate by no more than 4 deg at the edge of the field. For a point-source image in the absence of seeing, the corrector delivers 80 per cent encircled energy within 0.71 arcsec, corresponding to a full width at half-maximum (FWHM) of 0.41 arcsec. The effect of differential image distortion with changing zenith distance is 0.13 arcsec across the field between 0- and 65-deg zenith distance.
## 2.2 The WEAVE fibres and fibre positioner

Behind the prime-focus corrector system, fibres are deployed in the focal plane using a buffered positioner system developed from the 2dF concept (Lewis et al. 2002): the positioner can deploy up to 1008 fibre buttons on each of two field plates using a pair of Cartesian robots sharing a common x-axis. Each science-fibre button is 5 mm × 2 mm × 7 mm in size and terminates in a 1.5 mm × 1.5 mm prism with a concave upper surface to expand the telescope beam to f/3.2. Each MOS science fibre carries a 85 µm core fibre that subtends 1.3 arcsec on the sky. Eight buttons on each field hold coherent fibre imaging bundles to be used for acquisition and guiding. On the first plate ('plate A'), each of the eight guide-fibre bundles is packaged so that each bundle replaces six potential science fibres, leaving 960 science fibres available. On the second plate ('plate B'), 10 groups of six science fibres are replaced by pairs of deployable mIFUs. On plate B, the guide-fibre bundles each replace only a single science fibre to leave 940 science fibres available. The two MOS plates are at either end of the positioner, such that when one plate is being used for the current observation, the other can be set up for the upcoming observation. A MOS field can be configured in under one hour, thus setting the length of a typical WEAVE observation. Once the current observation has finished, the positioner tumbles to reverse the roles of the two plates (Lewis et al. 2014). This results in a marked improvement in observing efficiency as compared with AF2, as there will, in principle, be no loss in observing time due to field configurations when switching between the two MOS plates for observations with the MOS or mIFU modes with WEAVE. Table 2 summarizes the key parameters of the four focal-plane modes of WEAVE.

The 20 mIFUs are packaged similarly to the MOS fibres, but with slightly larger buttons (7 mm × 5 mm × 7 mm in size), each holding a hexagonal close-packed array of 37 fibres with the same size and aperture as the MOS fibres. The outer diameter of the fibre buffer layer is 120 µm, so each mIFU covers roughly 11 × 12 arcsec² on the sky with a filling factor of ≈0.50. The buffered arrangement of the two field plates also affords an intermediate location (i.e. at 90° to the MOS plates), which accommodates the LIFU that contains a hexagonal array of 547 fibres with 170 µm cores, equivalent to 2.6 arcsec on the sky, and hence subtending 90 × 78 arcsec² on the sky with a filling factor of ≈0.54, along with eight peripheral bundles of seven fibres, arranged in a ring of radius 150 arcsec from the centre of the array, to be used for sky subtraction. The LIFU package includes a discrete guiding camera mounted within the same physical structure as the LIFU head.

Each MOS observation is prepared in advance using a purpose-built software tool (CONFIGURE: Terrett et al. 2014, see Fig. 2), which builds on the simulated annealing approach of Miszalski et al. (2006) with some elements of the original 'Oxford' algorithm (Colless et al. 2001) to facilitate an optimal, on-the-fly assignment of fibres for sky-background determination without impacting the results of the annealing process. The output of CONFIGURE is an XML file containing all of the information required to specify an observation, including observing constraints, spectrograph configuration, and calibration lamp sequences. Observations for the mIFU and LIFU modes follow a similar but slightly simpler approach. Using CONFIGURE, the mIFUs in a given field can be manually allocated to targets such that the central fibre of each mIFU is located at a desired target position. Final sky coordinates for the remaining 36 fibres are output by CONFIGURE, based on the sky position angle of the mIFU field resulting from this allocation. Each mIFU-field set-up requires at least one of the mIFUs to be dedicated for calibration purposes. The central seven fibres are

identified with the primary calibration target (i.e. a white dwarf), and the remaining 30 fibres are labelled as sky positions. LIFU observations are prepared using a similar tool, with the output XML file providing information on guide-star positions and the precise on-sky position for each fibre within the LIFU.

## 2.3 The WEAVE spectrograph

All four sets of science fibres pass through a cable wrap around the prime-focus instrument rotator, along the vanes of the prime-focus support spider and down the telescope structure, to where they pass over the elevation axis and into the Groundbased High Resolution Imaging Laboratory (GHRIL) instrument room<fnref n="8" /> on the Nasmyth platform that houses the WEAVE spectrograph (see Fig. 3).

The spectrograph (Rogers et al. 2014) provides full wavelength coverage over the range 366–959 nm at a nominal resolving power of $R \sim 5000$ (the low-resolution, 'LR', mode) or a pair of restricted wavelength ranges at $R \sim 20000$ (the high-resolution, 'HR', mode; note that these resolving powers are halved when using the LIFU with its two-times larger fibres). This is achieved by a dual-beam design with an off-axis catadioptric<fnref n="9" /> $f/3.1$ collimator, which delivers a 190 nm collimated beam. The dichroic is located before the collimator correcting lenses so that these can be optimized for transmission in each arm. Each arm feeds an $f/1.8$ transmissive camera consisting of eight lenses (one asphere on the first lens), designed such that the lens prescriptions are common to both cameras, and only the coatings and lens spacings differ between the two. The useful focal plane of each camera is 8192 (spectral) × 6144 (spatial) pixels, populated by a pair of e2V CCD 231-C6 CCDs, with the physical size of the pixels being 15 µm. This oversize of the detector avoids use of a custom format, and also allows the gap in the spectral direction to be chosen in each camera to fall in a region of low interest. At the same time, placing this gap close to the centre of the image plane makes the control of Littrow ghosts easy by using Volume Phase Holographic (VPH) gratings with slightly slanted fringes (Burgh et al. 2007; Bianco et al. 2018), such that the image of the ghost is formed in the gap. Each arm of the spectrograph contains a rolling shutter mechanism and a grating exchange mechanism to allow the choice between two resolution modes, with the associated change of beam angles accommodated by articulation of the two cameras. The available modes and associated nominal spectral coverage are listed in Table 3.

Direct illumination of each camera's focal plane is achieved by a series of LEDs that are mounted on the spectrograph's shutter mechanisms. These are used to provide a direct measure of the pixel-to-pixel response variations of the detector, and a measure of the non-linearity present in the detector read-out electronics.

## 2.4 Calibration unit

The WHT has a set of projector lamps mounted at one of the broken-Cassegrain foci. These can be used to direct light to the primary focus via the tertiary mirror. Investigations during the development of WEAVE have shown that the illumination is adequate – but not perfect – out to the edge of the WEAVE field of view (Domínguez et al. 2016). Observations of a quartz-halogen lamp are used to determine the traces of the individual fibre spectra on the detector mosaic for each camera, and to provide a rough indication of the

<footnote n="8">⁸https://www.ing.iac.es//PR/inst.php?tel=wht&inst = GHRIL</footnote>

<footnote n="9">⁹A catadioptric optical system combines both reflection and refraction to reduce aberration effects.</footnote>
## 2.2 The WEAVE fibres and fibre positioner

Behind the prime-focus corrector system, fibres are deployed in the focal plane using a buffered positioner system developed from the 2dF concept (Lewis et al. 2002): the positioner can deploy up to 1008 fibre buttons on each of two field plates using a pair of Cartesian robots sharing a common x-axis. Each science-fibre button is 5 mm × 2 mm × 7 mm in size and terminates in a 1.5 mm × 1.5 mm prism with a concave upper surface to expand the telescope beam to f/3.2. Each MOS science fibre carries a 85 µm core fibre that subtends 1.3 arcsec on the sky. Eight buttons on each field hold coherent fibre imaging bundles to be used for acquisition and guiding. On the first plate ('plate A'), each of the eight guide-fibre bundles is packaged so that each bundle replaces six potential science fibres, leaving 960 science fibres available. On the second plate ('plate B'), 10 groups of six science fibres are replaced by pairs of deployable mIFUs. On plate B, the guide-fibre bundles each replace only a single science fibre to leave 940 science fibres available. The two MOS plates are at either end of the positioner, such that when one plate is being used for the current observation, the other can be set up for the upcoming observation. A MOS field can be configured in under one hour, thus setting the length of a typical WEAVE observation. Once the current observation has finished, the positioner tumbles to reverse the roles of the two plates (Lewis et al. 2014). This results in a marked improvement in observing efficiency as compared with AF2, as there will, in principle, be no loss in observing time due to field configurations when switching between the two MOS plates for observations with the MOS or mIFU modes with WEAVE. Table 2 summarizes the key parameters of the four focal-plane modes of WEAVE.

The 20 mIFUs are packaged similarly to the MOS fibres, but with slightly larger buttons (7 mm × 5 mm × 7 mm in size), each holding a hexagonal close-packed array of 37 fibres with the same size and aperture as the MOS fibres. The outer diameter of the fibre buffer layer is 120 µm, so each mIFU covers roughly 11 × 12 arcsec² on the sky with a filling factor of ≈0.50. The buffered arrangement of the two field plates also affords an intermediate location (i.e. at 90° to the MOS plates), which accommodates the LIFU that contains a hexagonal array of 547 fibres with 170 µm cores, equivalent to 2.6 arcsec on the sky, and hence subtending 90 × 78 arcsec² on the sky with a filling factor of ≈0.54, along with eight peripheral bundles of seven fibres, arranged in a ring of radius 150 arcsec from the centre of the array, to be used for sky subtraction. The LIFU package includes a discrete guiding camera mounted within the same physical structure as the LIFU head.

Each MOS observation is prepared in advance using a purpose-built software tool (CONFIGURE: Terrett et al. 2014, see Fig. 2), which builds on the simulated annealing approach of Miszalski et al. (2006) with some elements of the original 'Oxford' algorithm (Colless et al. 2001) to facilitate an optimal, on-the-fly assignment of fibres for sky-background determination without impacting the results of the annealing process. The output of CONFIGURE is an XML file containing all of the information required to specify an observation, including observing constraints, spectrograph configuration, and calibration lamp sequences. Observations for the mIFU and LIFU modes follow a similar but slightly simpler approach. Using CONFIGURE, the mIFUs in a given field can be manually allocated to targets such that the central fibre of each mIFU is located at a desired target position. Final sky coordinates for the remaining 36 fibres are output by CONFIGURE, based on the sky position angle of the mIFU field resulting from this allocation. Each mIFU-field set-up requires at least one of the mIFUs to be dedicated for calibration purposes. The central seven fibres are

identified with the primary calibration target (i.e. a white dwarf), and the remaining 30 fibres are labelled as sky positions. LIFU observations are prepared using a similar tool, with the output XML file providing information on guide-star positions and the precise on-sky position for each fibre within the LIFU.

## 2.3 The WEAVE spectrograph

All four sets of science fibres pass through a cable wrap around the prime-focus instrument rotator, along the vanes of the prime-focus support spider and down the telescope structure, to where they pass over the elevation axis and into the Groundbased High Resolution Imaging Laboratory (GHRIL) instrument room<fnref n="8" /> on the Nasmyth platform that houses the WEAVE spectrograph (see Fig. 3).

The spectrograph (Rogers et al. 2014) provides full wavelength coverage over the range 366–959 nm at a nominal resolving power of $R \sim 5000$ (the low-resolution, 'LR', mode) or a pair of restricted wavelength ranges at $R \sim 20000$ (the high-resolution, 'HR', mode; note that these resolving powers are halved when using the LIFU with its two-times larger fibres). This is achieved by a dual-beam design with an off-axis catadioptric<fnref n="9" /> $f/3.1$ collimator, which delivers a 190 mm collimated beam. The dichroic is located before the collimator correcting lenses so that these can be optimized for transmission in each arm. Each arm feeds an $f/1.8$ transmissive camera consisting of eight lenses (one asphere on the first lens), designed such that the lens prescriptions are common to both cameras, and only the coatings and lens spacings differ between the two. The useful focal plane of each camera is 8192 (spectral) × 6144 (spatial) pixels, populated by a pair of e2V CCD 231-C6 CCDs, with the physical size of the pixels being 15 µm. This oversize of the detector avoids use of a custom format, and also allows the gap in the spectral direction to be chosen in each camera to fall in a region of low interest. At the same time, placing this gap close to the centre of the image plane makes the control of Littrow ghosts easy by using Volume Phase Holographic (VPH) gratings with slightly slanted fringes (Burgh et al. 2007; Bianco et al. 2018), such that the image of the ghost is formed in the gap. Each arm of the spectrograph contains a rolling shutter mechanism and a grating exchange mechanism to allow the choice between two resolution modes, with the associated change of beam angles accommodated by articulation of the two cameras. The available modes and associated nominal spectral coverage are listed in Table 3.

Direct illumination of each camera's focal plane is achieved by a series of LEDs that are mounted on the spectrograph's shutter mechanisms. These are used to provide a direct measure of the pixel-to-pixel response variations of the detector, and a measure of the non-linearity present in the detector read-out electronics.

## 2.4 Calibration unit

The WHT has a set of projector lamps mounted at one of the broken-Cassegrain foci. These can be used to direct light to the primary focus via the tertiary mirror. Investigations during the development of WEAVE have shown that the illumination is adequate – but not perfect – out to the edge of the WEAVE field of view (Domínguez et al. 2016). Observations of a quartz-halogen lamp are used to determine the traces of the individual fibre spectra on the detector mosaic for each camera, and to provide a rough indication of the

<footnote n="8">⁸https://www.ing.iac.es//PR/inst.php?tel=wht&inst = GHRIL</footnote>

<footnote n="9">⁹A catadioptric optical system combines both reflection and refraction to reduce aberration effects.</footnote>
<table><caption>**Table 2.** Key parameters of the four focal-plane modes of WEAVE. MOS-B and mIFU modes both use plate B, but cannot be employed simultaneously.</caption><thead><tr><th>Focal-plane mode</th><th>MOS fibres (MOS-A)</th><th>MOS fibres (MOS-B)</th><th>mini-IFUs (mIFUs)</th><th>Large IFU (LIFU)</th></tr></thead><tbody><tr><td>Tumbler position</td><td>0 deg</td><td>180 deg</td><td>180 deg</td><td>90 deg</td></tr><tr><td>Diameter of individual fibres</td><td>1.3 arcsec (85 µm)</td><td>1.3 arcsec (85 µm)</td><td>1.3 arcsec (85 µm)</td><td>2.6 arcsec (170 µm)</td></tr><tr><td>Multiplex</td><td>960 fibres</td><td>940 fibres</td><td>20 IFUs</td><td>1 IFU</td></tr><tr><td>Diameter of field over which deployable</td><td>2 deg</td><td>2 deg</td><td>2 deg</td><td>on-axis</td></tr><tr><td>Minimum separation on sky</td><td>~60 arcsec</td><td>~60 arcsec</td><td>~60 arcsec</td><td>~60 arcsec</td></tr><tr><td>Fibres per IFU</td><td>~60 arcsec</td><td>~60 arcsec</td><td>37</td><td>547</td></tr><tr><td>IFU field of view</td><td>~60 arcsec</td><td>~60 arcsec</td><td>11 × 12 arcsec<sup>2</sup></td><td>90 × 78 arcsec<sup>2</sup></td></tr><tr><td>IFU filling factor</td><td>~60 arcsec</td><td>~60 arcsec</td><td>0.50</td><td>0.55</td></tr><tr><td>Fibres for auto-guiding</td><td>8 × 3 arcsec<sup>2</sup> coherent bundles</td><td>8 × 3 arcsec<sup>2</sup> coherent bundles</td><td>8 × 3 arcsec<sup>2</sup> coherent bundles</td><td>Separate camera<br>(4 × 3.7 arcmin<sup>2</sup> field of view)</td></tr><tr><td>Fibres for sky subtraction</td><td>~5–10 per cent of science fibres</td><td>~5–10 per cent of science fibres</td><td>One of the mIFUs</td><td>8 peripheral bundles of 7 fibres each</td></tr><tr><td>Configuration time</td><td>~55 min</td><td>~55 min</td><td>&lt;20 min</td><td>~1 min</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>**Figure 2.** Configured field for OB #3182 from OpR3b (see Section 6) as an example of a representative WEAVE-Survey MOS field. The black, dashed, outermost circle indicates WEAVE’s 2-deg-diameter field of view. MOS fibres and targets are colour-coded as follows – black: science (821); cyan: calibration stars (19); blue: blank sky (100); green: guide stars (8). The two faint inner circles denote the maximum extent of the outer two tiers of MOS fibres. As this particular field is a configuration on plate B, the ‘park’ locations of the 10 pairs of mIFU bundles can also be seen along the periphery. Note that the mIFUs cannot be used concurrently with the MOS fibres.</figcaption></figure>

illumination variations for each fibre. Observations of a ThArCr lamp are used to determine a wavelength solution for each fibre. With this lamp, we find that it is possible to obtain a sufficient density of lines in both low- and high-resolution modes.

