# MAVIS on the VLT: A Powerful, Synergistic ELT Complement in the Visible

François Rigaut, Richard Mcdermid, Giovanni Cresci, Guido Agapito, Matteo Aliverti, Simone Antoniucci, Andrea Balestra, Andrea Baruffolo, Olivier Beltramo-Martin, Maria Bergomi, et al.

<figure><img src="image_1.png" /></figure>

To cite this version:

François Rigaut, Richard Mcdermid, Giovanni Cresci, Guido Agapito, Matteo Aliverti, et al. "MAVIS on the VLT: A Powerful, Synergistic ELT Complement in the Visible." The Messenger, 2021, 185, pp.7-11. 10.18727/0722-6691/5245. hal-03512362

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# MAVIS on the VLT: A Powerful, Synergistic ELT Complement in the Visible

François Rigaut¹
Richard McDermid²
Giovanni Cresci³
Guido Agapito³
Matteo Aliverti³
Simone Antoniucci³
Andrea Balestra³
Andrea Baruffolo³
Olivier Beltramo-Martin⁴
Maria Bergomi³
Andrea Bianco³
Marco Bonaglia³
Giuseppe Bono³,⁵
Jean-Claude Bouret⁴
David Brodrick¹
Lorenzo Busoni³
Giulio Capasso³
Elena Carolo³
Simonetta Chinellato³
Mirko Colapietro³
Robert Content²
Jesse Cranney¹
Gayandhi de Silva²
Sergio D'Orsi³
Simon Ellis²
Daniela Fantinel³
Thierry Fusco⁴,⁷
Antony Galla¹
Gaston Gausachs¹
Damien Gratadour¹
Davide Greggio³
Marco Gullieuszik³
Pierre Haguenauer⁶
Dionne Haynes¹
Nick Herrald¹
Anthony Horton²
Devika Kamath²
Laura Magrini³
Antonino Marasco³
Luca Marafatto³
Davide Massari³
Helen McGregor²
Trevor Mendel¹
Stephanie Monty¹
Benoit Neichel⁴
Enrico Pinna³
Cédric Planetet³
Elisa Portaluri³
David Robertson²
Bernardo Salasnich³
Salvatore Savarese³
Pietro Schipani³
Christian Schwab²
Scott Smedley²
Rosanna Sordo³
Stefan Ströbele⁶
Annino Vaccarella¹
Daniele Vassallo³
Valentina Viotto³

Lew Waller²
Alessio Zanutta³
Hao Zhang¹
Ulf Seemann⁶
Harald Kuntschner⁶
Robin Arsenault⁶

¹ Australian National University, Canberra, Australia

² Macquarie University, Sydney, Australia

³ INAF, Italy

⁴ Laboratoire d'Astrophysique de Marseille (LAM), France

⁵ Tor Vergata University of Rome, Italy

⁶ ESO

⁷ Département d'Optique et Techniques Avancées (DOTA), Office National d'Etudes et de Recherches Aérospatial (ONERA), Paris, France

On 1 June 2021 ESO and a consortium of Australian, Italian and French institutions signed an agreement for the design and construction of the MCAO Assisted Visible Imager and Spectrograph (MAVIS). This Very Large Telescope (VLT) instrument will push the frontier of new astronomical instrument technologies to provide, for the first time, wide-field, diffraction-limited angular resolution at visible wavelengths. In combination with the VLT Adaptive Optics Facility, it will use multi-conjugate adaptive optics (MCAO) to feed a 4k × 4k imager covering 30 × 30 arcseconds, as well as an Integral Field Spectrograph (IFS). Angular resolution down to 18 milliarcseconds will be achieved at a wavelength of 550 nm (V band). The IFS will provide four spectral modes, with spectral resolutions from 4000 to over 15 000 between 370 and 950 nm. This will enable a wide variety of science cases, spanning themes that include the emergence of the Hubble sequence, resolving the contents of nearby galaxies, star clusters over cosmic time and the birth, life, and death of stars and their planets. Delivering visible images and integral-field spectroscopy at an angular resolution two to three times better than that of the Hubble Space Telescope will make MAVIS a powerful complement at visible wavelengths to future facilities like the James Webb Space Telescope and the 30–40-metre-class

ground-based telescopes currently under construction, which are all optimised for science at infrared wavelengths.

