Prime Focus Spectrograph for the
Subaru telescope: massively
multiplexed optical and near-infraredfiber spectrographHajime Sugai
Naoyuki Tamura
Hiroshi Karoji
Atsushi Shimono
Naruhisa Takato
Masahiko Kimura
Youichi Ohyama
Akitoshi Ueda
Hrand Aghazarian
Marcio Vital de Arruda
Robert H. Barkhouser
Charles L. Bennett
Steve Bickerton
Alexandre BozierDavid F. Braun
Khanh Bui
Christopher M. Capocasale
Michael A. Carr
Bruno Castilho
Yin-Chang Chang
Hsin-Yo Chen
Richard C. Y. Chou
Olivia R. Dawson
Richard G. Dekany
Eric M. Ek
Richard S. Ellis
Robin J. English
Didier FerrandDécio FerreiraCharles D. Fisher
Mirek Golebiowski
James E. Gunn
Murdock Hart
Timothy M. Heckman
Paul T. P. Ho
Stephen Hope
Larry E. Hovland
Shu-Fu Hsu
Yen-Shan Hu
Pin Jie Huang
Marc Jaquet
Jennifer E. Karr
Jason G. Kempenaar
Matthew E. King
Olivier Le Fèvre
David Le Mignant
Hung-Hsu Ling
Craig Loomis
Robert H. Lupton
Fabrice MadecPeter Mao
Lucas Souza Marrara
Brice MénardChaz Morantz
Hitoshi Murayama
Graham J. Murray

Antonio Cesar de Oliveira
Claudia Mendes de Oliveira
Ligia Souza de Oliveira
Joe D. Orndorff
Rodrigo de Paiva Vilaça
Eamon J. Partos
Sandrine Pascal
Thomas Pegot-Ogier
Daniel J. Reiley
Reed Riddle
Leandro Santos
Jesulino Bispo dos Santos
Mark A. Schwochert
Michael D. Seiffert
Stephen A. Smee
Roger M. Smith
Ronald E. Steinkraus
Laerte Sodré, Jr.
David N. Spergel
Christian Surace
Laurence Tresse
Clément Vidal
Sebastien Vives
Shiang-Yu Wang
Chih-Yi Wen
Amy C. Wu
Rosie Wyse
Chi-Hung Yan

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Journal of Astronomical Telescopes, Instruments, and Systems 1(3), 035001 (Jul–Sep 2015)

Prime Focus Spectrograph for the Subaru telescope:massively multiplexed optical and near-infraredfiber spectrographHajime Sugai,a,* Naoyuki Tamura,a Hiroshi Karoji,a Atsushi Shimono,a Naruhisa Takato,b Masahiko Kimura,cYouichi Ohyama,c Akitoshi Ueda,d Hrand Aghazarian,e Marcio Vital de Arruda,f Robert H. Barkhouser,gCharles L. Bennett,g Steve Bickerton,a Alexandre Bozier,h David F. Braun,e Khanh Bui,I
Christopher M. Capocasale,e Michael A. Carr,j Bruno Castilho,f Yin-Chang Chang,c Hsin-Yo Chen,cRichard C. Y. Chou,c Olivia R. Dawson,e Richard G. Dekany,k Eric M. Ek,e Richard S. Ellis,i Robin J. English,eDidier Ferrand,h Décio Ferreira,f Charles D. Fisher,e Mirek Golebiowski,g James E. Gunn,j Murdock Hart,gTimothy M. Heckman,g Paul T. P. Ho,c Stephen Hope,g Larry E. Hovland,e Shu-Fu Hsu,c Yen-Shan Hu,cPin Jie Huang,c Marc Jaquet,h Jennifer E. Karr,c Jason G. Kempenaar,e Matthew E. King,e Olivier Le Fèvre,hDavid Le Mignant,h Hung-Hsu Ling,c Craig Loomis,j Robert H. Lupton,j Fabrice Madec,h Peter Mao,iLucas Souza Marrara,f Brice Ménard,g Chaz Morantz,e Hitoshi Murayama,a Graham J. Murray,lAntonio Cesar de Oliveira,f Claudia Mendes de Oliveira,m Ligia Souza de Oliveira,f Joe D. Orndorff,gRodrigo de Paiva Vilaça,f Eamon J. Partos,e Sandrine Pascal,h Thomas Pegot-Ogier,h Daniel J. Reiley,iReed Riddle,i Leandro Santos,f Jesulino Bispo dos Santos,f Mark A. Schwochert,e Michael D. Seiffert,e,iStephen A. Smee,g Roger M. Smith,k Ronald E. Steinkraus,e Laerte Sodré Jr.,m David N. Spergel,jChristian Surace,h Laurence Tresse,h Clément Vidal,h Sebastien Vives,h Shiang-Yu Wang,c Chih-Yi Wen,cAmy C. Wu,e Rosie Wyse,g and Chi-Hung Yanca