Mapping of the wings of the fibre profiles on the detector is achieved by configuring three fields for each plate, each sampling every third fibre along the slit and observing high signal-to-noise arc and flat-field lamp exposures. As the fibres in these configurations can all be placed on a circle at around a 20-arcmin radius, these observations can also be used to determine fibre-to-fibre intrinsic transmission variations.

<figure><img src="image_3.png" /><figcaption>**Figure 3.** A CAD representation of the WEAVE spectrograph installed in the GHRIL instrument room on the WHT Nasmyth platform. The spectrograph cameras are pictured in a configuration unused in practice, with the blue camera at the LR position and the red camera at the HR position. As a scale reference, the optical bench is approximately 3 m long.</figcaption></figure>

<table><caption>**Table 3.** Available resolution modes (LR: low resolution; HR: high resolution) and nominal useful spectral coverage for the WEAVE spectrograph, including the wavelength gap between the chips.</caption><thead><tr><th>Spectrograph mode</th><th>R<sup>a</sup></th><th>λ<sub>low</sub> (Å)</th><th>λ<sub>high</sub> (Å)</th><th>Gap in λ coverage (Å)</th></tr></thead><tbody><tr><td>LR Blue</td><td>5000</td><td>3660</td><td>6060</td><td>5491–5539</td></tr><tr><td>LR Red</td><td>5000</td><td>5790</td><td>9590</td><td>7590–7669</td></tr><tr><td>HR Blue-1<sup>b</sup></td><td>20 000</td><td>4040</td><td>4650</td><td>4525–4536</td></tr><tr><td>HR Blue-2<sup>c</sup></td><td>20 000</td><td>4730</td><td>5450</td><td>5302–5315</td></tr><tr><td>HR Red</td><td>20 000</td><td>5950</td><td>6850</td><td>6412–6431</td></tr></tbody></table>

<footnote><sup>a</sup>Due to the larger fibre cores of the LIFU, the resolution in this mode is halved with respect to the MOS/mIFU cases.</footnote>

<footnote><sup>b</sup>‘Blue’ HR grating.</footnote>

<footnote><sup>c</sup>‘Green’ HR grating. Note that the two HR Blue modes cannot be used simultaneously.</footnote>
<table><caption>**Table 2.** Key parameters of the four focal-plane modes of WEAVE. MOS-B and mIFU modes both use plate B, but cannot be employed simultaneously.</caption><thead><tr><th>Focal-plane mode</th><th>MOS fibres (MOS-A)</th><th>MOS fibres (MOS-B)</th><th>mini-IFUs (mIFUs)</th><th>Large IFU (LIFU)</th></tr></thead><tbody><tr><td>Tumbler position</td><td>0 deg</td><td>180 deg</td><td>180 deg</td><td>90 deg</td></tr><tr><td>Diameter of individual fibres</td><td>1.3 arcsec (85 µm)</td><td>1.3 arcsec (85 µm)</td><td>1.3 arcsec (85 µm)</td><td>2.6 arcsec (170 µm)</td></tr><tr><td>Multiplex</td><td>960 fibres</td><td>940 fibres</td><td>20 IFUs</td><td>1 IFU</td></tr><tr><td>Diameter of field over which deployable</td><td>2 deg</td><td>2 deg</td><td>2 deg</td><td>on-axis</td></tr><tr><td>Minimum separation on sky</td><td>~60 arcsec</td><td>~60 arcsec</td><td>~60 arcsec</td><td>~60 arcsec</td></tr><tr><td>Fibres per IFU</td><td>~60 arcsec</td><td>~60 arcsec</td><td>37</td><td>547</td></tr><tr><td>IFU field of view</td><td>~60 arcsec</td><td>~60 arcsec</td><td>11 × 12 arcsec<sup>2</sup></td><td>90 × 78 arcsec<sup>2</sup></td></tr><tr><td>IFU filling factor</td><td>~60 arcsec</td><td>~60 arcsec</td><td>0.50</td><td>0.55</td></tr><tr><td>Fibres for auto-guiding</td><td>8 × 3 arcsec<sup>2</sup> coherent bundles</td><td>8 × 3 arcsec<sup>2</sup> coherent bundles</td><td>8 × 3 arcsec<sup>2</sup> coherent bundles</td><td>Separate camera<br>(4 × 3.7 arcmin<sup>2</sup> field of view)</td></tr><tr><td>Fibres for sky subtraction</td><td>~5–10 per cent of science fibres</td><td>~5–10 per cent of science fibres</td><td>One of the mIFUs</td><td>8 peripheral bundles of 7 fibres each</td></tr><tr><td>Configuration time</td><td>~55 min</td><td>~55 min</td><td>&lt;20 min</td><td>~1 min</td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Configured field for OB #3182 from OpR3b (see Section 6) as an example of a representative WEAVE-Survey MOS field. The black, dashed, outermost circle indicates WEAVE’s 2-deg-diameter field of view. MOS fibres and targets are colour-coded as follows – black: science (821); cyan: calibration stars (19); blue: blank sky (100); green: guide stars (8). The two faint inner circles denote the maximum extent of the outer two tiers of MOS fibres. As this particular field is a configuration on plate B, the ‘park’ locations of the 10 pairs of mIFU bundles can also be seen along the periphery. Note that the mIFUs cannot be used concurrently with the MOS fibres.</figcaption></figure>

illumination variations for each fibre. Observations of a ThArCr lamp are used to determine a wavelength solution for each fibre. With this lamp, we find that it is possible to obtain a sufficient density of lines in both low- and high-resolution modes.

Mapping of the wings of the fibre profiles on the detector is achieved by configuring three fields for each plate, each sampling every third fibre along the slit and observing high signal-to-noise arc and flat-field lamp exposures. As the fibres in these configurations can all be placed on a circle at around a 20-arcmin radius, these observations can also be used to determine fibre-to-fibre intrinsic transmission variations.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** A CAD representation of the WEAVE spectrograph installed in the GHRIL instrument room on the WHT Nasmyth platform. The spectrograph cameras are pictured in a configuration unused in practice, with the blue camera at the LR position and the red camera at the HR position. As a scale reference, the optical bench is approximately 3 m long.</figcaption></figure>

<table><caption>**Table 3.** Available resolution modes (LR: low resolution; HR: high resolution) and nominal useful spectral coverage for the WEAVE spectrograph, including the wavelength gap between the chips.</caption><thead><tr><th>Spectrograph mode</th><th>R<sup>a</sup></th><th>λ<sub>low</sub> (Å)</th><th>λ<sub>high</sub> (Å)</th><th>Gap in λ coverage (Å)</th></tr></thead><tbody><tr><td>LR Blue</td><td>5000</td><td>3660</td><td>6060</td><td>5491–5539</td></tr><tr><td>LR Red</td><td>5000</td><td>5790</td><td>9590</td><td>7590–7669</td></tr><tr><td>HR Blue-1<sup>b</sup></td><td>20 000</td><td>4040</td><td>4650</td><td>4525–4536</td></tr><tr><td>HR Blue-2<sup>c</sup></td><td>20 000</td><td>4730</td><td>5450</td><td>5302–5315</td></tr><tr><td>HR Red</td><td>20 000</td><td>5950</td><td>6850</td><td>6412–6431</td></tr></tbody></table>

<footnote><sup>a</sup>Due to the larger fibre cores of the LIFU, the resolution in this mode is halved with respect to the MOS/mIFU cases.</footnote>

<footnote><sup>b</sup>‘Blue’ HR grating.</footnote>

<footnote><sup>c</sup>‘Green’ HR grating. Note that the two HR Blue modes cannot be used simultaneously.</footnote>
Flux calibration is achieved on-sky using calibrator stars within each WEAVE field (Section 4.3.1).

## 2.5 Observatory Control System

WEAVE introduces a new top-level Observatory Control System (OCS) to the WHT that permits queue-based observations (Picó et al. 2018). The XML files generated by CONFIGURE are converted to observing blocks (OBs), which are stored in a database and can be scheduled in a queue for the operator, based on the prevailing conditions and target priorities. At the heart of the OCS is a sequencer task that coordinates the actions of the telescope, spectrograph, positioner, and calibration system using a CORBA-based<fnref n="10" /> architecture and a centralized noticeboard. In this context, the positioner has two functions: one for the observation of the current field and performing any actions required of the focal-plane imager, and one for the positioning robots concerning the parallel setting up of the next observation.

The ‘WEAVE observation queue scheduler’ (SCHEDULER: Fariña et al. 2018) is bespoke software written for use by the WEAVE operator to make informed decisions about which OBs in the database are most suitable for observation, at least one hour ahead of time, given the typical observation length (which in turn is driven by the configuration time of a MOS field). Requirements set by the OBs on instrument configuration (i.e. observing mode) and observing conditions (e.g. sky brightness, seeing, etc.) are combined with current and predicted weather conditions and current instrument configuration to allocate scores to OBs in the OB database. For example, poor seeing conditions might trigger a LIFU observation, during which no configuration is possible on either field plate. If this occurs during the night, the most efficient subsequent observation, once the seeing conditions start improving, is a mIFU field using plate B, as configuring a mIFU field takes less time (under 20 min) compared with a MOS field (just under one hour), during which plate A can be set up for a MOS observation. If such a LIFU observation takes place at the start of the night, both field plates will already have been set up with suitable fields before the beginning of the night. Note that each MOS field configuration is valid only for a specific length of time, depending on its elevation, due to the distortion introduced by the effects of differential refraction.

## 2.6 WEAVE data processing and archiving

From the outset, it was envisaged that WEAVE would require a dedicated data-processing pipeline to ensure the best outcomes from the facility. Given the scale and complexity of this task, the activities of this Science Processing and Analysis (SPA) system have been divided into three distinct sub-systems: the Core Processing System (CPS) for basic data reduction and calibration, the Advanced Processing System (APS) for producing derived data products, and the WEAVE Archive System (WAS) for data archiving.

All WEAVE data frames, including data for individual open-time programmes, are retrieved from the observatory in near real time by the CPS (Walton et al. 2014) and processed to remove instrumental signatures, calibrate wavelength and flux scales (see Section 4.3.1), and subtract sky background. A subset of the CPS pipeline will run at the telescope to provide a quick-look analysis tool (Peralta de Arriba et al. 2019) for the observers for the purposes of quality control during observations. For each OB, the output from the CPS consists

of a multispectrum image for each science frame in each arm (with sky and variance components) and a stacked multispectrum image for each arm. These data are passed to the WAS, together with the raw frames and processed calibration data.

The APS picks up the processed science frames and produces joined spectra in both the low and high-resolution modes, before providing an analysis of each spectrum using a variety of methods (e.g. FERRE, redrock,<fnref n="11" /> PPXF, and GANDALF: Allende Prieto et al. 2006; Bolton & Schlegel 2010; Cappellari 2012; Sarzi et al. 2017, the last two of which are wrapped in GIST: Bittner et al. 2019a, b). For extragalactic sources, the analysis yields redshift, galaxy classification, velocity dispersion, spectral indices, and equivalent widths for a list of standard emission lines. For each stellar source, the analysis produces velocity, temperature, surface gravity, and abundance measures for a variety of elements. For IFU spectra, the APS produces maps of velocity, velocity dispersion, and abundances. The APS also allows for additional analysis software ('Contributed Software') to be developed by the science teams and incorporated into the processing chain. All data products produced by the APS are stored in FITS tables and sent to the WAS.

The WAS is the official archive for WEAVE data products (Guerra et al. 2016). It is based on Apache Solr<fnref n="12" /> and provides interactive front-end access to WEAVE data and data products for WEAVE data users. The WAS not only stores the results from WEAVE’s two main pipelines, namely the CPS and the APS, but also stores Contributed Data Products (CDPs), which are specific data products produced by WEAVE Science Team members in addition to the APS products. To preserve and ensure the high quality of data available to WAS users, CDPs are agreed in advance by the Science Team with the Science Executive (see Section 3). As well as search and download options, the WAS user interface also offers plotting tools for the visualization of spectra and other data products. The WAS manages both internal and public data releases. Internal data releases are expected on a regular basis, at least once a semester, while public data releases will be yearly following the first public data release two years after the start of Survey operations. Data from the public data releases will also be published in the Virtual Observatory<fnref n="13" /> using the Table Access Protocol.<fnref n="14" />

## 3 WEAVE’S ORGANIZATIONAL STRUCTURE

The work and workforce within WEAVE are divided into two entities: the WEAVE Instrument Consortium and the WEAVE Survey Consortium. The former, also commonly referred to as ‘the WEAVE Project’, is responsible for the delivery of the WEAVE facility to the ING. The latter, also commonly referred to as ‘the WEAVE Science Team’, is responsible for preparing and executing a five-year survey. A simplified view of the organizational structure is shown in Fig. 4.

The WEAVE Project is responsible for the delivery of the complete instrument along with data-processing software and associated hardware for the data-processing pipelines, a data archive system (along with a user interface), and other useful tools such as a quick-look GUI, an operational repository, OB templates, and a file-submission platform (see Sections 2 and 6). The WEAVE Project is overseen by a Project Board with representatives from the funding parties,

<footnote n="10">¹⁰https://www.corba.org</footnote>

<footnote n="11">¹¹https://github.com/desihub/redrock</footnote>

<footnote n="12">¹²http://lucene.apache.org/solr/</footnote>

<footnote n="13">¹³https://ivoa.net/</footnote>

<footnote n="14">¹⁴https://www.ivoa.net/documents/TAP/20190927/REC-TAP-1.1.html</footnote>
Flux calibration is achieved on-sky using calibrator stars within each WEAVE field (Section 4.3.1).

## 2.5 Observatory Control System

WEAVE introduces a new top-level Observatory Control System (OCS) to the WHT that permits queue-based observations (Picó et al. 2018). The XML files generated by CONFIGURE are converted to observing blocks (OBs), which are stored in a database and can be scheduled in a queue for the operator, based on the prevailing conditions and target priorities. At the heart of the OCS is a sequencer task that coordinates the actions of the telescope, spectrograph, positioner, and calibration system using a CORBA-based<fnref n="10" /> architecture and a centralized noticeboard. In this context, the positioner has two functions: one for the observation of the current field and performing any actions required of the focal-plane imager, and one for the positioning robots concerning the parallel setting up of the next observation.

The ‘WEAVE observation queue scheduler’ (SCHEDULER: Fariña et al. 2018) is bespoke software written for use by the WEAVE operator to make informed decisions about which OBs in the database are most suitable for observation, at least one hour ahead of time, given the typical observation length (which in turn is driven by the configuration time of a MOS field). Requirements set by the OBs on instrument configuration (i.e. observing mode) and observing conditions (e.g. sky brightness, seeing, etc.) are combined with current and predicted weather conditions and current instrument configuration to allocate scores to OBs in the OB database. For example, poor seeing conditions might trigger a LIFU observation, during which no configuration is possible on either field plate. If this occurs during the night, the most efficient subsequent observation, once the seeing conditions start improving, is a mIFU field using plate B, as configuring a mIFU field takes less time (under 20 min) compared with a MOS field (just under one hour), during which plate A can be set up for a MOS observation. If such a LIFU observation takes place at the start of the night, both field plates will already have been set up with suitable fields before the beginning of the night. Note that each MOS field configuration is valid only for a specific length of time, depending on its elevation, due to the distortion introduced by the effects of differential refraction.

## 2.6 WEAVE data processing and archiving

From the outset, it was envisaged that WEAVE would require a dedicated data-processing pipeline to ensure the best outcomes from the facility. Given the scale and complexity of this task, the activities of this Science Processing and Analysis (SPA) system have been divided into three distinct sub-systems: the Core Processing System (CPS) for basic data reduction and calibration, the Advanced Processing System (APS) for producing derived data products, and the WEAVE Archive System (WAS) for data archiving.

All WEAVE data frames, including data for individual open-time programmes, are retrieved from the observatory in near real time by the CPS (Walton et al. 2014) and processed to remove instrumental signatures, calibrate wavelength and flux scales (see Section 4.3.1), and subtract sky background. A subset of the CPS pipeline will run at the telescope to provide a quick-look analysis tool (Peralta de Arriba et al. 2019) for the observers for the purposes of quality control during observations. For each OB, the output from the CPS consists

of a multispectrum image for each science frame in each arm (with sky and variance components) and a stacked multispectrum image for each arm. These data are passed to the WAS, together with the raw frames and processed calibration data.

The APS picks up the processed science frames and produces joined spectra in both the low and high-resolution modes, before providing an analysis of each spectrum using a variety of methods (e.g. FERRE, redrock,<fnref n="11" /> PPXF, and GANDALF: Allende Prieto et al. 2006; Bolton & Schlegel 2010; Cappellari 2012; Sarzi et al. 2017, the last two of which are wrapped in GIST: Bittner et al. 2019a, b). For extragalactic sources, the analysis yields redshift, galaxy classification, velocity dispersion, spectral indices, and equivalent widths for a list of standard emission lines. For each stellar source, the analysis produces velocity, temperature, surface gravity, and abundance measures for a variety of elements. For IFU spectra, the APS produces maps of velocity, velocity dispersion, and abundances. The APS also allows for additional analysis software ('Contributed Software') to be developed by the science teams and incorporated into the processing chain. All data products produced by the APS are stored in FITS tables and sent to the WAS.