## About MAVIS

Pushing adaptive optics (AO) technologies to the visible region has been the holy grail of high angular resolution observations since the inception of AO. It is indeed a very challenging proposition: enabling compensation at shorter wavelengths means deformable mirrors with more actuators (proportional to $λ^{-2.4}$), faster systems ($λ^{-1.2}$) and a correction valid over smaller isoplanatic patches ($λ^{-2.4}$). As a result the overall complexity grows as $λ^{-6}$. Having more actuators also means smaller subapertures in the wavefront sensors (WFSs), which requires more powerful lasers. It all becomes more complicated — and more expensive. Some extreme AO systems have been working in the visible, generally focused on high-contrast applications, but these systems generally have very low sky coverage, with access to only a few hundred targets and small fields of view (MagAO at the Magellan Telescopes, the First Light Adaptive Optics [FLAO] system at the Large Binocular Telescope, and the Spectro-Polarimetric High-contrast Exoplanet REsearch [SPHERE] instrument at ESO's Very Large Telescope [VLT]).

AO has matured over the last decades, and recently it has become possible to consider building a wide-field, high-sky-coverage facility instrument to serve a large number of science cases in the visible (Esposito et al., 2016). This was made possible by a number of key developments.

- The maturation of key technologies, including: (a) large, high-actuator-density deformable mirrors and deformable secondary mirrors; (b) high-power commercial lasers to create sodium guide stars like the ones used in the VLT Adaptive Optics Facility (AOF); and (c) high-throughput, low-latency real-time computers, and in particular the variant based on a graphics processing unit, which has
seen major performance and function-
ality improvements in the past decade.

- New control techniques. AO control got stuck with linear integrator controllers for a long time, but the past decade has seen the emergence of many promising techniques, such as Minimum Mean Square Error, Learn and Apply and the various incarnations of predictive controllers such as Linear Quadratic Gaussian control, together with the emergence of tricks like super-resolution.

- The success of on-sky demonstrators (for example, the Multi-conjugate Adaptive-optics Demonstrator [MAD] at ESO), or first-generation facility systems like GeMS, the MCAO system at the Gemini telescope, that could be considered as the precursor of MAVIS, with its five laser guide stars (LGSs) and three deformable mirrors. The consortium is also drawing from the experience of the AOF and more recently, the InfraRed Low Order Sensor (IRLOS) WFS upgrade for the Multi Unit Spectroscopic Explorer (MUSE).

MAVIS will go at the Nasmyth A focus of
the VLT AOF (at Unit Telescope 4), oppo-
site MUSE (see Figure 1). MAVIS is com-
posed of four main modules (Rigaut et al.,
2020): the AO module, that processes
the light and compensates for most of
the atmospheric-induced aberrations; a
4k × 4k imager equipped with a set of
narrow- and broadband filters; an integral
field spectrograph (IFS) with two modes,
one with a field of view of 3.6 × 2.5 arc-
seconds and spaxels of 25 milliarcsec-
onds, the other with a field of view of 7.2
× 5 arcseconds and spaxels of 50 milli-
arcseconds, and multiple spectral resolu-
tions; and the calibration unit, which pro-
vides what is needed for the calibration of
the AO module and the instruments. Both
the imager and the spectrograph cover
the wavelength range 370–950 nm in the
baseline design, and have been informed
by a comprehensive science case built in
consultation with the ESO user commu-
nity (McDermid et al., 2020).