The University of Tokyo, Kavli Institute for the Physics and Mathematics of the Universe (WPI), 5-1-5, Kashiwanoha, Kashiwa 277-8583, Japan
National Astronomical Observatory of Japan, Subaru Telescope, 650 North A`ohoku Place, Hilo, Hawaii 96720, United States
Academia Sinica, Institute of Astronomy and Astrophysics, P.O. Box 23-141, Taipei, TaiwandNational Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japane
Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109, United StatesfLaboratório Nacional de Astrofisica, MCTI, Rua Estados Unidos, 154, Bairro das Nações, Itajubá, Minas Gerais, BrazilgJohns Hopkins University, Department of Physics and Astronomy, 3400 North Charles Street, Baltimore, Maryland 21218, United Statesh
Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, Marseille 13388, Francei
California Institute of Technology, Astronomy Department, 1200 East California Boulevard, Pasadena, California 91125, United StatesjPrinceton University, Department of Astrophysical Sciences, Princeton, New Jersey 08544, United Statesk
Caltech Optical Observatories, 1201 East California Boulevard, Pasadena, California 91125, United Statesl
Durham University, Centre For Advanced Instrumentation, Physics Department, Rochester Building, South Road, Durham DH1 3LE,
United KingdommUniversidade de São Paulo, Instituto de Astronomia, Geofisica e Ciencias Atmosfericas, Rua do Matão, 1226–Cidade Universitária 05508-090,
Brazilbc

Abstract. The Prime Focus Spectrograph (PFS) is an optical/near-infrared multifiber spectrograph with 2394
science fibers distributed across a 1.3-deg diameter field of view at the Subaru 8.2-m telescope. The wide wavelength coverage from 0.38 μm to 1.26 μm, with a resolving power of 3000, simultaneously strengthens its ability
to target three main survey programs: cosmology, galactic archaeology and galaxy/AGN evolution. A medium
resolution mode with a resolving power of 5000 for 0.71 μm to 0.89 μm will also be available by simply exchanging dispersers. We highlight some of the technological aspects of the design. To transform the telescope focal
ratio, a broad-band coated microlens is glued to each fiber tip. A higher transmission fiber is selected for the
longest part of the cable system, optimizing overall throughput; a fiber with low focal ratio degradation is selected
for the fiber-positioner and fiber-slit components, minimizing the effects of fiber movements and fiber bending.
Fiber positioning will be performed by a positioner consisting of two stages of piezo-electric rotary motors. The
positions of these motors are measured by taking an image of artificially back-illuminated fibers with the metrology camera located in the Cassegrain container; the fibers are placed in the proper location by iteratively
measuring and then adjusting the positions of the motors. Target light reaches one of the four identical fastSchmidt spectrograph modules, each with three arms. The PFS project has passed several project-wide design
reviews and is now in the construction phase. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported
License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. [DOI: 10.1117/1.JATIS.1.3.035001]

Keywords: spectrographs; fiber applications; optical systems; infrared systems; astronomy.
Paper 14035P received Dec. 7, 2014; accepted for publication May 22, 2015; published online Jun. 23, 2015.