The WAS is the official archive for WEAVE data products (Guerra et al. 2016). It is based on Apache Solr<fnref n="12" /> and provides interactive front-end access to WEAVE data and data products for WEAVE data users. The WAS not only stores the results from WEAVE's two main pipelines, namely the CPS and the APS, but also stores Contributed Data Products (CDPs), which are specific data products produced by WEAVE Science Team members in addition to the APS products. To preserve and ensure the high quality of data available to WAS users, CDPs are agreed in advance by the Science Team with the Science Executive (see Section 3). As well as search and download options, the WAS user interface also offers plotting tools for the visualization of spectra and other data products. The WAS manages both internal and public data releases. Internal data releases are expected on a regular basis, at least once a semester, while public data releases will be yearly following the first public data release two years after the start of Survey operations. Data from the public data releases will also be published in the Virtual Observatory<fnref n="13" /> using the Table Access Protocol.<fnref n="14" />

# 3 WEAVE’S ORGANIZATIONAL STRUCTURE

The work and workforce within WEAVE are divided into two entities: the WEAVE Instrument Consortium and the WEAVE Survey Consortium. The former, also commonly referred to as ‘the WEAVE Project’, is responsible for the delivery of the WEAVE facility to the ING. The latter, also commonly referred to as ‘the WEAVE Science Team’, is responsible for preparing and executing a five-year survey. A simplified view of the organizational structure is shown in Fig. 4.

The WEAVE Project is responsible for the delivery of the complete instrument along with data-processing software and associated hardware for the data-processing pipelines, a data archive system (along with a user interface), and other useful tools such as a quick-look GUI, an operational repository, OB templates, and a file-submission platform (see Sections 2 and 6). The WEAVE Project is overseen by a Project Board with representatives from the funding parties,

<footnote n="10">¹⁰https://www.corba.org</footnote>

<footnote n="11">¹¹https://github.com/desihub/redrock</footnote>

<footnote n="12">¹²http://lucene.apache.org/solr/</footnote>

<footnote n="13">¹³https://ivoa.net/</footnote>

<footnote n="14">¹⁴https://www.ivoa.net/documents/TAP/20190927/REC-TAP-1.1.html</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 4.** A simplified version of the WEAVE Project Management Team structure (the 'WEAVE organogram'). Arrows point in the direction of reporting. PM: Project Manager; PI: Principal Investigator; PS: Project Scientist; CPS: Core Processing System; APS: Advanced Processing System; WAS: WEAVE Archive System; SWG: Survey Working Group; QAG: Quality Assurance Group.</figcaption></figure>

and managed through the Project Executive.<fnref n="15" /> The latter comprises the Project Manager and Principal Investigators (PIs) from each of the communities in WEAVE-funding countries. These PIs also currently compose the Science Executive, which oversees the Survey Consortium.

The WEAVE Survey Consortium – also known as the WEAVE Science Team – is composed of eight individual science teams, preparing to execute the eight respective surveys. Each team is led by a Science Team Lead (STL) and each survey has its own science case and survey plan. The WEAVE Survey Consortium currently consists of over 500 astronomers<fnref n="16" /> spread over 12 countries, where membership is contingent on satisfying eligibility requirements, e.g. any astronomer working in an ING country may request to join the Survey Consortium. An individual joins the Survey Consortium by becoming a member of one or more of the eight science teams. WEAVE membership entitles its members to proprietary WEAVE-data access during Survey operations, while for certain science teams, this also includes ancillary data provided through a Memorandum of Understanding (MoU) with another survey. The component surveys comprising the global WEAVE Survey are described further in Section 4.

## 3.1 Survey Working Group

During Survey operations, the main responsibility of the Survey Working Group (SWG) is to provide catalogues of targets and configured fields in the form of XML files using CONFIGURE (see Section 2.2) on a trimesterly basis, i.e. every 3 months. The SWG is composed of representatives from the WEAVE Science Team, each of whom reports to the relevant STL, and is currently led by the Project Scientist and Deputy Project Scientist. The SWG has been fundamental in helping to define the structure and details of key data-flow processes, testing many steps of the Survey preparation process from the creation of input target catalogues and testing of the CONFIGURE software for creating configured fields to trialling the

trimesterly XML submission process via the WEAVE Automated Submission Platform (WASP) in the Operational Rehearsals (see Section 6).

## 3.2 On-Island Survey Management Team

The On-Island Survey Management Team (OISMT) at the ING is responsible for many aspects of the running of the WEAVE facility, ranging from the commissioning of the WEAVE instrument to ensuring the day-to-day running of WEAVE in the contexts of both the WEAVE Survey and PI programmes<fnref n="17" /> once the instrument is on sky. With support from the OISMT, a dedicated WEAVE instrument specialist will be responsible for managing the execution of nightly OBs, aided by the observation queue created by the SCHEDULER based on current and predicted weather conditions and the observing requirements of OBs in the OB database, held at the OCS. Regular communication between the OISMT and the SWG is key for ensuring the smooth and optimized scheduling of OBs from the WEAVE Survey and PI programmes combined, and this interface has been tested extensively during the most recent Operational Rehearsal (see Section 6).

## 3.3 Quality Assurance Group

Although various checks are performed on the spectra being processed by the CPS and APS before reaching the WAS, some unexpected results requiring closer inspection are anticipated, especially at the start of Survey operations. To allow such spectra to be caught and investigated quickly, the Quality Assurance Group (QAG) of each science team will perform regular checks on data taken for their survey to ensure that the spectra are of sufficient quality.

The work of the QAG will also be instrumental in decisions to be made on timescales of several months to a year. Although each of the eight science teams comprising the global WEAVE Survey has an internally and externally reviewed five-year survey plan based on its individual science case prior to start of Survey operations, the detailed planning of which fields/targets are submitted for observations (and when) is a real-time decision-making operation that will continue to take place on a trimesterly cycle. The QAG will report its findings of their quality-assessment work to the STL and SWG member(s) of their team on a regular basis. This, in turn, will form the basis of decisions made by the STL and SWG on any necessary strategic planning for upcoming submissions on how to proceed with their survey. Reports from the QAG will also be combined with individual survey completion rates and other considerations such as right-ascension pressure for top-level Survey-planning decisions to be made by the Science Executive.

The QAG workflow and its tasks have been tested intensively during Operational Rehearsals involving the WEAVE Science Team (see Section 6).

## 3.4 IFU Working Group

The IFU Working Group (IWG) deals with planning observational and software aspects of WEAVE's integral-field modes. The IWG consists of members of the software systems, SWG, QAG, and

<footnote n="17">¹⁷WEAVE will be available for community PI programmes in the same manner as for instruments on other ING telescopes, via the usual ING call for proposals. Further details can be found at https://www.ing.iac.es/astronomy/observing/INGinfo_home.html.</footnote>

<footnote n="15">¹⁵https://ingconfluence.ing.iac.es/confluence/display/WEAV/Project+Executive</footnote>

<footnote n="16">¹⁶https://ingconfluence.ing.iac.es/confluence/display/WEAV/Science+Team</footnote>
<figure><img src="image_5.png" /><figcaption>**Figure 4.** A simplified version of the WEAVE Project Management Team structure (the 'WEAVE organogram'). Arrows point in the direction of reporting. PM: Project Manager; PI: Principal Investigator; PS: Project Scientist; CPS: Core Processing System; APS: Advanced Processing System; WAS: WEAVE Archive System; SWG: Survey Working Group; QAG: Quality Assurance Group.</figcaption></figure>

and managed through the Project Executive.<fnref n="15" /> The latter comprises the Project Manager and Principal Investigators (PIs) from each of the communities in WEAVE-funding countries. These PIs also currently compose the Science Executive, which oversees the Survey Consortium.

The WEAVE Survey Consortium – also known as the WEAVE Science Team – is composed of eight individual science teams, preparing to execute the eight respective surveys. Each team is led by a Science Team Lead (STL) and each survey has its own science case and survey plan. The WEAVE Survey Consortium currently consists of over 500 astronomers<fnref n="16" /> spread over 12 countries, where membership is contingent on satisfying eligibility requirements, e.g. any astronomer working in an ING country may request to join the Survey Consortium. An individual joins the Survey Consortium by becoming a member of one or more of the eight science teams. WEAVE membership entitles its members to proprietary WEAVE-data access during Survey operations, while for certain science teams, this also includes ancillary data provided through a Memorandum of Understanding (MoU) with another survey. The component surveys comprising the global WEAVE Survey are described further in Section 4.

## 3.1 Survey Working Group

During Survey operations, the main responsibility of the Survey Working Group (SWG) is to provide catalogues of targets and configured fields in the form of XML files using CONFIGURE (see Section 2.2) on a trimesterly basis, i.e. every 3 months. The SWG is composed of representatives from the WEAVE Science Team, each of whom reports to the relevant STL, and is currently led by the Project Scientist and Deputy Project Scientist. The SWG has been fundamental in helping to define the structure and details of key data-flow processes, testing many steps of the Survey preparation process from the creation of input target catalogues and testing of the CONFIGURE software for creating configured fields to trialling the

trimesterly XML submission process via the WEAVE Automated Submission Platform (WASP) in the Operational Rehearsals (see Section 6).

## 3.2 On-Island Survey Management Team

The On-Island Survey Management Team (OISMT) at the ING is responsible for many aspects of the running of the WEAVE facility, ranging from the commissioning of the WEAVE instrument to ensuring the day-to-day running of WEAVE in the contexts of both the WEAVE Survey and PI programmes<fnref n="17" /> once the instrument is on sky. With support from the OISMT, a dedicated WEAVE instrument specialist will be responsible for managing the execution of nightly OBs, aided by the observation queue created by the SCHEDULER based on current and predicted weather conditions and the observing requirements of OBs in the OB database, held at the OCS. Regular communication between the OISMT and the SWG is key for ensuring the smooth and optimized scheduling of OBs from the WEAVE Survey and PI programmes combined, and this interface has been tested extensively during the most recent Operational Rehearsal (see Section 6).

## 3.3 Quality Assurance Group

Although various checks are performed on the spectra being processed by the CPS and APS before reaching the WAS, some unexpected results requiring closer inspection are anticipated, especially at the start of Survey operations. To allow such spectra to be caught and investigated quickly, the Quality Assurance Group (QAG) of each science team will perform regular checks on data taken for their survey to ensure that the spectra are of sufficient quality.

The work of the QAG will also be instrumental in decisions to be made on timescales of several months to a year. Although each of the eight science teams comprising the global WEAVE Survey has an internally and externally reviewed five-year survey plan based on its individual science case prior to start of Survey operations, the detailed planning of which fields/targets are submitted for observations (and when) is a real-time decision-making operation that will continue to take place on a trimesterly cycle. The QAG will report its findings of their quality-assessment work to the STL and SWG member(s) of their team on a regular basis. This, in turn, will form the basis of decisions made by the STL and SWG on any necessary strategic planning for upcoming submissions on how to proceed with their survey. Reports from the QAG will also be combined with individual survey completion rates and other considerations such as right-ascension pressure for top-level Survey-planning decisions to be made by the Science Executive.

The QAG workflow and its tasks have been tested intensively during Operational Rehearsals involving the WEAVE Science Team (see Section 6).

## 3.4 IFU Working Group

The IFU Working Group (IWG) deals with planning observational and software aspects of WEAVE's integral-field modes. The IWG consists of members of the software systems, SWG, QAG, and

<footnote n="17">¹⁷WEAVE will be available for community PI programmes in the same manner as for instruments on other ING telescopes, via the usual ING call for proposals. Further details can be found at https://www.ing.iac.es/astronomy/observing/INGinfo_home.html.</footnote>

<footnote n="15">¹⁵https://ingconfluence.ing.iac.es/confluence/display/WEAV/Project+Executive</footnote>

<footnote n="16">¹⁶https://ingconfluence.ing.iac.es/confluence/display/WEAV/Science+Team</footnote>
ad-hoc members of the science teams as required, and discusses issues ranging from observation preparation to dithering strategies, data reduction, and analysis techniques. Given the complexity of designing IFU observations within the context of the WEAVE observation preparation system, the IWG has been instrumental in the development and testing of the 'IFU workflow' software for preparation of IFU observations, which in turn has also significantly contributed to the parallel development of the 'MOS workflow' software for preparation of the MOS observations. During Survey operations, the work carried out by the IWG will serve as additional input to the SWG and the QAG of science teams that include IFU observations.

# 4 THE WEAVE SURVEY

The WEAVE Survey is composed of eight individual surveys, each with its own dedicated science team, whose science goals span a wide range of topics covering various fields of stellar, Galactic, and extragalactic astronomy. The surveys will together use approximately 1150 nights over the course of five years of WHT time. Their science goals play crucial roles in complementing major space- and ground-based programmes in the current and coming decade, including Gaia, LOFAR, and Apertif, by providing a dedicated wide-field optical spectroscopic instrument in the Northern hemisphere. WEAVE will also be accessible to the wider astronomical community through open calls outside of the allocated survey time.

In this section, an overview of each of the planned surveys of the global WEAVE Survey is provided, starting from surveys focusing on the nearby Universe and extending towards higher redshifts (see also Table 4).

## 4.1 The WEAVE Galactic Archaeology Survey

How did our Galaxy and its components assemble and evolve over time? This question is key to the general understanding of galaxy formation over cosmic times, as the Milky Way is the system for which we can hope to constrain in most detail the physical processes that play a role in the formation and evolution of galaxies. These processes leave behind specific signatures that are encoded in the location, kinematics, and chemistry of stars (e.g. Freeman & Bland-Hawthorn 2002). The ultimate goal of Galactic Archaeology as a field of study is to obtain a comprehensive census of the positions, orbits, ages, and chemical compositions of stars in all major stellar structures of our own Galaxy to enable a complete reconstruction of its formation and subsequent evolution.

Many questions remain open in understanding how the complex structure of the Milky Way was assembled, with its stellar populations occupying a bulge, a thin and thick disc, and a halo. Whether the different stellar populations have truly different origins, what the relative importance is of internal (in-situ star formation, secular evolution of the disc, etc.) and external (accretion events, gas accretion through filaments, etc.) processes in forming and shaping the Galaxy, and how universal these processes are in the evolution of galaxies in general, are questions to which Galactic Archaeology as a field of astronomical study tries to find answers. The most outstanding open questions currently include: What is the assembly history of the Milky Way mass? What is the role of the structure currently known as the Milky Way thick disc at the earliest times, and what is its relationship with the Milky Way bulge and halo? What are the relative fractions and properties of the Galactic stellar halo, formed within the Milky Way (in situ)

and accreted from 'building blocks' of satellite systems? What is the general shape of the potential of the Milky Way's halo out to large distances, and how lumpy is this potential? Is the Galactic disc duality (that refers to the existence of a thin and thick disc) real, and what is the cause of this duality if so? What causes the [α/Fe] bimodality in the Galactic disc? What is the Galactic disc potential in detail, and how important are deviations from axisymmetry and stationarity? What are the origins of the chemical elements in the Galaxy?

The European Space Agency mission Gaia (Gaia Collaboration 2016) was launched in 2013 December. During its nominal five-year mission (and up to 10 years pending approvals of mission extensions), Gaia is already producing – and will continue to produce – the most accurate astrometric data (positions, parallaxes, and hence geometrical distances and proper motions) ever produced in the optical domain for more than a billion stars in the Milky Way. In addition, Gaia is providing very accurate photometry in three bandpasses (Riello et al. 2021), complemented since its DR3 by shallower photometry in a fourth bandpass ($G_{RVS}$; Sartoretti et al. 2023), as well as spectrophotometric information from the Gaia BP/RP spectrophotometer (De Angeli et al. 2023).

However, for a number of objectives, Gaia will not be sufficient by itself, and ground-based spectroscopic information provided by large-scale surveys such as WEAVE are mandatory. Gaia's on-board Radial Velocity Spectrograph (RVS), with a resolution of ~11 500 over the wavelength range 845–872 nm (Cropper et al. 2018; Katz et al. 2023), will not reach the same depth as the rest of the Gaia instruments,<fnref n="18" /> leaving the vast majority of the survey without the third (radial) velocity dimension. Furthermore, the RVS's limited wavelength coverage and resolution will not allow for detailed studies of a large variety of elemental abundances, with reliable detailed elemental abundances only for sources brighter than $G \approx 12^{19}$ (Recio-Blanco et al. 2023).