The MAVIS consortium¹ is made up of
ASTRALIS² (Australia, formerly the AAO
Consortium), INAF (Italy), the Laboratoire
d'Astrophysique de Marseille (LAM,
France) and ESO. ASTRALIS (the AAO
and ANU nodes) provides the Project

Office, the imager and spectrograph, the
LGS WFS, the Real-Time Controller, the
detectors, and the calibration unit. INAF
is responsible for the AO module, as well
as the natural guide star (NGS) WFS,
the performance simulations, and the
Instrument Control System. LAM provides
AO expertise and point spread function
reconstruction. Finally, ESO is responsi-
ble for driving the deformable mirror
development and procurement, the LGS
Facility upgrade to split the four existing
LGS beams into eight beacons, the inter-
face with the VLT, and the commission-
ing. ESO will also provide nine smALI
vlsible CamEra (ALICE) cameras and up
to five Next Generation Controller II
detector controllers. The COVID-19 pan-
demic posed a major challenge to the
smooth operation of such a distributed
consortium. In particular, the closure of
Australia's borders (likely to stay in effect
until sometime in 2022) has without a
doubt impaired the efficiency of commu-
nication within the consortium.

Phase A of MAVIS began in February
2019 and the Phase A final review was
passed with flying colours in May 2020.
The ESO follow-up team organised the
start of Phase B and coordinated the
various approval steps (by the Scientific

A period of slightly more than one year is planned for each of the preliminary design and final design. Preliminary Design Review is expected in July 2022 and Final Design Review in August 2023. The Preliminary Acceptance Europe milestone is scheduled for 2027 and could take place in Australia (to be decided). First light is currently scheduled for the second semester of 2027, in line with first light of ESO's Extremely Large Telescope (ELT).

<figure><img src="image_3.png" /><figcaption>Figure 1. A rendering of MAVIS on the Nasmyth platform. The circle represents the rotator-adaptor interface (to which MAVIS is not attached). The various volumes are coloured as follows: yellow — adaptive optics module; orange — laser guide star wavefront sensor; red — natural guide star wavefront sensor; blue (behind) — imager; purple — spectrograph;</figcaption></figure>

green — calibration unit; grey — electronics cabi-
nets. MAVIS will be fitted with a light-tight enclosure,
but will rely on passive thermal management, with
the possible exception of the spectrograph which
might be actively temperature controlled (currently
being studied).

Technical Committee, the Finance Com-
mittee and Council) with the Paranal
Instrumentation Programme. The agree-
ment for Phases B to E was signed on
1 June 2021.

Science drivers

By probing the frontiers of angular resolu-
tion and sensitivity across a large portion
of the observable sky (~ 50% at the
galactic pole) at visible wavelengths,
MAVIS will enable progress on an array of
scientific topics, from studies of the Solar
System to planetary systems around
other stars, and from the physics of star
formation in the Milky Way to the first star
clusters in the Universe.
The point-source imaging sensitivity of
MAVIS will exceed that of the Hubble
Space Telescope (HST), giving an order
of magnitude greater depth, and with
higher angular resolution. In 10 hours of
integration, MAVIS will detect point
sources and compact galaxies down to
30.4 AB in I band, about 1 magnitude
deeper than the Hubble Ultra-Deep Field
in i775, despite its integration time a
factor of 10 longer (96 hrs). This will allow
MAVIS to produce the deepest optical
images ever obtained on a large field of
view. This will open a new window onto
the study of the structural properties of
the first galaxies and of compact galaxies
across cosmic time, as illustrated by Fig-
ure 2.

Spectroscopically, the combination of
high spatial resolution, large wavelength
coverage and high spectral resolution
(R > 5000) will allow MAVIS to finally trace
the evolution of the interstellar medium
(ISM) during the critical galaxy transfor-
mation in the redshift range 0.2 < z < 0.8,
from the clumpy and turbulent discs
observed at high redshift into the thin
discs of local late-type galaxies, resolving
the intrinsic velocity dispersion and the
chemical and physical properties of the
ISM.