*Address all correspondence to: Hajime Sugai, E-mail: hajime.sugai@ipmu.jp

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1Introduction
1

The Prime Focus Spectrograph (PFS) is an optical/near-infraredmultifiber spectrograph with 2394 science fibers (Fig. 1), and isscheduled to be mounted on the Subaru 8.2-m telescope. The
position of each fiber is controlled by a fiber positioner, called“Cobra,” consisting of two-staged piezo-electric rotary squiggle
motors. Each fiber positioner unit samples a circular patrol
region of 9.5-mm diameter at the telescope focal plane, andthese positioners are arrayed into a hexagonal pattern with 8mm central distances between adjacent positioners. The wholearray extends to a single hexagonal field of view (FOV) whose
effective diameter is 1.3 deg. The hexagonal FOV has an advantage for efficient tiling of survey areas, compared with e.g., acircular FOV. The fibers are divided into four groups, each ofwhich enters one of the four spectrograph modules in a slit comprising 600 or 597 science fibers in a single row. Each spectrograph module has three color arms to cover a wide wavelength
region from 0.38 μm to 1.26 μm, with a mean resolving powerλ∕δλ of 3000. This resolution is optimal for spectroscopic surveys of fainter galaxies and stars, targeting cosmology, Galacticarchaeology and galaxy/AGN evolution. To simplify the designof the spectrograph, the F∕2.2 beam from the telescope wide
field corrector (WFC) is increased to F∕2.8 by a microlens
bonded to each fiber. A double Schmidt design is used forthe spectrograph modules, which are F∕2.5 at the fiber sideand F∕1.09 at the detector side. These apertures are slightly
larger than the nominal F∕#, allowing for some focal ratio degradation (FRD) in the fiber. A medium spectral resolution modewith resolving power of 5000 for the red arms (0.71 μm to0.89 μm) is also included. This addition particularly benefits
important Galactic archaeology studies. This new mode isrealized by using a simple grating/grism exchange mechanism
without changing any other design elements. The basic characteristics of PFS are summarized in Table 1. The total efficiency
from the atmosphere to the detector is expected to be around∼10%–20%,1 depending on the wavelength and observing conditions such as telescope elevation and fiber position in FOV.
Together, PFS and hyper suprime-cam (HSC)2 comprise theSubaru measurement of the images and redshifts project. PFS
will accomplish the spectroscopic aspect of this project, and
HSC has been implementing the imaging part since first lightin 2012. The scientific case for the PFS surveys is describedin an earlier paper.3 PFS has now entered the construction phase,and Fig. 2 shows the integration and test flow. Generally speaking, PFS comprises two main components: the prime focus
instrument (PFI) and the spectrographs, with the fiber system
connecting the two parts. This paper provides the first technicaloverview of the instrument, describing the essential components
as well as their intensive test results. Further details on individual PFS components can be found in separate, componentspecific SPIE conference papers viz.: fiber system,4–9 fiber positioner10 and prime focus instrument,11 spectrograph,12–14 Dewar
and detector,15–18 and metrology camera.19 The subsystems are
discussed in roughly the same order as the science light paththrough the instrument. The Subaru telescope and WFC focus
light through a microlens; this microlens, which is described inSec. 2.1, increases the F∕# of the beam to ease the design
requirements on the spectrographs. The light then enters themultimode fibers, which are part of a fiber system describedin Sec. 2.2 that also includes connectors and fiber arrays. In
the telescope focal plane, these fibers are mounted in a seriesof positioners, described in Sec. 2.3, which connect the fibers

Fig. 1 Schematic overview of the Prime Focus Spectrograph (PFS) instrument (not to scale). Subaru’s
primary mirror focuses light through the wide field corrector (WFC) and a flat glass field element, thenonto fibers mounted in an array of fiber positioners. These positioners enable the fiber to be placed on theimage of an astronomical object. The fiber positioners as well as other components are mounted into
a telescope interface called POpt2. These elements near the telescope focal plane collectively comprisethe prime focus instrument (PFI). The fibers are gathered into a fiber cable to route the light to four spectrographs. Each spectrograph also contains backillumination to enable the fiber positions to be measured
by the metrology camera.

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Table 1 Basic characteristics for Prime Focus Spectrograph (PFS).