The WEAVE Galactic Archaeology (GA) survey is tailored to complement Gaia in two primary ways:

(i) by providing measurements based on low-resolution spectra of radial velocities (expected precision ±1–2 km s⁻¹), effective temperature, surface gravity, and metallicity (expected precision ±0.2 dex) for ~1.8–2.6 million targets in the faint part of the Gaia catalogues with magnitudes in the range 16 < *G* < 20.7, which are too faint for the RVS (see Fig. 5);

(ii) by yielding accurate measurements based on high-resolution spectra of abundance ratios covering the main nucleosynthetic channels (light, α-, Fe-peak, and s- and r-process neutron-capture elements, to better than ±0.05–0.2 dex), depending on the element and type of star, and radial velocities (to better than ~ ±0.5 km s⁻¹) for ~1.1–1.6 million stars in the magnitude range 12 < *G* < 16, for which the RVS will again not be able to provide the relevant data.

Studying the origin and evolution of the main different stellar components of the Milky Way (halo, thin and thick discs) calls for massive spectroscopic surveys to provide line-of-sight velocities, distances, and chemical abundances of several million stars, and

<footnote n="18">¹⁸Gaia will deliver astrometry down to $G \sim 21$, while Gaia RVS will deliver radial velocities only down to $G \sim 16–17$.</footnote>

<footnote n="19">¹⁹This limiting magnitude was obtained by extracting from the DR3 Astrophysical Parameters catalogue in the Gaia archive all stars with at least a [Ca/Fe] abundance measurement with a corresponding flag of 0, which shows a strong drop in number count for $G > 12.5$.</footnote>
ad-hoc members of the science teams as required, and discusses issues ranging from observation preparation to dithering strategies, data reduction, and analysis techniques. Given the complexity of designing IFU observations within the context of the WEAVE observation preparation system, the IWG has been instrumental in the development and testing of the 'IFU workflow' software for preparation of IFU observations, which in turn has also significantly contributed to the parallel development of the 'MOS workflow' software for preparation of the MOS observations. During Survey operations, the work carried out by the IWG will serve as additional input to the SWG and the QAG of science teams that include IFU observations.

# 4 THE WEAVE SURVEY

The WEAVE Survey is composed of eight individual surveys, each with its own dedicated science team, whose science goals span a wide range of topics covering various fields of stellar, Galactic, and extragalactic astronomy. The surveys will together use approximately 1150 nights over the course of five years of WHT time. Their science goals play crucial roles in complementing major space- and ground-based programmes in the current and coming decade, including Gaia, LOFAR, and Apertif, by providing a dedicated wide-field optical spectroscopic instrument in the Northern hemisphere. WEAVE will also be accessible to the wider astronomical community through open calls outside of the allocated survey time.

In this section, an overview of each of the planned surveys of the global WEAVE Survey is provided, starting from surveys focusing on the nearby Universe and extending towards higher redshifts (see also Table 4).

## 4.1 The WEAVE Galactic Archaeology Survey

How did our Galaxy and its components assemble and evolve over time? This question is key to the general understanding of galaxy formation over cosmic times, as the Milky Way is the system for which we can hope to constrain in most detail the physical processes that play a role in the formation and evolution of galaxies. These processes leave behind specific signatures that are encoded in the location, kinematics, and chemistry of stars (e.g. Freeman & Bland-Hawthorn 2002). The ultimate goal of Galactic Archaeology as a field of study is to obtain a comprehensive census of the positions, orbits, ages, and chemical compositions of stars in all major stellar structures of our own Galaxy to enable a complete reconstruction of its formation and subsequent evolution.

Many questions remain open in understanding how the complex structure of the Milky Way was assembled, with its stellar populations occupying a bulge, a thin and thick disc, and a halo. Whether the different stellar populations have truly different origins, what the relative importance is of internal (in-situ star formation, secular evolution of the disc, etc.) and external (accretion events, gas accretion through filaments, etc.) processes in forming and shaping the Galaxy, and how universal these processes are in the evolution of galaxies in general, are questions to which Galactic Archaeology as a field of astronomical study tries to find answers. The most outstanding open questions currently include: What is the assembly history of the Milky Way mass? What is the role of the structure currently known as the Milky Way thick disc at the earliest times, and what is its relationship with the Milky Way bulge and halo? What are the relative fractions and properties of the Galactic stellar halo, formed within the Milky Way (in situ)

and accreted from 'building blocks' of satellite systems? What is the general shape of the potential of the Milky Way's halo out to large distances, and how lumpy is this potential? Is the Galactic disc duality (that refers to the existence of a thin and thick disc) real, and what is the cause of this duality if so? What causes the [α/Fe] bimodality in the Galactic disc? What is the Galactic disc potential in detail, and how important are deviations from axisymmetry and stationarity? What are the origins of the chemical elements in the Galaxy?

The European Space Agency mission Gaia (Gaia Collaboration 2016) was launched in 2013 December. During its nominal five-year mission (and up to 10 years pending approvals of mission extensions), Gaia is already producing – and will continue to produce – the most accurate astrometric data (positions, parallaxes, and hence geometrical distances and proper motions) ever produced in the optical domain for more than a billion stars in the Milky Way. In addition, Gaia is providing very accurate photometry in three bandpasses (Riello et al. 2021), complemented since its DR3 by shallower photometry in a fourth bandpass ($G_{RVS}$; Sartoretti et al. 2023), as well as spectrophotometric information from the Gaia BP/RP spectrophotometer (De Angeli et al. 2023).

However, for a number of objectives, Gaia will not be sufficient by itself, and ground-based spectroscopic information provided by large-scale surveys such as WEAVE are mandatory. Gaia's on-board Radial Velocity Spectrograph (RVS), with a resolution of ~11 500 over the wavelength range 845–872 nm (Cropper et al. 2018; Katz et al. 2023), will not reach the same depth as the rest of the Gaia instruments,<fnref n="18" /> leaving the vast majority of the survey without the third (radial) velocity dimension. Furthermore, the RVS's limited wavelength coverage and resolution will not allow for detailed studies of a large variety of elemental abundances, with reliable detailed elemental abundances only for sources brighter than $G \approx 12^{19}$ (Recio-Blanco et al. 2023).

The WEAVE Galactic Archaeology (GA) survey is tailored to complement Gaia in two primary ways:

(i) by providing measurements based on low-resolution spectra of radial velocities (expected precision ±1–2 km s⁻¹), effective temperature, surface gravity, and metallicity (expected precision ±0.2 dex) for ~1.8–2.6 million targets in the faint part of the Gaia catalogues with magnitudes in the range 16 < *G* < 20.7, which are too faint for the RVS (see Fig. 5);

(ii) by yielding accurate measurements based on high-resolution spectra of abundance ratios covering the main nucleosynthetic channels (light, α-, Fe-peak, and s- and r-process neutron-capture elements, to better than ±0.05–0.2 dex), depending on the element and type of star, and radial velocities (to better than ~ ±0.5 km s⁻¹) for ~1.1–1.6 million stars in the magnitude range 12 < *G* < 16, for which the RVS will again not be able to provide the relevant data.

Studying the origin and evolution of the main different stellar components of the Milky Way (halo, thin and thick discs) calls for massive spectroscopic surveys to provide line-of-sight velocities, distances, and chemical abundances of several million stars, and

<footnote n="18">¹⁸Gaia will deliver astrometry down to $G \sim 21$, while Gaia RVS will deliver radial velocities only down to $G \sim 16–17$.</footnote>

<footnote n="19">¹⁹This limiting magnitude was obtained by extracting from the DR3 Astrophysical Parameters catalogue in the Gaia archive all stars with at least a [Ca/Fe] abundance measurement with a corresponding flag of 0, which shows a strong drop in number count for $G > 12.5$.</footnote>
<table><caption>**Table 4.** WEAVE survey parameters, listed in order of survey appearance in the text. Numbers provided are best estimates at the time of writing for a seven-year WEAVE Survey.</caption><thead><tr><th>WEAVE survey (short hand)</th><th>Main targets</th><th>Number of objects</th><th>Area (deg<sup>2</sup>)</th><th>Survey fraction<sup>a</sup></th><th>Observing mode(s)</th><th>Resolution modes(s)</th><th>Redshift</th></tr></thead><tbody><tr><td>Galactic Archaeology (GA-LRhighlat)</td><td>MSTO<sup>b</sup> stars, RGB<sup>b</sup> stars, BHB<sup>b</sup> stars, RR Lyrae</td><td>~1.6 × 10<sup>6</sup></td><td>8750</td><td>0.168</td><td>MOS</td><td>LR</td><td>0</td></tr><tr><td>Galactic Archaeology (GA-LRdisc)</td><td>Red Clump stars, RGB stars</td><td>~1.1 × 10<sup>6</sup></td><td>1370</td><td>0.110</td><td>MOS</td><td>LR</td><td>0</td></tr><tr><td>Galactic Archaeology (GA-HR)</td><td>Main sequence and RGB stars</td><td>~1.6 × 10<sup>6</sup></td><td>5650</td><td>0.309</td><td>MOS</td><td>HR</td><td>0</td></tr><tr><td>Galactic Archaeology (GA-OC)</td><td>Stars in open clusters and star-forming regions</td><td>~1 × 10<sup>5</sup></td><td>375</td><td>0.029</td><td>MOS</td><td>HR</td><td>0</td></tr><tr><td>Stellar, Circumstellar, and Interstellar Physics (SCIP)</td><td>OBA stars, ionized nebulae, young stars, compact objects</td><td>~4 × 10<sup>5</sup></td><td>1230</td><td>0.069</td><td>MOS, LIFU</td><td>LR, HR</td><td>0</td></tr><tr><td>White Dwarfs</td><td>White dwarfs</td><td>≥5 × 10<sup>4</sup></td><td>≥10 000</td><td>~0.012</td><td>MOS, mIFU<sup>c</sup>, LIFU<sup>c</sup></td><td>LR, HR</td><td>0</td></tr><tr><td>WEAVE-Apertif</td><td>H I-detected, mostly late-type galaxies</td><td>400 (LR), 100 (HR)</td><td>0.2<sup>d</sup></td><td>0.061</td><td>LIFU</td><td>LR, HR</td><td>&lt;0.04</td></tr><tr><td>WEAVE Galaxy Clusters</td><td>Galaxies in dense environments</td><td>~2 × 10<sup>5</sup></td><td>1350</td><td>0.064</td><td>MOS, mIFU, LIFU</td><td>LR</td><td>&lt;0.5</td></tr><tr><td>Stellar Populations at intermediate redshifts Survey (StePS)</td><td>Field galaxies</td><td>~2.5 × 10<sup>4</sup></td><td>25</td><td>0.026</td><td>MOS</td><td>LR</td><td>0.3–0.7</td></tr><tr><td>WEAVE-LOFAR (WL)</td><td>150 MHz sources</td><td>~7 × 10<sup>5</sup></td><td>8950</td><td>0.109</td><td>MOS, mIFU, LIFU</td><td>LR</td><td>&lt;6.9</td></tr><tr><td>WEAVE-QSO (WQ)</td><td>Bright, r &lt; 21.5: z &gt; 2.2; 21.5 &lt; r &lt; 23.5: 2.5 &lt; z &lt; 3<sup>e</sup></td><td>~4 × 10<sup>5</sup></td><td>8950</td><td>0.056</td><td>MOS</td><td>LR, HR</td><td>&gt;2.2</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Figure 5.** Expected radial velocity accuracies for WEAVE in its *R* = 5000 mode (thick solid coloured lines) for different stellar effective temperature ranges, based on the WEAVE ‘Operational Rehearsal’ (OpR) simulations (specifically OpR2.5 – see also Section 6; final improvements to the APS pipeline based on OpR3 data have confirmed the results presented here, albeit on a smaller sample), together with the post-launch predictions for *Gaia* at the end of the nominal five-year mission (thin solid coloured lines) for different stellar types. Dashed coloured lines represent *Gaia* end-of-mission photometric limits for astrometry of different spectral types.</figcaption></figure>

WEAVE is a cornerstone for this endeavour, as one of the first – and for its high-resolution capability, the only – high-multiplex wide-field optical multi-object spectrograph in the Northern hemisphere. These spectroscopic surveys complement *Gaia*'s superb astrometry while benefitting from *Gaia* for optimal target selection. A WEAVE survey starting in the coming months will be particularly timely, benefitting from *Gaia* Data Release 3 in 2022 June (DR3: Gaia Collaboration 2023a), and producing its final science in the same time-frame as the *Gaia* end-of-mission catalogues (the *Gaia* legacy release, planned for 2030). *Gaia*'s third data release (DR3, released as the EDR3 on 2020 December 4 and DR3 on 2022 June 13: Gaia Collaboration 2023a) are being used for WEAVE target selection, providing a homogeneous source for astrometry (positions, parallaxes, proper motions – Lindegren et al. 2021) and broad-band photometry (Riello et al. 2021), to enhance the capabilities of WEAVE to build large source catalogues tailored to studying specific aspects of the Milky Way’s stellar populations, with well-controlled and reproducible target selection criteria.

The WEAVE GA surveys fall into four independent categories listed below, according to the spectral resolution of the observations (LR or HR) and the stellar populations or Galactic locations targeted, as illustrated in Figs 6 and 7. These complementary surveys, together with the SCIP and White Dwarfs surveys (see Sections 4.2 and 4.3, respectively), nearly completely sample the Milky Way’s stellar populations accessible from the Northern hemisphere.

(i) **The high-latitude LR survey ('LR-highlat')**: This low-resolution wide-area survey of 6000–8500 deg<sup>2</sup> at high Galactic latitude will observe 1.10 × 10<sup>6</sup> (five-year survey) to 1.55 × 10<sup>6</sup> (seven-year survey) stars in the magnitude range 15 < *r*<sub>SDSS</sub> < 20–

<footnote><sup>a</sup>'Survey fraction' denotes the fraction of total WEAVE-Survey fibres hours currently planned for use for a given WEAVE survey.</footnote>

<footnote><sup>b</sup>MSTO: main-sequence turnoff; RGB: red giant branch; BHB: blue horizontal branch.</footnote>

<footnote><sup>c</sup>White dwarfs used purely for IFU-mode calibrations.</footnote>

<footnote><sup>d</sup>Area coverage calculated using the field of view of the LIFU, not the field of view of WEAVE.</footnote>

<footnote><sup>e</sup>Applies to WQ-Wide (when J-PAS target selection is available). WQ-HighDens will target r < 23.5 for z > 2.2 over ~420 deg<sup>2</sup>.</footnote>
<table><caption>**Table 4.** WEAVE survey parameters, listed in order of survey appearance in the text. Numbers provided are best estimates at the time of writing for a seven-year WEAVE Survey.</caption><thead><tr><th>WEAVE survey (short hand)</th><th>Main targets</th><th>Number of objects</th><th>Area (deg²)</th><th>Survey fraction<sup>a</sup></th><th>Observing mode(s)</th><th>Resolution modes(s)</th><th>Redshift</th></tr></thead><tbody><tr><td>Galactic Archaeology (GA-LRhighlat)</td><td>MSTO<sup>b</sup> stars, RGB<sup>b</sup> stars, BHB<sup>b</sup> stars, RR Lyrae</td><td>~1.6 × 10<sup>6</sup></td><td>8750</td><td>0.168</td><td>MOS</td><td>LR</td><td>0</td></tr><tr><td>Galactic Archaeology (GA-LRdisc)</td><td>Red Clump stars, RGB stars</td><td>~1.1 × 10<sup>6</sup></td><td>1370</td><td>0.110</td><td>MOS</td><td>LR</td><td>0</td></tr><tr><td>Galactic Archaeology (GA-HR)</td><td>Main sequence and RGB stars</td><td>~1.6 × 10<sup>6</sup></td><td>5650</td><td>0.309</td><td>MOS</td><td>HR</td><td>0</td></tr><tr><td>Galactic Archaeology (GA-OC)</td><td>Stars in open clusters and star-forming regions</td><td>~1 × 10<sup>5</sup></td><td>375</td><td>0.029</td><td>MOS</td><td>HR</td><td>0</td></tr><tr><td>Stellar, Circumstellar, and Interstellar Physics (SCIP)</td><td>OBA stars, ionized nebulae, young stars, compact objects</td><td>~4 × 10<sup>5</sup></td><td>1230</td><td>0.069</td><td>MOS, LIFU</td><td>LR, HR</td><td>0</td></tr><tr><td>White Dwarfs</td><td>White dwarfs</td><td>≥5 × 10<sup>4</sup></td><td>≥10 000</td><td>~0.012</td><td>MOS, mIFU<sup>c</sup>, LIFU<sup>c</sup></td><td>LR, HR</td><td>0</td></tr><tr><td>WEAVE-Apertif</td><td>H I-detected, mostly late-type galaxies</td><td>400 (LR), 100 (HR)</td><td>0.2<sup>d</sup></td><td>0.061</td><td>LIFU</td><td>LR, HR</td><td>&lt;0.04</td></tr><tr><td>WEAVE Galaxy Clusters</td><td>Galaxies in dense environments</td><td>~2 × 10<sup>5</sup></td><td>1350</td><td>0.064</td><td>MOS, mIFU, LIFU</td><td>LR</td><td>&lt;0.5</td></tr><tr><td>Stellar Populations at intermediate redshifts Survey (StePS)</td><td>Field galaxies</td><td>~2.5 × 10<sup>4</sup></td><td>25</td><td>0.026</td><td>MOS</td><td>LR</td><td>0.3–0.7</td></tr><tr><td>WEAVE-LOFAR (WL)</td><td>150 MHz sources</td><td>~7 × 10<sup>5</sup></td><td>8950</td><td>0.109</td><td>MOS, mIFU, LIFU</td><td>LR</td><td>&lt;6.9</td></tr><tr><td>WEAVE-QSO (WQ)</td><td>Bright, r &lt; 21.5: z &gt; 2.2; 21.5 &lt; r &lt; 23.5: 2.5 &lt; z &lt; 3<sup>e</sup></td><td>~4 × 10<sup>5</sup></td><td>8950</td><td>0.056</td><td>MOS</td><td>LR, HR</td><td>&gt;2.2</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Expected radial velocity accuracies for WEAVE in its *R* = 5000 mode (thick solid coloured lines) for different stellar effective temperature ranges, based on the WEAVE ‘Operational Rehearsal’ (OpR) simulations (specifically OpR2.5 – see also Section 6; final improvements to the APS pipeline based on OpR3 data have confirmed the results presented here, albeit on a smaller sample), together with the post-launch predictions for *Gaia* at the end of the nominal five-year mission (thin solid coloured lines) for different stellar types. Dashed coloured lines represent *Gaia* end-of-mission photometric limits for astrometry of different spectral types.</figcaption></figure>