The astrometric precision and magnitude
limits of MAVIS will exceed those of

the HST and will extend the ultra-precise
local framework from the Gaia mission
into the densest and most crowded
fields, out to the furthest reaches of the
Milky Way and into the Local Group.
MAVIS will provide proper motion accura-
cies of 5–10 km s⁻¹ out to distances of
~ 100 kpc over five-year timescales,
approaching the radial velocity accuracy
from MAVIS spectroscopy. The combined
spectral and imaging capabilities will
provide full 6D phase-space information
of individual stars, giving a new precision
to studies of intermediate mass black
holes in globular clusters (Monty et al.,
2021), and unique dynamical information
to constrain the dark matter properties
of local group dwarf galaxies.

MAVIS will enable access to the new
regimes of faint sources and complex
crowded fields that will become com-
monplace at longer wavelengths in the
era of ELTs, but with the diagnostic
power of optical wavelengths. This will
be particularly important in the low-red-
shift Universe (z < 0.5), where the best-
calibrated and understood physical diag-
nostics are found at wavelengths below
1 µm. In this way, MAVIS will be a crucial
complement to the capabilities of ESO's
ELT, delivering angular resolution in the
optical comparable to that delivered by
ELTs in the infrared. A key example of
this synergy is in studying resolved stellar
populations beyond the Local Group.
Whilst ELTs will be able to detect individ-
ual stars in galaxies beyond several
Mpc with modest integration times, infra-
red wavelengths probe mainly the Ray-
leigh-Jeans tail of the stellar black body
spectrum, making infrared colours largely
degenerate to key stellar population
parameters such as age and metallicity.
By contrast, the shallower depths acces-
sible on an 8-metre telescope are fully
compensated by the increased diagnos-

tic power of optical colours, making
MAVIS a crucial tool for capitalising on
ELT science.

The unprecedented combination of sen-
sitivity, angular resolution, blue coverage
and moderately-high-resolution spectros-
copy (R > 12 000) afforded by MAVIS and
the AOF will allow, for the first time, the
characterisation of the cores of globular
clusters. It will be possible to explore
abundances anomalies, multiple stellar
populations, and core binary fractions:
the missing link to understanding the
formation scenario and subsequent dyna-
mical and chemical evolution of these
systems.

Finally, MAVIS will be transformational for
our understanding of the life and death
of stars and their planets, by revealing
uncharted details of their close environ-
ment, such as protoplanetary discs, jets
and companions. This includes our own
Solar System, where MAVIS will provide
spectroscopy and high-contrast imag-
ing of planetary atmospheres, cometary
activity and Kuiper Belt Objects.

Given this broad range of foreseen scien-
tific applications, combined with a robust
and flexible operational model, MAVIS
will represent the general purpose facility
instrument that will fill the gap for high-
spatial-resolution optical capabilities in
the post-HST era. Moreover, MAVIS oper-
ations will closely overlap with the new
generation of giant telescopes optimised
for near-IR observations, providing their
fundamental complement in the optical
regime.

Instrument design concept

Informed by a number of trade-off studies
of the most crucial elements, the MAVIS

**Figure 2.** Example of a morphological study of a high-z galaxy as observed in the I band with the HST, the Multi-AO Imaging Camera for Deep Observations (MICADO) at ESO's ELT, NIRCam on the James Webb Space Telescope and MAVIS. Based on the z = 5 high-resolution simulated "Althaea" galaxy (Pallottini et al., 2017), shown in the first panel, the other panels show how the same target of 25 AB magnitude would appear in the I band if observed with different facilities for a fixed exposure time of 1 hour. In the VLT/MAVIS image, clumpy regions as faint as 29 AB magnitude are detected with a signal-to-noise ratio of five.

<figure><img src="image_4.png" /></figure>
design went through several iterations
during Phase A, to finally converge on
the current baseline. The considerable
expertise within the consortium in AO
and post-focal instrumentation helped in
converging on a healthy design, in partic-
ular the optical design, which maximises
throughput and minimises optical aberra-
tions and field distortion, whilst allowing
easy alignment and modular integration
(Viotto et al., 2020; Ellis et al., 2020).