Field element
Shape

Plane; thickness 54 mm

Fiber
Number2394 science fibers

Diameter (Core/Cladding/Buffer)

Cable C: 127 μm∕165 μm∕189 μm; Core size corresponds to 1”.12 at field center and 1”.02 at
corner when a microlens attached
Cable B: 128 μm∕169 μm∕189 μm
Cable A: 129 μm∕168 μm∕189 μm

LengthCable C: ∼5 m; Cable B: ∼50 m; Cable A: ∼2 m

Bundling

Cable C: 21 segmented tubes protected by 1 expandable tube
Cable B: 21 segmented tubes bundled and fixed by an adhesive tape around 1 supporting tube
Cable A: 600 science fibers arranged linearly in each of 4 slits

Connectors

Two positions: on telescope spider & on spectrograph benches

MicrolensShape

Plano-concave (spherical); thickness 3 mm; glued to fiber input edge

F-ratio transformation

F/2.2 to F/2.8

Fiber positioner
Positioning mechanism

Two stages of piezo-electric rotary squiggle motors

Positioner distribution

Hexagonal pattern

Distance from neighboring positioners

8.0 mmPatrol region

9.5 mm diameter circle for each fiber

Field shape & size
Field shape

HexagonField size/area

Diagonal line length & areai) Hexagon defined by fiber positioner centers (i.e., twice of distance from field center tofarthest fiber positioner centers): 448.00 mm ¼ 1.366 deg on sky. 1.21 deg2 area.
ii) Hexagonal patrol region within which any astronomical target can be accessed at least
with one fiber: 453.92 mm ¼ 1.383 deg on sky. 1.24 deg2 area.
When obscured areas of calibration dots mentioned in subsection 2.3 are considered,
the above areas should be reduced by 4%.

Spectrograph
Number4 spectrograph modules, each with a slit of 600 or 597 science fibers & 3-color arms:
located on fourth floor infrared side

Slit length∼140 mm, with center-to-center fiber spacing of 213.93 μm

F ratios

All-Schmidt type: collimator F/2.5 & camera F/1.09

Grating

VPH; diameter 280 mm

Wavelength region380-1260 nm (blue: 380-650 nm; red: 630-970 nm; NIR 940-1260 nm)

Spectral resolution∼2.7 A (∼1.6 A for red-arm medium resolution mode in 710-885 nm)

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Table 1 (Continued).
Dewar & Detector
Dewar window

Camera Schmidt corrector

Pixel size

15 μm

Detector

A pair of 2 K × 4 K fully depleted CCDs for each of blue & red arms; 4 K × 4 K HgCdT e(1.7 μm cutoff) for NIR arm

Metrology camera
At Cassegrain

Magnification

0.0366

Camera aperturesize

380 mmDetector

50 M 3.2 μm-pixel CMOS sensor

Design,build

Location

System softwareinterface simulators
System software
coding & test
against simulators(incl gen2 & TSC)

Data
reduction

Gen2 simulator
available
Fibercable C

cameraTelescope
integration & verification

Pre-delivery integration
verificationFibercable B
Module 1 integ. &data characteri.
Integrate
softwaresystems

Module 2 integ.
Module 3 integ.

Cable C assembly topositioner, including
fiducial fibers

Module 4 integ.
Install inspectrograph room,
function test

Install system & DR software& test with real gen2 & TSC

Install cable
B to telInstall metrology
cam to tel

Install POpt to top end, align metrology cam
On-sky target acquisition verification
Final system verification - throughput, observing efficiency etc.

Fig. 2 PFS integration and test flow. This flow highlights the modular nature of PFS; the Spectrographand PFI components can be built and tested independently. The fiber system connects thesecomponents.

to the celestial images. The fibers transmit the science light toone of the four spectrograph modules, described in Sec. 2.4. The
spectrograph modules also provide a means for back-illumination, which enables the metrology camera system, described inSec. 2.5, to measure the positions of the fibers. The relevant software and observational execution processes are described inSec. 2.6. PFS technical first light is scheduled on Subaru in
early 2018 with the start of the associated Subaru strategic program survey expected in 2019.
The PFS collaboration is organized from Kavli IPMU (WPI)
with PI Hitoshi Murayama and a PFS project office. Other
members of the consortium include astronomers and engineersfrom Caltech/JPL, Princeton and JHU in the USA, LAM in
France, USP/LNA in Brazil, and ASIAA in Taiwan. In 2014
January, NAOJ/Subaru formally joined the project, and MPA,
Germany joined later in 2014.
Journal of Astronomical Telescopes, Instruments, and Systems