WEAVE is a cornerstone for this endeavour, as one of the first – and for its high-resolution capability, the only – high-multiplex wide-field optical multi-object spectrograph in the Northern hemisphere. These spectroscopic surveys complement *Gaia*'s superb astrometry while benefitting from *Gaia* for optimal target selection. A WEAVE survey starting in the coming months will be particularly timely, benefitting from *Gaia* Data Release 3 in 2022 June (DR3: Gaia Collaboration 2023a), and producing its final science in the same time-frame as the *Gaia* end-of-mission catalogues (the *Gaia* legacy release, planned for 2030). *Gaia*'s third data release (DR3, released as the EDR3 on 2020 December 4 and DR3 on 2022 June 13: Gaia Collaboration 2023a) are being used for WEAVE target selection, providing a homogeneous source for astrometry (positions, parallaxes, proper motions – Lindegren et al. 2021) and broad-band photometry (Riello et al. 2021), to enhance the capabilities of WEAVE to build large source catalogues tailored to studying specific aspects of the Milky Way’s stellar populations, with well-controlled and reproducible target selection criteria.

The WEAVE GA surveys fall into four independent categories listed below, according to the spectral resolution of the observations (LR or HR) and the stellar populations or Galactic locations targeted, as illustrated in Figs 6 and 7. These complementary surveys, together with the SCIP and White Dwarfs surveys (see Sections 4.2 and 4.3, respectively), nearly completely sample the Milky Way’s stellar populations accessible from the Northern hemisphere.

(i) **The high-latitude LR survey ('LR-highlat')**: This low-resolution wide-area survey of 6000–8500 deg² at high Galactic latitude will observe 1.10 × 10⁶ (five-year survey) to 1.55 × 10⁶ (seven-year survey) stars in the magnitude range 15 < *r*<sub>SDSS</sub> < 20–

<footnote><sup>a</sup>‘Survey fraction’ denotes the fraction of total WEAVE-Survey fibres hours currently planned for use for a given WEAVE survey.</footnote>

<footnote><sup>b</sup>MSTO: main-sequence turnoff; RGB: red giant branch; BHB: blue horizontal branch.</footnote>

<footnote><sup>c</sup>White dwarfs used purely for IFU-mode calibrations.</footnote>

<footnote><sup>d</sup>Area coverage calculated using the field of view of the LIFU, not the field of view of WEAVE.</footnote>

<footnote><sup>e</sup>Applies to WQ-Wide (when J-PAS target selection is available). WQ-HighDens will target r < 23.5 for z > 2.2 over ~420 deg².</footnote>
<figure><img src="image_6.png" /><figcaption>**Figure 6.** Footprint of the WEAVE GA surveys on the sky in Galactic coordinates (ℓ, b) in an Aitoff projection overlaid on the Gaia DR3 (Gaia Collaboration 2023a) density contours of stars with parallax measures, $G < 16$, and $-10 \deg < \delta < 80 \deg$ (in stars deg$^{-2}$). HR chemo-dynamical survey: black dots; Open Clusters survey: red stars; disc-dynamics LR survey: dark green dots; high-latitude LR survey (shared with the WL and WQ surveys): thick yellow outline, where the region hashed using thinner yellow lines shows the provisional area to be surveyed at ~100 per cent coverage factor, while the remaining area for this survey will be surveyed with an ~30 per cent coverage factor. Known stellar streams and dwarf spheroidal galaxies covered within the pointed part of the high-latitude LR survey: light green stars. The available declination range arises from the impact of differential atmospheric refraction on targets near the edge of the field on the typical 1-hour WEAVE-Survey OB. Near-polar targets like NGC 188 require short exposures outside of the normal WEAVE-Survey OBs.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 7.** Expected coverage of the WEAVE GA sub-surveys in the (X, Z) plane (Cartesian coordinates centred on the Sun's position in the Galaxy) with the Galactic Centre located at (−8.5, 0) kpc. This is based on the target-selection-scheme footprints of each of the sub-surveys, applied to simulations performed with GUMS (Gaia Universe Model Snapshot; Robin et al. 2012) and Galaxia (Sharma et al. 2011), taking into account the expected uncertainties on the Gaia astrometric data used in the target selection, estimated using the PyGaia toolkit (for Gaia performances, see https://www.cosmos.esa.int/web/gaia/science-performance).</figcaption></figure>

21$^{20}$ to complement Gaia with line-of-sight velocities and metallicities for stars too faint for the RVS, and thereby yield the best-yet constraints on the halo potential’s lumpiness and total mass to large distances (~100 kpc; e.g. Cautun et al. 2020), address the formation of the Galactic halo, and characterize its progenitors (including substructures such as streams and dwarf galaxies). The search for traces of accretion events has recently been exemplified by Gaia’s finding of a significant merger, known as Gaia Enceladus or the Gaia Sausage, that has been shown to make up a significant fraction of the inner Galactic halo (e.g. Belokurov et al. 2018; Haywood et al. 2018; Helmi et al. 2018; Gallart et al. 2019; Myeong et al. 2019; Naidu et al. 2020; Malhan et al. 2022). A number of new cold streams have also been discovered thanks to Gaia astrometry (e.g. Malhan, Ibata & Martin 2018; Ibata et al. 2021), which anticipates the wealth of discoveries awaiting us with the combination of Gaia and WEAVE. While the main target of this survey is the Galactic halo, the survey will also probe the thick disc of the Milky Way, complementing the HR survey (see section below). To achieve these goals, the LR-highlat wide-area survey will target the following: colour- and magnitude-selected main-sequence turnoff (MSTO) stars; red giant branch (RGB) stars selected by colour, magnitude, and Gaia astrometric data (parallaxes and proper motions, to reject local red main-sequence stars from the selection); extremely metal-poor star candidates selected from narrow-band photometry (e.g. Pristine: Starkenburg et al. 2017), broad-band photometry (e.g. SDSSS; Abolfathi et al. 2018), or Gaia’s on-board spectrophotometry (Gaia Data Release

<footnote>$^{20}$Magnitudes with subscript ‘SDSS’ refer to Sloan Digital Sky Survey magnitudes (see Albareti et al. 2017).</footnote>
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Footprint of the WEAVE GA surveys on the sky in Galactic coordinates (ℓ, b) in an Aitoff projection overlaid on the Gaia DR3 (Gaia Collaboration 2023a) density contours of stars with parallax measures, $G < 16$, and $-10 \deg < \delta < 80 \deg$ (in stars deg$^{-2}$). HR chemo-dynamical survey: black dots; Open Clusters survey: red stars; disc-dynamics LR survey: dark green dots; high-latitude LR survey (shared with the WL and WQ surveys): thick yellow outline, where the region hashed using thinner yellow lines shows the provisional area to be surveyed at ~100 per cent coverage factor, while the remaining area for this survey will be surveyed with an ~30 per cent coverage factor. Known stellar streams and dwarf spheroidal galaxies covered within the pointed part of the high-latitude LR survey: light green stars. The available declination range arises from the impact of differential atmospheric refraction on targets near the edge of the field on the typical 1-hour WEAVE-Survey OB. Near-polar targets like NGC 188 require short exposures outside of the normal WEAVE-Survey OBs.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 7.** Expected coverage of the WEAVE GA sub-surveys in the (X, Z) plane (Cartesian coordinates centred on the Sun's position in the Galaxy) with the Galactic Centre located at (−8.5, 0) kpc. This is based on the target-selection-scheme footprints of each of the sub-surveys, applied to simulations performed with GUMS (Gaia Universe Model Snapshot; Robin et al. 2012) and Galaxia (Sharma et al. 2011), taking into account the expected uncertainties on the Gaia astrometric data used in the target selection, estimated using the PyGaia toolkit (for Gaia performances, see https://www.cosmos.esa.int/web/gaia/science-performance).</figcaption></figure>

21$^{20}$ to complement Gaia with line-of-sight velocities and metallicities for stars too faint for the RVS, and thereby yield the best-yet constraints on the halo potential’s lumpiness and total mass to large distances (~100 kpc; e.g. Cautun et al. 2020), address the formation of the Galactic halo, and characterize its progenitors (including substructures such as streams and dwarf galaxies). The search for traces of accretion events has recently been exemplified by Gaia’s finding of a significant merger, known as Gaia Enceladus or the Gaia Sausage, that has been shown to make up a significant fraction of the inner Galactic halo (e.g. Belokurov et al. 2018; Haywood et al. 2018; Helmi et al. 2018; Gallart et al. 2019; Myeong et al. 2019; Naidu et al. 2020; Malhan et al. 2022). A number of new cold streams have also been discovered thanks to Gaia astrometry (e.g. Malhan, Ibata & Martin 2018; Ibata et al. 2021), which anticipates the wealth of discoveries awaiting us with the combination of Gaia and WEAVE. While the main target of this survey is the Galactic halo, the survey will also probe the thick disc of the Milky Way, complementing the HR survey (see section below). To achieve these goals, the LR-highlat wide-area survey will target the following: colour- and magnitude-selected main-sequence turnoff (MSTO) stars; red giant branch (RGB) stars selected by colour, magnitude, and Gaia astrometric data (parallaxes and proper motions, to reject local red main-sequence stars from the selection); extremely metal-poor star candidates selected from narrow-band photometry (e.g. Pristine: Starkenburg et al. 2017), broad-band photometry (e.g. SDSSS; Abolfathi et al. 2018), or Gaia’s on-board spectrophotometry (Gaia Data Release

<footnote>$^{20}$Magnitudes with subscript ‘SDSS’ refer to Sloan Digital Sky Survey magnitudes (see Albareti et al. 2017).</footnote>
3 and onwards; see Sartoretti et al. 2023); blue horizontal branch (BHB) stars and blue stragglers (using, for example, a combination of colours including the *u* band, e.g. Deason et al. 2012); RR Lyrae stars (e.g. Sesar et al. 2017; Holl et al. 2018; Iorio & Belokurov 2021; Clementini et al. 2023); hyper-velocity stars (e.g. Brown et al. 2018; Contigiani, Rossi & Marchetti 2019; Evans, Marchetti & Rossi 2022; Marchetti, Evans & Rossi 2022). This survey is foreseen to share fibres on the sky with the WL and WQ surveys (see Sections 4.7 and 4.8, respectively), and hence share the same footprint. In addition to this wide-area component, the LR-highlat survey will drill a few deeper (down to at least $r_{SDSS} = 21$) 4-hour pointings (and even up to 7 hours in some specific cases) around specific objects of interest, namely dwarf spheroidal galaxies and known stellar streams, where the multi-epoch data will allow for the detection of binary stars to refine the line-of-sight velocity dispersion profiles of these objects and to understand better the dark matter distribution (e.g. Posti & Helmi 2019; Cautun et al. 2020). This pointed survey is expected to cover a total of ~200 deg² on the sky.

(ii) **The disc-dynamics LR survey ('LR-disc')**: This low-resolution, high radial-velocity accuracy survey will observe ~500 lines of sight through the Galactic disc at very low latitudes to probe fundamental Galactic disc dynamics processes in a volume inaccessible to Gaia alone. The survey will observe $7 \times 10^5$ (five-year survey) to $1.1 \times 10^6$ (seven-year survey) stars selected mainly to be red-clump giants with Pan-STARRS1 (Chambers et al. 2016) r-band magnitudes in the range $15 < r_{PanSTARRS} < 19$,<fnref n="21" /> spanning evenly the distance range ~2–8 kpc from the Sun, or from ~2 to 15 kpc in Galactocentric radius. Towards the outer disc, all RGB stars will be allowed into the selection function. The fundamental goal of this survey is to understand the phase-space distribution of stars of the Galactic disc, in particular those effects associated with mergers and/or resonances of non-axisymmetric perturbations (bar and spiral arms), both in the inner and outer disc (see also Famaey et al. 2016). This survey is likely to provide unique insights into the effects of various internal and external perturbers on the secular evolution of the Galactic disc at large scales, in a similar fashion to the local non-stationarity found locally in Gaia DR2 (e.g. Antoja et al. 2018; Gaia Collaboration 2018b; Bennett & Bovy 2019; Bovy et al. 2019; Ruiz-Lara et al. 2020; Bland-Hawthorn & Tepper-García 2021). Thanks to the Northern location of WEAVE, this survey will also provide a unique view of the Galactic Anticentre (cf. Gaia Collaboration 2021).

(iii) **The HR chemo-dynamical survey ('HR')**: This high-resolution survey, covering ~4000–5600 deg² at intermediate and high Galactic latitudes, will target stars with $12 < G < 16$ to complement Gaia's phase-space information with full chemical information, thereby opening the full 15+ dimensional space of the chemo-dynamical picture of the three major Galactic populations: the thin and thick discs and the halo. The thick disc will be sampled to large distances, allowing us to study its connection to the thin disc and the inner halo, together with its age-density and age-kinematics relations through mono-abundance and mono-age groups (e.g. Mackereth et al. 2017; Mackereth & Bovy 2020). Ancient accretions that have now dissolved in the halo and discs will be detected chemo-dynamically through the wealth of abundance information for different nucleosynthetic channels that will be provided by the HR spectra (e.g. Di Matteo et al. 2019; Mackereth et al. 2019). In the

volume within which Gaia parallaxes are most accurate (i.e. accurate to better than 10 per cent), high-quality stellar parameters (effective temperature, gravity, metallicity, and [α/Fe]) for $>7 \times 10^5$ (five-year survey) to $1.1 \times 10^6$ (seven-year survey) MSTO and sub-giant stars will also allow us to constrain stellar ages to exquisite precision (Bland-Hawthorn et al. 2019; Kordopatis et al. 2023) within a sphere of radius ~2 kpc. The target selection function, based on absolute magnitudes and their associated uncertainties from Gaia DR3 (or from later releases), will include all stars with absolute magnitudes $M_G < 4.5$, thus also selecting intrinsically bright targets such as RGB stars that will probe the Milky Way's thin and thick discs and halo to distances of ~10 kpc and beyond. Targets of special interest will be given higher priority to ensure proper sampling. These include very and extremely metal-poor stars with prior information on metallicity (e.g. Pristine: Starkenburg et al. 2017), RR Lyrae (see e.g. Clementini et al. 2023), and Cepheid (both in this HR survey, as well as in the LRdisc survey above; see e.g. Ripepi et al. 2023) variable stars, known exoplanet hosts, bright members of known streams (e.g. Martin et al. 2022), and stars with high tangential velocities to increase the halo fraction in the sample (from Gaia DR3; see e.g. Gaia Collaboration 2018a; Koppelman & Helmi 2021).

(iv) **The Open Clusters survey ('OC')**: This is a survey (mostly in HR) of a sizeable sample (~120) of old and young open clusters (OCs) and tiling three cluster-formation regions, aimed at obtaining homogeneous information on radial velocities and chemical information. This will complement Gaia's superb distances and proper motions of individual stars in known and newly discovered OCs (e.g. Castro-Ginard et al. 2020, 2022, and references therein) up to 5 kpc from the Sun, with a precision in distance of 1 per cent up to a distance of 1.5 kpc and 10 per cent for more distant targets, leading to an accurate and reliable definition of membership even for the most distant objects. This survey will constrain our understanding of the formation and disruption processes of OCs (Carrera et al. 2019b; Cantat-Gaudin 2022, and references therein), the chemical evolution of the Milky Way disc using OCs as probes (Carrera et al. 2019a; Cantat-Gaudin et al. 2020; Spina et al. 2022), and provide insights into star formation and early stellar evolution (Valentini et al. 2019; Magrini et al. 2021).