The current MAVIS design can be sum-
marised as follows:

- A largely transmissive design across
the board (AO module, imager, spec-
trograph), using only on-axis optics.
The intention is to minimise field distor-
tions, as well as to make the optical
alignment more tolerant.

- A gravity-invariant design, in which the science and NGS WFSs are commonly de-rotated optically and the LGS WFSs use mechanical de-rotation to track the LGS constellation.

- Three deformable mirrors: the AOF sec-
ondary (1170 actuators) and two post-
focal deformable mirrors, optically con-
jugated to 6 and 13.5 kilometres, for a
grand total of 5420 actuators.

- Eight laser guide stars created from
splitting the four existing AOF lasers,
feeding eight 40 × 40 Shack-Hartmann
WFSs. The current concept makes use
of diffraction gratings to split each laser
and is minimally invasive to the LGS
Facility, maintaining full compatibility
with existing AOF operation modes.

- Three near-infrared NGS WFSs, provid-
ing tip-tilt and focus information (each
has a selectable 1 × 1 or 2 × 2 lenslet
array). These WFSs use SAPHIRA
detectors over the J and H bands and
NGCII controllers for maximum sensi-
tivity, leading to a limiting magnitude of
more than V = 18.5.

- An imager covering the 30 × 30 arc-
second field of view, better-than-Nyquist
sampled in the V band (7.36 milliarc-
seconds pixel⁻¹, for images of 18 milli-
arcseconds full width at half maximum
[FWHM] in good conditions). Note that
for relatively obvious geometric rea-
sons, the image quality performance is
specified over the disc of radius 15 arc-
seconds, but we are looking for ways
to also optimise the field corners.

- A very compact, high-throughput mon-
olithic IFS, covering 370 to 950 nm

<figure><img src="image_5.png" /></figure>

Figure 3. (Above) The four MAVIS IFS spectral reso-
lution modes and corresponding dynamical scales,
chosen to address key science areas. These include:
the study of chemical abundances and radial veloci-
ties in crowded fields (HR-BLUE); exploring the evo-
lution of ISM turbulence in galaxy discs and hunting
intermediate-mass black holes (HR-RED); probing
the extremes of mass and metallicity of young stars
and untangling the complexity of ionised gas in gal-
axies (LR-BLUE); and resolving Lyman-alpha emit-
ters at z > 6.5, and studying stellar dynamics in z < 1
 galaxies (LR-RED). MUSE is shown in grey for com-
 parison.

Figure 4. (Below) The MAVIS focal plane, with eight laser guide stars (orange) and three natural guide stars (red). The imager covers/defines the “science field” (green). The integral field spectrograph (IFS) field is in purple (coarse mode) and blue (fine mode). The exact shape and location of the IFS field of view are still being optimised. Having the IFS off-centred with respect to the technical field increases the capture range for natural guide stars, and thus the sky coverage.

<figure><img src="image_6.png" /></figure>

(possibly to 1000 nm), with four spec-
tral modes, providing spectral resolu-
tions from 4000 through over 15 000
(see Figure 3) and two spaxel scales
(approximately 25 and 50 milliarcsec-
onds spaxel⁻¹), corresponding to fields
of view of 3.6 × 2.5 arcseconds and

7.2 × 5 arcseconds. The current design
uses a field of view offset from the
optical axis by about 8 arcseconds
(see Figure 4), which slightly improves
the probability of finding NGSs. We
are looking into the option of using the
imager and the spectrograph simulta-
neously, which will provide interesting opportunities to extract information from the IFS when in crowded fields.

## Performance

The system performance requirements were specified for typical Paranal turbulence conditions, i.e., a seeing of 0.87 arcseconds at a zenith angle of 30 degrees. There has been a huge push during phase A and the start of phase B on numerical simulations (Agapito et al., 2020). These were instrumental in driving the instrument AO design: how many actuators were needed? how many deformable mirrors? how many laser guide stars and wavefront sensors? natural guide stars? All of these design parameters were optimised on the basis of the initial call for proposal requirements. Once the baseline design was settled, numerical simulations were then used to fine tune performance estimates. The consortium is still in this phase, and we have embarked on statistical performance estimates and sensitivity analyses, based on the recently released database of turbulence profiles for Paranal.