A careful validation and accurate calibration of the derived atmospheric parameters, measured abundance ratios, and radial velocities will ensure that the data from the aforementioned four GA sub-surveys will be on the same scale. The WEAVE GA sub-surveys will therefore include dedicated observations for this purpose, including the observation of globular clusters, well-studied OCs, field giant stars with stellar parameters derived from asteroseismic data from the CoRoT (e.g. Anders et al. 2017) or Kepler (e.g. Borucki et al. 2010; Kallinger et al. 2010; Yu et al. 2018) missions, and field stars (Gaia benchmark stars; Heiter et al. 2015; Hawkins et al. 2016), as well as stars in previous high-resolution spectroscopy studies (e.g. Soubiran et al. 2016). This will allow us to combine the results of the GA sub-surveys both among themselves and with several other stellar Galactic surveys including Gaia-ESO (Gilmore et al. 2012), GALAH (Buder et al. 2021), APOGEE (Majewski et al. 2017), 4MOST (de Jong et al. 2019), and others; see e.g. Soubiran, Brouillet & Casamiquela (2022) for a comparison of metallicity determinations for FGK stars across public data releases from several such spectroscopic surveys.

In addition to dedicated calibration fields, it is highly desirable that a significant overlap is built between the high and low spectral resolution (HR and LR, respectively) GA surveys in order to ensure

<footnote n="21">²¹Magnitudes with subscript 'PanSTARRS' refer to PanSTARRS PS1 magnitudes (Tonry et al. 2012).</footnote>
3 and onwards; see Sartoretti et al. 2023); blue horizontal branch (BHB) stars and blue stragglers (using, for example, a combination of colours including the *u* band, e.g. Deason et al. 2012); RR Lyrae stars (e.g. Sesar et al. 2017; Holl et al. 2018; Iorio & Belokurov 2021; Clementini et al. 2023); hyper-velocity stars (e.g. Brown et al. 2018; Contigiani, Rossi & Marchetti 2019; Evans, Marchetti & Rossi 2022; Marchetti, Evans & Rossi 2022). This survey is foreseen to share fibres on the sky with the WL and WQ surveys (see Sections 4.7 and 4.8, respectively), and hence share the same footprint. In addition to this wide-area component, the LR-highlat survey will drill a few deeper (down to at least $r_{SDSS} = 21$) 4-hour pointings (and even up to 7 hours in some specific cases) around specific objects of interest, namely dwarf spheroidal galaxies and known stellar streams, where the multi-epoch data will allow for the detection of binary stars to refine the line-of-sight velocity dispersion profiles of these objects and to understand better the dark matter distribution (e.g. Posti & Helmi 2019; Cautun et al. 2020). This pointed survey is expected to cover a total of ~200 deg² on the sky.

(ii) **The disc-dynamics LR survey ('LR-disc')**: This low-resolution, high radial-velocity accuracy survey will observe ~500 lines of sight through the Galactic disc at very low latitudes to probe fundamental Galactic disc dynamics processes in a volume inaccessible to Gaia alone. The survey will observe $7 \times 10^5$ (five-year survey) to $1.1 \times 10^6$ (seven-year survey) stars selected mainly to be red-clump giants with Pan-STARRS1 (Chambers et al. 2016) r-band magnitudes in the range $15 < r_{PanSTARRS} < 19$,<fnref n="21" /> spanning evenly the distance range ~2–8 kpc from the Sun, or from ~2 to 15 kpc in Galactocentric radius. Towards the outer disc, all RGB stars will be allowed into the selection function. The fundamental goal of this survey is to understand the phase-space distribution of stars of the Galactic disc, in particular those effects associated with mergers and/or resonances of non-axisymmetric perturbations (bar and spiral arms), both in the inner and outer disc (see also Famaey et al. 2016). This survey is likely to provide unique insights into the effects of various internal and external perturbers on the secular evolution of the Galactic disc at large scales, in a similar fashion to the local non-stationarity found locally in Gaia DR2 (e.g. Antoja et al. 2018; Gaia Collaboration 2018b; Bennett & Bovy 2019; Bovy et al. 2019; Ruiz-Lara et al. 2020; Bland-Hawthorn & Tepper-García 2021). Thanks to the Northern location of WEAVE, this survey will also provide a unique view of the Galactic Anticentre (cf. Gaia Collaboration 2021).

(iii) **The HR chemo-dynamical survey ('HR')**: This high-resolution survey, covering ~4000–5600 deg² at intermediate and high Galactic latitudes, will target stars with $12 < G < 16$ to complement Gaia's phase-space information with full chemical information, thereby opening the full 15+ dimensional space of the chemo-dynamical picture of the three major Galactic populations: the thin and thick discs and the halo. The thick disc will be sampled to large distances, allowing us to study its connection to the thin disc and the inner halo, together with its age-density and age-kinematics relations through mono-abundance and mono-age groups (e.g. Mackereth et al. 2017; Mackereth & Bovy 2020). Ancient accretions that have now dissolved in the halo and discs will be detected chemo-dynamically through the wealth of abundance information for different nucleosynthetic channels that will be provided by the HR spectra (e.g. Di Matteo et al. 2019; Mackereth et al. 2019). In the

volume within which Gaia parallaxes are most accurate (i.e. accurate to better than 10 per cent), high-quality stellar parameters (effective temperature, gravity, metallicity, and [α/Fe]) for $>7 \times 10^5$ (five-year survey) to $1.1 \times 10^6$ (seven-year survey) MSTO and sub-giant stars will also allow us to constrain stellar ages to exquisite precision (Bland-Hawthorn et al. 2019; Kordopatis et al. 2023) within a sphere of radius ~2 kpc. The target selection function, based on absolute magnitudes and their associated uncertainties from Gaia DR3 (or from later releases), will include all stars with absolute magnitudes $M_G < 4.5$, thus also selecting intrinsically bright targets such as RGB stars that will probe the Milky Way's thin and thick discs and halo to distances of ~10 kpc and beyond. Targets of special interest will be given higher priority to ensure proper sampling. These include very and extremely metal-poor stars with prior information on metallicity (e.g. Pristine: Starkenburg et al. 2017), RR Lyrae (see e.g. Clementini et al. 2023), and Cepheid (both in this HR survey, as well as in the LRdisc survey above; see e.g. Ripepi et al. 2023) variable stars, known exoplanet hosts, bright members of known streams (e.g. Martin et al. 2022), and stars with high tangential velocities to increase the halo fraction in the sample (from Gaia DR3; see e.g. Gaia Collaboration 2018a; Koppelman & Helmi 2021).

(iv) **The Open Clusters survey ('OC')**: This is a survey (mostly in HR) of a sizeable sample (~120) of old and young open clusters (OCs) and tiling three cluster-formation regions, aimed at obtaining homogeneous information on radial velocities and chemical information. This will complement Gaia's superb distances and proper motions of individual stars in known and newly discovered OCs (e.g. Castro-Ginard et al. 2020, 2022, and references therein) up to 5 kpc from the Sun, with a precision in distance of 1 per cent up to a distance of 1.5 kpc and 10 per cent for more distant targets, leading to an accurate and reliable definition of membership even for the most distant objects. This survey will constrain our understanding of the formation and disruption processes of OCs (Carrera et al. 2019b; Cantat-Gaudin 2022, and references therein), the chemical evolution of the Milky Way disc using OCs as probes (Carrera et al. 2019a; Cantat-Gaudin et al. 2020; Spina et al. 2022), and provide insights into star formation and early stellar evolution (Valentini et al. 2019; Magrini et al. 2021).

A careful validation and accurate calibration of the derived atmospheric parameters, measured abundance ratios, and radial velocities will ensure that the data from the aforementioned four GA sub-surveys will be on the same scale. The WEAVE GA sub-surveys will therefore include dedicated observations for this purpose, including the observation of globular clusters, well-studied OCs, field giant stars with stellar parameters derived from asteroseismic data from the CoRoT (e.g. Anders et al. 2017) or Kepler (e.g. Borucki et al. 2010; Kallinger et al. 2010; Yu et al. 2018) missions, and field stars (Gaia benchmark stars; Heiter et al. 2015; Hawkins et al. 2016), as well as stars in previous high-resolution spectroscopy studies (e.g. Soubiran et al. 2016). This will allow us to combine the results of the GA sub-surveys both among themselves and with several other stellar Galactic surveys including Gaia-ESO (Gilmore et al. 2012), GALAH (Buder et al. 2021), APOGEE (Majewski et al. 2017), 4MOST (de Jong et al. 2019), and others; see e.g. Soubiran, Brouillet & Casamiquela (2022) for a comparison of metallicity determinations for FGK stars across public data releases from several such spectroscopic surveys.

In addition to dedicated calibration fields, it is highly desirable that a significant overlap is built between the high and low spectral resolution (HR and LR, respectively) GA surveys in order to ensure

<footnote n="21">²¹Magnitudes with subscript 'PanSTARRS' refer to PanSTARRS PS1 magnitudes (Tonry et al. 2012).</footnote>
homogeneity of the stellar parameters and individual chemical abundances that are derived between these two instrumental modes. While calibration stars (cluster and reference field stars observed specifically with both modes) provide the absolute anchor to these quantities through comparisons with independent estimates, it is desirable to build an overlap of stars surveyed in both LR and HR that would enable us to better map the systematics between stellar parameters and abundances derived from these two types of WEAVE spectra. Furthermore, it is expected that it will not be possible to measure the abundances of some elements in the restricted wavelength range of the HR mode; among these are, for example, nitrogen (and carbon in the green+red HR mode). A programme to utilize sub-optimal and/or relatively undersubscribed observing conditions has thus been devised to observe a fraction of the GA-HR survey observing blocks in LR mode, to be executed in weather conditions when the main surveys cannot be executed. Thanks to the limited depth of the GA-HR survey ($12 \le G \le 16$), this programme is expected to be executed in bright time and modest seeing conditions (seeing < 2 arcsec), thus complementing the suite of programmes with observations suitable for the relatively undersubscribed bright time. This GA programme will allow us: (i) to map systematics between the LR and HR analysis of WEAVE spectra over a large part of the Hertzsprung-Russell diagram, and reflecting the target selection of the GA surveys, with sufficiently large statistics to allow this mapping to be worked out with machine-learning methods (e.g. Nandakumar et al. 2022); and (ii) to potentially build a significant sample in which the HR suite of elemental abundances is complemented by the few elements that are only accessible (or better measured) in LR.

## 4.2 The WEAVE Stellar, Circumstellar, and Interstellar Physics Survey

Advancements in characterizing the stellar and interstellar processes currently at work in the Milky Way are helping us to better understand the physics that shape the appearances, content, and evolutionary histories of galaxies in the wider and younger Universe. Only locally can we easily access individual examples of the least well-described (and often short-lived) phases in stellar evolution down to sub-solar luminosity, and study the relationship between stars and the interstellar medium at maximum angular resolution. Topics of current significance include: environmental factors in the formation of stars (Vincke & Pfalzner 2016; Guarcello et al. 2021); mapping the ISM in terms of its extinction and its ionization (Maíz Apellániz & Barbá 2018; Rathjen et al. 2021; Dharmawardena et al. 2022); filling in the Galactic massive-star Hertzsprung-Russell diagram (Berlanas et al. 2020; Holgado et al. 2020); how young stellar objects disperse into the field (Kounkel & Covey 2019; Wright 2020); the approach to stellar end states via cooling white dwarfs, X-ray, and other interacting binaries (Pala et al. 2020; Inight et al. 2021).

SCP will use the products of the INT Photometric H-Alpha Survey (IPHAS: Drew et al. 2005; Barentsen et al. 2014; Monguió et al. 2020) and UV-Excess (UVEX) Survey of the Northern Galactic Plane (Groot et al. 2009; Monguió et al. 2020) executed on the Isaac Newton Telescope and (to a lesser extent) the VST Photometric H-Alpha Survey of the Southern Galactic Plane and Bulge (VPHAS+: Drew et al. 2014) at the Very Large Telescope (VLT) Survey Telescope, for much of its target selection. Between them, these surveys map the whole Galactic plane at ~1 arcsec resolution within $|b| < 5$ deg and in five optical bands. These photometric surveys have captured more than half a billion point sources in the Milky Way's main mass component. Well-tuned samples of a range of object classes are

<figure><img src="image_8.png" /><figcaption>**Figure 8.** The SCP survey footprint, mapped in Galactic coordinates, starting from $\ell \simeq 25$ deg (top right) and ending finally at $\ell \simeq 210$ deg (bottom left), with an island of pointings that encloses the Rosette Nebula, NGC 2264, and the Monoceros supernova remnant. The regular pattern of LR pointings, covering ~1200 deg$^2$, is traced by the orange circles representing the WEAVE field of view. In the second Galactic quadrant ($95 \deg < \ell < 170 \deg$), the footprint is shifted towards positive $b$ to better track the warp of the Galactic disc. The regions to be studied intensively using the blue and red HR gratings are outlined in blue: these are in Cygnus (top panel, seven tailored pointings that are different from the LR survey) and the Galactic Anticentre (region indicated by rectangular outlines, and using the same field centres as the LR survey).</figcaption></figure>

being extracted from them for WEAVE, mostly within the magnitude ranges $11 < i < 18$ and $13 < B < 20$. SCP will pursue a number of scientifically distinct goals in Galactic astronomy, permitting full use of the available fibres in every pointing. The SCP survey footprint is shown in Fig. 8.

The SCP survey targets the high-mass and young/old extremes of stellar evolution along with the ISM on a large scale, capturing both the emissive, ionized nebulous component, and the ISM seen in absorption against background starlight. Since over half of the planned targets are relatively luminous OBA stars, the total space volume accessed will be very large, creating the opportunity to map Galactic disc structure out to heliocentric distances of ~10 kpc. The ~200 000 BA stars to be surveyed, falling within the SCP footprint, will provide a comprehensive account of young stellar kinematics in the Galactic disc on combining them with Gaia proper motions. The feasibility of this via fibre spectroscopy has been tested by Harris et al. (2018), where it was shown that stellar parameters of sufficient quality can be derived from the far-red calcium-triplet region of the spectrum alone. WEAVE in its $R \sim 5000$ mode captures this region well, and is accessible for Galactic plane extinctions up to at least $A_V \sim 6$. The ~20 000 O and B stars to be targeted will be most abundant in the first Galactic quadrant ($\ell < 90$ deg). These spectra will build a more complete picture of the Galactic massive-star Hertzsprung-Russell diagram, and inform data-starved stellar evolution models. Parameter determination will proceed via the methods presented by Simón-Díaz (2010), Holgado et al. (2018), Carneiro et al. (2019), and others.

Pan-optical multi-object spectroscopy of stellar sight-lines at $R \sim 5000$ provides many types of information, including: radial velocities to precisions of a few km s$^{-1}$; reliable stellar effective temperatures, and surface gravities; overall metallicity and first evidence of chemical peculiarity; detection of the more obvious
homogeneity of the stellar parameters and individual chemical abundances that are derived between these two instrumental modes. While calibration stars (cluster and reference field stars observed specifically with both modes) provide the absolute anchor to these quantities through comparisons with independent estimates, it is desirable to build an overlap of stars surveyed in both LR and HR that would enable us to better map the systematics between stellar parameters and abundances derived from these two types of WEAVE spectra. Furthermore, it is expected that it will not be possible to measure the abundances of some elements in the restricted wavelength range of the HR mode; among these are, for example, nitrogen (and carbon in the green+red HR mode). A programme to utilize sub-optimal and/or relatively undersubscribed observing conditions has thus been devised to observe a fraction of the GA-HR survey observing blocks in LR mode, to be executed in weather conditions when the main surveys cannot be executed. Thanks to the limited depth of the GA-HR survey ($12 \le G \le 16$), this programme is expected to be executed in bright time and modest seeing conditions (seeing < 2 arcsec), thus complementing the suite of programmes with observations suitable for the relatively undersubscribed bright time. This GA programme will allow us: (i) to map systematics between the LR and HR analysis of WEAVE spectra over a large part of the Hertzsprung-Russell diagram, and reflecting the target selection of the GA surveys, with sufficiently large statistics to allow this mapping to be worked out with machine-learning methods (e.g. Nandakumar et al. 2022); and (ii) to potentially build a significant sample in which the HR suite of elemental abundances is complemented by the few elements that are only accessible (or better measured) in LR.