Because of the very tight wavefront error allowance — of the order of 110 nm — resulting from the fact that MAVIS targets correction at visible wavelengths, the MAVIS simulation and control group has had to make use of both proven (for example, Learn and Apply or Minimum Mean Square Error) and more novel control techniques. The latter include predictive control (Cranney et al., 2020) and “super-resolution”, a technique to increase wavefront sensing diversity (or reduce redundancy) by shifting/rotating the WFSs with respect to each other.

Figure 5 presents the long-exposure Strehl ratio (well known to AO connoisseurs, the Strehl is a measure of how close the image is to the diffraction limit). These results include only atmospheric and AO error terms. The average Strehl over the disc of radius 15 arcseconds is 28.9%, which provides a very healthy margin over the specified 15% to accommodate other error sources such as optics, vibrations, etc. The absolute Strehl standard deviation over the same circular field is 1.3% (4.5% relative), indi-

<figure><img src="image_7.png" /><figcaption>Figure 5. Strehl radial profile as a function of the off-axis distance, for various controllers (Minimum Mean Square Error or Learn and Apply, with or without predictive control and super-resolution). The Strehl performance specifications are defined over the</figcaption></figure>

cating a very uniform image quality — a trademark of multi-conjugate AO. At this level of Strehl, the images appear essentially diffraction-limited, i.e., close to $\lambda/D$ in FWHM. When including vibrations, and various detector effects, we are expecting image sizes of 18 milliarcseconds.

The choice of near-infrared wavelengths for the NGS WFS provides a welcome boost to the sky coverage of MAVIS, where sources in the partially-corrected NGS patrol field are diffraction-limited at 1.6 µm, with moderate to high Strehl. Combined with the use of near-infrared avalanche photodiode arrays, this allows it to reach high sky coverage values; over 50% of the pointings at the galactic pole will provide images with better-than-HST angular resolution (< 50 milliarcseconds).

Finally, a preliminary exposure time calculator has been developed and is available on github³. MAVISIM⁴, a tool to generate synthetic images, has also been developed. It employs a user-provided object model (currently only point sources) and a database of field-varying point spread functions from end-to-end numerical AO simulations. A future version will offer compatibility with extended objects. Both the exposure time calculator and

disc of radius 15 arcseconds (not over the 30 x 30 arcsecond-square field). This set of simulations uses three bright natural guide stars on a triangle, at 20 arcsecond radius from the centre of the field.

MAVISIM are under active development, with updated versions expected throughout phase B.

## Acknowledgements

The MAVIS consortium labour is funded by NCRIS (Australia, funding managed by Astronomy Australia Limited), INAF (Italy), CNRS (France) and ESO (for the ESO MAVIS team members). We want to thank ESO for a very fruitful and positive collaboration to date, and the many astronomers, both within and beyond the MAVIS consortium, who have contributed to the development of the MAVIS science case.

## References

Agapito, G. et al. 2020, Proc. SPIE, 11448, 114483R
Cranney, J. et al. 2020, Proc. SPIE, 11448, 114482L
Ellis, S. et al. 2020, Proc. SPIE, 11447, 11447AO
Esposito, S. et al. 2016, Proc. SPIE, 9909, 99093U
McDermid, R. M. et al. 2020, arXiv: 2009.09242
Monty, S. et al. 2021, MNRAS, 507, 2192
Pallottini, A. et al. 2017, MNRAS, 471, 4128
Rigaut, F. et al. 2020, Proc. SPIE, 11447, 114471R
Viotto, V. et al. 2020, Proc. SPIE, 11448, 114480D

## Links

1. The MAVIS consortium: mavis-ao.org/consortium

2. AUSTRALIS: https://astralis.org.au/

3. MAVIS exposure time calculator: https://github.com/jtmendel/mavisetc

4. MAVISIM image generation tool: https://github.com/smonty93/MAVISIM