## 4.2 The WEAVE Stellar, Circumstellar, and Interstellar Physics Survey

Advancements in characterizing the stellar and interstellar processes currently at work in the Milky Way are helping us to better understand the physics that shape the appearances, content, and evolutionary histories of galaxies in the wider and younger Universe. Only locally can we easily access individual examples of the least well-described (and often short-lived) phases in stellar evolution down to sub-solar luminosity, and study the relationship between stars and the interstellar medium at maximum angular resolution. Topics of current significance include: environmental factors in the formation of stars (Vincke & Pfalzner 2016; Guarcello et al. 2021); mapping the ISM in terms of its extinction and its ionization (Maíz Apellániz & Barbá 2018; Rathjen et al. 2021; Dharmawardena et al. 2022); filling in the Galactic massive-star Hertzsprung-Russell diagram (Berlanas et al. 2020; Holgado et al. 2020); how young stellar objects disperse into the field (Kounkel & Covey 2019; Wright 2020); the approach to stellar end states via cooling white dwarfs, X-ray, and other interacting binaries (Pala et al. 2020; Inight et al. 2021).

SCP will use the products of the INT Photometric H-Alpha Survey (IPHAS: Drew et al. 2005; Barentsen et al. 2014; Monguió et al. 2020) and UV-Excess (UVEX) Survey of the Northern Galactic Plane (Groot et al. 2009; Monguió et al. 2020) executed on the Isaac Newton Telescope and (to a lesser extent) the VST Photometric H-Alpha Survey of the Southern Galactic Plane and Bulge (VPHAS+: Drew et al. 2014) at the Very Large Telescope (VLT) Survey Telescope, for much of its target selection. Between them, these surveys map the whole Galactic plane at ~1 arcsec resolution within $|b| < 5$ deg and in five optical bands. These photometric surveys have captured more than half a billion point sources in the Milky Way's main mass component. Well-tuned samples of a range of object classes are

<figure><img src="image_9.png" /><figcaption>**Figure 8.** The SCP survey footprint, mapped in Galactic coordinates, starting from $\ell \simeq 25$ deg (top right) and ending finally at $\ell \simeq 210$ deg (bottom left), with an island of pointings that encloses the Rosette Nebula, NGC 2264, and the Monoceros supernova remnant. The regular pattern of LR pointings, covering ~1200 deg$^2$, is traced by the orange circles representing the WEAVE field of view. In the second Galactic quadrant ($95 \deg < \ell < 170 \deg$), the footprint is shifted towards positive $b$ to better track the warp of the Galactic disc. The regions to be studied intensively using the blue and red HR gratings are outlined in blue: these are in Cygnus (top panel, seven tailored pointings that are different from the LR survey) and the Galactic Anticentre (region indicated by rectangular outlines, and using the same field centres as the LR survey).</figcaption></figure>

being extracted from them for WEAVE, mostly within the magnitude ranges $11 < i < 18$ and $13 < B < 20$. SCP will pursue a number of scientifically distinct goals in Galactic astronomy, permitting full use of the available fibres in every pointing. The SCP survey footprint is shown in Fig. 8.

The SCP survey targets the high-mass and young/old extremes of stellar evolution along with the ISM on a large scale, capturing both the emissive, ionized nebulous component, and the ISM seen in absorption against background starlight. Since over half of the planned targets are relatively luminous OBA stars, the total space volume accessed will be very large, creating the opportunity to map Galactic disc structure out to heliocentric distances of ~10 kpc. The ~200 000 BA stars to be surveyed, falling within the SCP footprint, will provide a comprehensive account of young stellar kinematics in the Galactic disc on combining them with Gaia proper motions. The feasibility of this via fibre spectroscopy has been tested by Harris et al. (2018), where it was shown that stellar parameters of sufficient quality can be derived from the far-red calcium-triplet region of the spectrum alone. WEAVE in its $R \sim 5000$ mode captures this region well, and is accessible for Galactic plane extinctions up to at least $A_V \sim 6$. The ~20 000 O and B stars to be targeted will be most abundant in the first Galactic quadrant ($\ell < 90$ deg). These spectra will build a more complete picture of the Galactic massive-star Hertzsprung-Russell diagram, and inform data-starved stellar evolution models. Parameter determination will proceed via the methods presented by Simón-Díaz (2010), Holgado et al. (2018), Carneiro et al. (2019), and others.

Pan-optical multi-object spectroscopy of stellar sight-lines at $R \sim 5000$ provides many types of information, including: radial velocities to precisions of a few km s$^{-1}$; reliable stellar effective temperatures, and surface gravities; overall metallicity and first evidence of chemical peculiarity; detection of the more obvious
<figure><img src="image_9.png" /><figcaption>**Figure 9.** The distribution of potential WEAVE diffuse-ISM targets for the Rosette/NGC 2264/Monoceros supernova remnant region, derived from IGAPS/IPHAS Hα + [N II] narrow-band imagery (see www.igapsimages.org and Greimel et al. 2021). The planned SCIP LR fields are superimposed as blue circles. The candidate fibre positions, before final downsizing for configuration, are plotted on a colour scale according to narrow-band surface brightness (in erg cm⁻² s⁻¹ arcsec⁻²). The brightest diffuse structure here is the Rosette Nebula, at the lower left. In the upper right is the NGC 2264 region, with the ~220-arcmin diameter Monoceros supernova remnant in between.</figcaption></figure>

spectroscopic binaries; interstellar absorption features; a suite of nebular/circumstellar emission line diagnostics; mass transfer/loss signatures; and markers for magnetic activity. In WEAVE's HR mode at R = 20 000, observations in the Cygnus region and the Galactic Anticentre will push even further, from the measurement of projected stellar rotation speeds down to lower limits, to the determination of individual heavy-element abundances. Greater precision of radial velocity measurements will better expose the velocity dispersions of the local environments. Spectroscopic binaries will be detected via careful programmes of repeat observations.

The SCIP programme will be split between a LR survey, taking roughly two-thirds of the SCIP survey time, and two HR components that will equally split the remainder in bright time.

(i) **The LR survey:** Around 1200 deg² of the northern Galactic Plane will be observed. The largest target group is comprised of the higher-mass stars: this includes OBA stars and their evolved counterparts (including red supergiants), emission-line objects, and Cepheid variables. The aim is to densely sample most Galactic longitudes in the thin disc accessible from La Palma. A novel feature of SCIP's target selection is the use of Hα imagery from IPHAS to identify, in addition, a web of Hα-bright diffuse-ISM positions spanning HII regions, supernova remnants, and other nebulae (Greimel et al. 2021, and Fig. 9). These will complement the O- and B-star targets, with spectroscopy of the associated ionized ISM. The targeted range is from surface brightnesses of 6 × 10⁻¹⁶ up to ~10⁻¹³ erg cm⁻² s⁻¹ arcsec⁻², with full abundance analysis achievable above 3–5 × 10⁻¹⁵ erg cm⁻² s⁻¹ arcsec⁻². Radial velocities will be obtained for all targets. A third focus is on the unbiased exploration of the incidence of young stellar objects in order to understand how they disperse across the Galactic field. The target selection draws significantly from infrared and optical photometric surveys and Gaia astrometry, yielding candidates of young stellar objects outside of the traditionally explored cores of OB associations and OCs. The majority of targets will be selected using the method

<figure><img src="image_10.png" /><figcaption>**Figure 10.** The discovery space for the high-resolution Cygnus component of the SCIP survey, expressed in Gaia BP magnitudes. Known OB stars are shown in orange (smaller of the two histograms), while those stars available for confirmation with WEAVE are shown in cyan (larger of the two histograms) after scaling down by a factor of 0.3. These fall within the seven fields planned for the survey in the Galactic longitude range 75 deg < l < 81 deg (see Fig. 8).</figcaption></figure>

of Wilson et al. (2023); for HAeBe stars, see Vioque et al. (2020). WEAVE spectroscopy will enable definitive classifications, as well as measures of radial velocities and accretion rates. A minority of fibres per pointing will also be allocated to compact binaries and white dwarfs (see also Section 4.3).

(ii) **The HR surveys:** These also place high priority on O, B, and A stars. In the Cygnus region, the aim is to obtain spectroscopy of prominent OB-associations (for a recent review, see Wright et al. 2022) to build a full picture of kinematics and elemental abundances across the region, extending the studies first initiated by Berlanas et al. (2018). As shown in Fig. 10, the discovery potential is very large. In the Anticentre fields, the emphasis shifts to observations of late-B and A stars, where the science aims concern both the stars themselves and describing Galactic disc kinematics as imprinted on this young, dynamically cold population. Measuring constraints on binarity via repeat exposures is an important aspect in both HR programme components. Most stars in the Universe form in binaries or higher-order multiples (Sana 2022). In their review, Duchêne & Kraus (2013) cite studies that indicate that ~50 per cent of B and A stars have binary companions (see also Moe & Di Stefano 2017; Banyard et al. 2022). In the case of O stars, it has been estimated that over half will experience binary mass exchange in their lifetimes (Sana et al. 2013, 2014).

The normal SCIP survey planning for MOS observations assumes an upper limit to the seeing of 1.2 arcsec, appropriate for point sources observed using fibres of diameter 1.3 arcsec. This constraint is unnecessary for extended sources, such as Galactic planetary nebulae (PNe), where abundance patterns as a function of position are an important goal (García-Rojas et al. 2022). In poor seeing conditions (~2 arcsec or more) that are unlikely to be usable for primary science objectives of the WEAVE surveys, SCIP will exploit the WEAVE (m/L) IFU capability to provide complete spatial chemodynamical maps of a significant sample of PNe. The key science aim is to probe the physical processes governing the late-stage evolution of intermediate-mass stars, and to determine how the abundances and dynamics of larger PNe are governed by their central stars (single or binary). PNe targets have been picked out that suit the 1.3–1.5-
<figure><img src="image_10.png" /><figcaption>**Figure 9.** The distribution of potential WEAVE diffuse-ISM targets for the Rosette/NGC 2264/Monoceros supernova remnant region, derived from IGAPS/IPHAS Hα + [N II] narrow-band imagery (see www.igapsimages.org and Greimel et al. 2021). The planned SCIP LR fields are superimposed as blue circles. The candidate fibre positions, before final downsizing for configuration, are plotted on a colour scale according to narrow-band surface brightness (in erg cm⁻² s⁻¹ arcsec⁻²). The brightest diffuse structure here is the Rosette Nebula, at the lower left. In the upper right is the NGC 2264 region, with the ~220-arcmin diameter Monoceros supernova remnant in between.</figcaption></figure>

spectroscopic binaries; interstellar absorption features; a suite of nebular/circumstellar emission line diagnostics; mass transfer/loss signatures; and markers for magnetic activity. In WEAVE's HR mode at R = 20 000, observations in the Cygnus region and the Galactic Anticentre will push even further, from the measurement of projected stellar rotation speeds down to lower limits, to the determination of individual heavy-element abundances. Greater precision of radial velocity measurements will better expose the velocity dispersions of the local environments. Spectroscopic binaries will be detected via careful programmes of repeat observations.

The SCIP programme will be split between a LR survey, taking roughly two-thirds of the SCIP survey time, and two HR components that will equally split the remainder in bright time.

(i) **The LR survey:** Around 1200 deg² of the northern Galactic Plane will be observed. The largest target group is comprised of the higher-mass stars: this includes OBA stars and their evolved counterparts (including red supergiants), emission-line objects, and Cepheid variables. The aim is to densely sample most Galactic longitudes in the thin disc accessible from La Palma. A novel feature of SCIP's target selection is the use of Hα imagery from IPHAS to identify, in addition, a web of Hα-bright diffuse-ISM positions spanning HII regions, supernova remnants, and other nebulae (Greimel et al. 2021, and Fig. 9). These will complement the O- and B-star targets, with spectroscopy of the associated ionized ISM. The targeted range is from surface brightnesses of 6 × 10⁻¹⁶ up to ~10⁻¹³ erg cm⁻² s⁻¹ arcsec⁻², with full abundance analysis achievable above 3–5 × 10⁻¹⁵ erg cm⁻² s⁻¹ arcsec⁻². Radial velocities will be obtained for all targets. A third focus is on the unbiased exploration of the incidence of young stellar objects in order to understand how they disperse across the Galactic field. The target selection draws significantly from infrared and optical photometric surveys and Gaia astrometry, yielding candidates of young stellar objects outside of the traditionally explored cores of OB associations and OCs. The majority of targets will be selected using the method

<figure><img src="image_11.png" /><figcaption>**Figure 10.** The discovery space for the high-resolution Cygnus component of the SCIP survey, expressed in Gaia BP magnitudes. Known OB stars are shown in orange (smaller of the two histograms), while those stars available for confirmation with WEAVE are shown in cyan (larger of the two histograms) after scaling down by a factor of 0.3. These fall within the seven fields planned for the survey in the Galactic longitude range 75 deg < l < 81 deg (see Fig. 8).</figcaption></figure>

of Wilson et al. (2023); for HAeBe stars, see Vioque et al. (2020). WEAVE spectroscopy will enable definitive classifications, as well as measures of radial velocities and accretion rates. A minority of fibres per pointing will also be allocated to compact binaries and white dwarfs (see also Section 4.3).

(ii) **The HR surveys:** These also place high priority on O, B, and A stars. In the Cygnus region, the aim is to obtain spectroscopy of prominent OB-associations (for a recent review, see Wright et al. 2022) to build a full picture of kinematics and elemental abundances across the region, extending the studies first initiated by Berlanas et al. (2018). As shown in Fig. 10, the discovery potential is very large. In the Anticentre fields, the emphasis shifts to observations of late-B and A stars, where the science aims concern both the stars themselves and describing Galactic disc kinematics as imprinted on this young, dynamically cold population. Measuring constraints on binarity via repeat exposures is an important aspect in both HR programme components. Most stars in the Universe form in binaries or higher-order multiples (Sana 2022). In their review, Duchêne & Kraus (2013) cite studies that indicate that ~50 per cent of B and A stars have binary companions (see also Moe & Di Stefano 2017; Banyard et al. 2022). In the case of O stars, it has been estimated that over half will experience binary mass exchange in their lifetimes (Sana et al. 2013, 2014).

The normal SCIP survey planning for MOS observations assumes an upper limit to the seeing of 1.2 arcsec, appropriate for point sources observed using fibres of diameter 1.3 arcsec. This constraint is unnecessary for extended sources, such as Galactic planetary nebulae (PNe), where abundance patterns as a function of position are an important goal (García-Rojas et al. 2022). In poor seeing conditions (~2 arcsec or more) that are unlikely to be usable for primary science objectives of the WEAVE surveys, SCIP will exploit the WEAVE (m/L) IFU capability to provide complete spatial dynamical maps of a significant sample of PNe. The key science aim is to probe the physical processes governing the late-stage evolution of intermediate-mass stars, and to determine how the abundances and dynamics of larger PNe are governed by their central stars (single or binary). PNe targets have been picked out that suit the 1.3–1.5-
arcmin field of view of the LIFU sampled by 2.6 arcsec fibres. These are large, bright nebulae where, in some, the central stars are known to be close binaries; such objects have markedly different chemical properties to other PNe (Wesson et al. 2018). Our list includes well-known objects that have yet to be mapped comprehensively (e.g. the Owl and Cat's-Eye nebulae). There will also be instances of several smaller nebulae in the field of view, for which the mIFU mode could also be usefully utilized.

## 4.3 The WEAVE White Dwarfs Survey

All stars born with masses $\le 8-10 M_{\odot}$ eventually become white dwarfs (Ritossa, García-Berro & Iben 1999; Doherty et al. 2017): Earth-sized electron-degenerate stellar embers. The short main-sequence lifetime of stars with $M \gtrsim 1.5 M_{\odot}$ implies that the majority of A/F-type stars formed throughout the history of the Galaxy are now white dwarfs. As such, white dwarfs play a central role across a variety of areas in astrophysics. Homogeneous samples of white dwarfs with accurate physical parameters are essential for constraining and calibrating stellar evolution theory, including mass-loss on the asymptotic giant branch (intimately linked to the initial-to-final mass relation, e.g. Williams, Bolte & Koester 2009; Cummings et al. 2018), internal rotation profiles and loss of angular momentum (Hermes et al. 2017), and fundamental nuclear reaction rates (Kunz et al. 2002), with important implications for stellar population synthesis and galaxy evolution theory (Maraston 1998; Kalirai, Marigo & Tremblay 2014). Because of their well-constrained cooling ages, white dwarfs provide an insight into the age of the Galactic disc (Winget et al. 1987; Oswalt et al. 1996), OCs (García-Berro et al. 2010) and globular clusters (Hansen et al. 2007), and can even trace variations in the Galactic star formation rate (Tremblay et al. 2014).

White dwarfs will be observed as flux calibrators in WEAVE Survey observations, and the resulting spectra will provide a rich database for white-dwarf-enabled science.

### 4.3.1 White dwarfs as flux calibrators

White dwarfs are routinely used as spectro-photometric flux standards (Bohlin, Dickinson & Calzetti 2001; Bohlin, Gordon & Tremblay 2014; Moehler et al. 2014), as their spectra are extremely simple (i.e. pure hydrogen atmospheres in most cases) and can easily be modelled to high precision (1–2 per cent across all wavelengths) with only two free parameters: effective temperature and surface gravity. Moreover, in a magnitude-limited sample, the majority of white dwarfs have effective temperatures $\gtrsim 8000$ K, i.e. they are blue objects with significant amounts of flux at the shortest wavelengths.

The surface density and magnitude distribution of the *Gaia* white-dwarf population (Pancino et al. 2012) is well matched to the field of view of WEAVE and the aperture of the WHT. In the MOS mode, 10–15 white dwarfs are expected to be observed per WEAVE pointing for calibration purposes in the LR configuration. The number of suitable white dwarfs will likely be around 3–5 for the HR MOS configuration due to the magnitude limit for this mode, dictated by the observational constraints set by the science targets in the field, being brighter than for a LR observation (see Section 4.3). In the mIFU and LIFU modes, constraints set by the number and placement of the IFU(s) in the field mean that no more than one white dwarf is expected to be observed per pointing; bootstrapping from other exposures or other information (such as known magnitudes) are required for these observations.

### 4.3.2 White dwarf science with WEAVE

A significant fraction of local white dwarfs are members of binaries (Toonen et al. 2017), providing a benchmark population for investigating complex interactions (e.g. Zorotovic et al. 2010), including the progenitors of Type Ia supernovae. Degenerate stars with precise cooling ages in wide binaries can also be used to calibrate main-sequence ages (Rebassa-Mansergas et al. 2016). The host stars of virtually all planetary systems – including the Sun – will evolve into white dwarfs, and a sizeable fraction of white dwarfs host the remnants of planetary systems (Farihi, Jura & Zuckerman 2009; Koester, Gänsicke & Farihi 2014). White dwarfs that accrete tidally disrupted planetesimals display photospheric trace metals, which provides a unique opportunity to measure the bulk composition of extra-solar planets (Zuckerman et al. 2007; Gänsicke et al. 2012; Xu et al. 2017). Finally, white dwarfs are laboratories of extreme physics that are unachievable on Earth, including atomic and molecular physics in the presence of strong magnetic fields (Guan 2006) and high-density plasmas (Kowalski 2006).

The extremely large surface gravities of white dwarfs, $\log g \simeq 8$, result in chemical stratification and atmospheric compositions dominated by hydrogen and/or helium (e.g. Eisenstein et al. 2006; Giammichele, Bergeron & Dufour 2012). About 20 per cent of white dwarfs display spectroscopic peculiarities, including metal pollution from accreted planetary debris or from dredge-up of core material, and Zeeman-splitting in magnetic fields of up to $10^9$ MG; see Fig. 11 for examples. Spectroscopy spanning the full optical range is therefore critically important for the study of these degenerate stellar remnants.

Because of their small size, white dwarfs are intrinsically faint, and difficult to distinguish from more distant main-sequence stars with similar colours. Consequently, most known white dwarfs that were identified as ultraviolet-excess objects (Green, Schmidt & Liebert 1986; Liebert, Bergeron & Holberg 2005; Kleinman et al. 2013) are young ($\le 1$ Gyr) and hot ($T_{\text{eff}} \gtrsim 10\,000$ K), and are utterly unrepresentative of the Galactic white dwarf population as a whole. *Gaia* DR2 finally provided the information necessary to break the degeneracy between nearby white dwarfs and background main-sequence stars: accurate parallaxes. Gentile Fusillo et al. (2019) assembled the first unbiased all-sky magnitude-limited ($G \simeq 20$) sample of 260 000 white dwarf candidates using *Gaia* DR2, followed by an updated catalogue using *Gaia* EDR3 data in Gentile Fusillo et al. (2021). While the *Gaia* data are sufficient for identifying white dwarfs with high confidence, follow-up spectroscopy is required (Fig. 11) to determine their physical properties (effective temperature, surface gravity, atmospheric composition, magnetic field strength, and multiplicity) and derive fundamental properties (mass, cooling age, and progenitor mass) that are necessary to address the science areas outlined above.

WEAVE will target $\simeq 100 000$ white dwarfs with $G \le 20$ (Fig. 12) for LR spectroscopy, and will roughly *triple* the number of white dwarfs with high-quality spectroscopy in the Northern hemisphere (Kleinman et al. 2013; Gentile Fusillo, Gänsicke & Greiss 2015). The WEAVE White Dwarfs survey will establish the first large and homogeneous spectroscopic sample that is not subject to complex selection effects, and is therefore ideally suited for detailed statistical analyses of white dwarfs in the context of galactic, stellar, and planetary structure and evolution. The combination of accurate *Gaia* parallaxes and photometry with spectroscopic mass determinations will result in an extremely stringent test of the mass–radius relation of white dwarfs (see e.g. Parsons et al. 2017; Tremblay et al. 2017, 2019; Genest-Beaulieu & Bergeron 2019). The blue coverage (down
arcmin field of view of the LIFU sampled by 2.6 arcsec fibres. These are large, bright nebulae where, in some, the central stars are known to be close binaries; such objects have markedly different chemical properties to other PNe (Wesson et al. 2018). Our list includes well-known objects that have yet to be mapped comprehensively (e.g. the Owl and Cat's-Eye nebulae). There will also be instances of several smaller nebulae in the field of view, for which the mIFU mode could also be usefully utilized.

## 4.3 The WEAVE White Dwarfs Survey

All stars born with masses $\le 8-10 M_{\odot}$ eventually become white dwarfs (Ritossa, García-Berro & Iben 1999; Doherty et al. 2017): Earth-sized electron-degenerate stellar embers. The short main-sequence lifetime of stars with $M \gtrsim 1.5 M_{\odot}$ implies that the majority of A/F-type stars formed throughout the history of the Galaxy are now white dwarfs. As such, white dwarfs play a central role across a variety of areas in astrophysics. Homogeneous samples of white dwarfs with accurate physical parameters are essential for constraining and calibrating stellar evolution theory, including mass-loss on the asymptotic giant branch (intimately linked to the initial-to-final mass relation, e.g. Williams, Bolte & Koester 2009; Cummings et al. 2018), internal rotation profiles and loss of angular momentum (Hermes et al. 2017), and fundamental nuclear reaction rates (Kunz et al. 2002), with important implications for stellar population synthesis and galaxy evolution theory (Maraston 1998; Kalirai, Marigo & Tremblay 2014). Because of their well-constrained cooling ages, white dwarfs provide an insight into the age of the Galactic disc (Winget et al. 1987; Oswalt et al. 1996), OCs (García-Berro et al. 2010) and globular clusters (Hansen et al. 2007), and can even trace variations in the Galactic star formation rate (Tremblay et al. 2014).

White dwarfs will be observed as flux calibrators in WEAVE Survey observations, and the resulting spectra will provide a rich database for white-dwarf-enabled science.

### 4.3.1 White dwarfs as flux calibrators

White dwarfs are routinely used as spectro-photometric flux standards (Bohlin, Dickinson & Calzetti 2001; Bohlin, Gordon & Tremblay 2014; Moehler et al. 2014), as their spectra are extremely simple (i.e. pure hydrogen atmospheres in most cases) and can easily be modelled to high precision (1–2 per cent across all wavelengths) with only two free parameters: effective temperature and surface gravity. Moreover, in a magnitude-limited sample, the majority of white dwarfs have effective temperatures $\gtrsim 8000$ K, i.e. they are blue objects with significant amounts of flux at the shortest wavelengths.

The surface density and magnitude distribution of the *Gaia* white-dwarf population (Pancino et al. 2012) is well matched to the field of view of WEAVE and the aperture of the WHT. In the MOS mode, 10–15 white dwarfs are expected to be observed per WEAVE pointing for calibration purposes in the LR configuration. The number of suitable white dwarfs will likely be around 3–5 for the HR MOS configuration due to the magnitude limit for this mode, dictated by the observational constraints set by the science targets in the field, being brighter than for a LR observation (see Section 4.3). In the mIFU and LIFU modes, constraints set by the number and placement of the IFU(s) in the field mean that no more than one white dwarf is expected to be observed per pointing; bootstrapping from other exposures or other information (such as known magnitudes) are required for these observations.

### 4.3.2 White dwarf science with WEAVE

A significant fraction of local white dwarfs are members of binaries (Toonen et al. 2017), providing a benchmark population for investigating complex interactions (e.g. Zorotovic et al. 2010), including the progenitors of Type Ia supernovae. Degenerate stars with precise cooling ages in wide binaries can also be used to calibrate main-sequence ages (Rebassa-Mansergas et al. 2016). The host stars of virtually all planetary systems – including the Sun – will evolve into white dwarfs, and a sizeable fraction of white dwarfs host the remnants of planetary systems (Farihi, Jura & Zuckerman 2009; Koester, Gänsicke & Farihi 2014). White dwarfs that accrete tidally disrupted planetesimals display photospheric trace metals, which provides a unique opportunity to measure the bulk composition of extra-solar planets (Zuckerman et al. 2007; Gänsicke et al. 2012; Xu et al. 2017). Finally, white dwarfs are laboratories of extreme physics that are unachievable on Earth, including atomic and molecular physics in the presence of strong magnetic fields (Guan 2006) and high-density plasmas (Kowalski 2006).

The extremely large surface gravities of white dwarfs, $\log g \simeq 8$, result in chemical stratification and atmospheric compositions dominated by hydrogen and/or helium (e.g. Eisenstein et al. 2006; Giammichele, Bergeron & Dufour 2012). About 20 per cent of white dwarfs display spectroscopic peculiarities, including metal pollution from accreted planetary debris or from dredge-up of core material, and Zeeman-splitting in magnetic fields of up to $10^9$ MG; see Fig. 11 for examples. Spectroscopy spanning the full optical range is therefore critically important for the study of these degenerate stellar remnants.

Because of their small size, white dwarfs are intrinsically faint, and difficult to distinguish from more distant main-sequence stars with similar colours. Consequently, most known white dwarfs that were identified as ultraviolet-excess objects (Green, Schmidt & Liebert 1986; Liebert, Bergeron & Holberg 2005; Kleinman et al. 2013) are young ($\le 1$ Gyr) and hot ($T_{\text{eff}} \gtrsim 10\,000$ K), and are utterly unrepresentative of the Galactic white dwarf population as a whole. *Gaia* DR2 finally provided the information necessary to break the degeneracy between nearby white dwarfs and background main-sequence stars: accurate parallaxes. Gentile Fusillo et al. (2019) assembled the first unbiased all-sky magnitude-limited ($G \simeq 20$) sample of 260 000 white dwarf candidates using *Gaia* DR2, followed by an updated catalogue using *Gaia* EDR3 data in Gentile Fusillo et al. (2021). While the *Gaia* data are sufficient for identifying white dwarfs with high confidence, follow-up spectroscopy is required (Fig. 11) to determine their physical properties (effective temperature, surface gravity, atmospheric composition, magnetic field strength, and multiplicity) and derive fundamental properties (mass, cooling age, and progenitor mass) that are necessary to address the science areas outlined above.

WEAVE will target $\simeq 100\,000$ white dwarfs with $G \le 20$ (Fig. 12) for LR spectroscopy, and will roughly *triple* the number of white dwarfs with high-quality spectroscopy in the Northern hemisphere (Kleinman et al. 2013; Gentile Fusillo, Gänsicke & Greiss 2015). The WEAVE White Dwarfs survey will establish the first large and homogeneous spectroscopic sample that is not subject to complex selection effects, and is therefore ideally suited for detailed statistical analyses of white dwarfs in the context of galactic, stellar, and planetary structure and evolution. The combination of accurate *Gaia* parallaxes and photometry with spectroscopic mass determinations will result in an extremely stringent test of the mass–radius relation of white dwarfs (see e.g. Parsons et al. 2017; Tremblay et al. 2017, 2019; Genest-Beaulieu & Bergeron 2019). The blue coverage (down
<figure><img src="image_11.png" /><figcaption>**Figure 11.** Examples showing a variety of white dwarf spectra in the range 3850–5550 Å. White dwarfs have large surface gravities (log g ≃ 8), which result in the chemical stratification of their constituents. Most white dwarfs have atmospheres dominated by hydrogen or helium, and their spectra contain only Balmer (DA) or He (DB) lines. However, ≃20 per cent of white dwarfs exhibit spectroscopic peculiarities. Accretion of disrupted planetesimals results in photospheric contamination by metals (DAZ, DBZ, DZ), providing a unique window into the bulk abundances of exoplanetary bodies. Cool white dwarfs with deep convective envelopes may dredge up carbon and oxygen from their cores (cDQ), allowing sensitive tests of stellar evolution. A small number of hotter white dwarfs with carbon-dominated atmospheres (hDQ) are thought to be products of white dwarf mergers, possibly descending from R Corona Borealis stars. Finally, up to 10 per cent show magnetic fields, across all atmospheric compositions (e.g. DAH, DBH), and serve as laboratories for atomic physics under extreme conditions. The WEAVE Survey will target ≃100 000 white dwarfs selected from Gaia DR3 (Fig. 12).</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 12.** Hertzsprung–Russell diagram (left) and magnitude distribution (right) of the 104 198 high-confidence white dwarf candidate targets (red, Dec > −10 deg and G ≤ 20), selected from Gaia DR2 (Gentile Fusillo et al. 2019). Synthetic photometry was computed for main-sequence stars, subgiants, and giants (the blue lines labelled V, IV, and III, respectively) using the spectral library of Pickles (1998), and for white dwarfs with masses of 0.2, 0.6, and 1.0 M⊙ (unlabelled black lines, from top to bottom) using the cooling sequences of Holberg & Bergeron (2006) and model spectra from Koester (2010).</figcaption></figure>

to 3700 Å) of WEAVE is essential for covering the higher Balmer lines, which are important diagnostics for the surface gravity (Kepler et al. 2006), and to probe for atmospheric pollution by planetary debris via Ca H/K absorption. Our large sample size will result in the identification of rare white dwarf species, tracing the extremes of parameter space of short-lived phases in their evolution, such as stellar remnants from stars near the core-collapse boundary (Gänsicke et al. 2010), products of thermonuclear supernovae (Vennes et al. 2017; Raddi et al. 2018a, b; Shen et al. 2018), of as-yet not fully understood binary interactions (Dufour et al. 2007; Kepler, Koester & Ourique 2016), and possibly examples of entirely new evolutionary channels (Marsh et al. 2016). WEAVE spectroscopy will also provide radial velocity measurements that are an important complement to the Gaia proper motions – most white dwarfs have no features in the Gaia RVS spectra, or are too faint for Gaia radial velocities altogether. This first large sample of white dwarfs with full 3D kinematics will allow us to distinguish them into their thin and thick disc, and halo populations (Pauli et al. 2003, 2006; Anguiano et al. 2017), with an expected ≃1 per cent of halo white dwarfs. Furthermore, the kinematics will provide constraints on the age–velocity dispersion relation, and insight into the mass distribution of white dwarfs that formed via binary mergers (Wegg & Pinney 2012).

Given their low luminosity, the surface density of white dwarfs varies only mildly, with an average ≃5 stars deg⁻², or ≃15 per WEAVE pointing. The WEAVE White Dwarfs survey is therefore multiplexed into the entire WEAVE Survey footprint and the white dwarfs will also be used for the spectro-photometric flux calibration of the entire WEAVE Survey (Section 4.3.1).

## 4.4 The WEAVE-Apertif Survey

The detailed analysis of galaxies in the local Universe is crucial to understanding galaxy evolution. However, recent local IFU surveys of galaxies (e.g. CALIFA, Sánchez et al. 2012; SAMI, Croom et al. 2012; MANGA, Bundy et al. 2015) suffer from an optical selection bias that makes it difficult to address the effects of environment on the evolution of galaxies. The gas content of galaxies is less strongly bound to the gravitational potential well than its stellar content. Thus, one possible approach to study the effect of environment on galaxy evolution is to characterize the morphology of galaxies seen in the atomic gas, which indicates interactions that have taken place, and to select galaxies with a range of features. To this end, a wide-area survey of resolved neutral atomic hydrogen (HⅠ) is needed.

### 4.4.1 Apertif and synergy with WEAVE LIFU observations

In the last decade, the Westerbork Synthesis Radio Telescope (WSRT) was upgraded with a new receiver: an innovative focal-plane array system named Apertif (Verheijen et al. 2008; van Cappellen et al. 2022) that allowed for the execution of wide-field radio surveys, and HⅠ surveys out to cosmological distances (z ~ 0.2). Apertif survey operations were conducted between 2019 July 1 and 2022 February 28, surveying a large portion of the Northern hemisphere in 1.4 GHz radio continuum, polarization, and the 21-cm spectral line of neutral atomic hydrogen. The Apertif instrument was the first working focal-plane array capable of full Westerbork resolution [15 arcsec × 15 sin (Dec) arcsec beam²] over a field of view of 6.5 deg² per pointing, after tiling, and with HⅠ spectral resolution of down to 2.6 km s⁻¹. The 300 MHz bandwidth could

<footnote n="22">²²The factor of sin(Dec) is distinctive to WSRT as an East–West array.</footnote>