2

Design and Production

Although the detailed design of the instrument has been accomplished through a number of iterations, the most important initial
optical parameters were based directly from the scientific caseand basic technical constraints. The fiber core diameter of about
100 μm was determined by the image scale at the prime focus
provided through WFC and the typical target galaxy size. The
numerical aperture, ∼0.22 of the fiber, was constrained by the Fratio of the beam provided by WFC. The spectrograph F-ratio of
F∕1.1 was constrained by the detector pixel size of 15 μm forboth the optical and near-infrared (NIR) arms. Since a large demagnification factor is required to avoid oversampling the fibercore images, the ∼2400 fibers are separated into four distinct
spectrograph modules in order to ensure the required imaging
capability of each camera. These first-order constraints were

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complicated by both F-ratio degradation of a fiber and WFC
effects such as nontelecentricity and vignetting; together, theseeffects require the addition of a microlens on each fiber in thetelescope focal plane to transform the input F-ratio from 2.2
to 2.8.

2.1

Microlens

A glass-molded microlens, produced by Panasonic Industrial
Devices Nitto, transforms the F-ratio F∕2.2 of the Subaru telescope plus WFC at prime focus into a slower F-ratio of F∕2.8.
This transformation eases the spectrograph design and also
ensures an efficient light acceptance cone for each fiber. Glassmolding is an attractive fabrication technique because it provides a wider selection of refractive indices than plastic molding
and better uniformity of mechanical dimensions than polishing.
We selected a high refractive index glass K-VC82 (nd ¼1.75550) for producing the 3-mm thickness microlenses with
a 4.764-mm curvature-radius concave at the entrance surface(Fig. 3). A thickness of 3 mm was chosen as a tradeoff between
manufacturing constraints and performance for extended targets.
The measured thickness uniformity of mass-produced microlenses was within the specification of 10 μm. The measured
curvature radius and outer diameter of mass-produced microlenses were 4.77  0.03 mm (1 sigma) and 1.486 mm  1.2 μm(1 sigma), respectively. The concave surface has been broadband coated (reflectance <1% between 380 nm and 1.26 μm)
and overcoated with MgF2 . The opposite flat side will be
glued to a fiber. The excellent mechanical uniformity of the
microlens enables passive alignment between the fiber and themicrolens. We use a black zirconia fiber arm for integrating afiber, a microlens and a fiber positioner motor shaft.

2.2

Fiber System

The fiber system consists of three components termed cables C,
B, and A. Cable C is connected to the fiber positioners, capturing the science light in the telescope focal plane. Cable B allows
this light to propagate through the telescope structure and building to a special room containing the spectrographs. Cable A isconnected to the spectrographs. These cables are linked by two
fiber connectors: one on the telescope spider and the other oneach spectrograph module bench. This arrangement allows mostof the total fiber lengths, cable B, to be permanently installed on
the telescope. It permits the focal plane assembly to be removed
so that other instruments can be mounted. Finally, it allows easier and safer installation of spectrograph modules and the systemseparability, as highlighted in Fig. 2. Furthermore, this arrangement simplifies future upgrades and flexibility; for example, ahigh resolution spectrograph could be realized relatively easily
if desired, making use of the existing Cobra fiber positioners andall of the fixed fiber systems.
Each cable is terminated with an assembly based on custom,
32-fiber, MTP connectors produced by US Conec, Ltd. On thetelescope spider, cable B is connected to cable C using 22
USCONEC connectors in an assembly called the “tower
connector”; four tower connectors are needed to connect all
2394 fibers. The tower connector is engineered to be particularly
thin to meet challenging restrictions on size and shape at theprime focus. At the spectrograph end, cable B is connected tocable A using 12 USCONEC connectors in an assembly of eightso-called “gang connectors.”
Fiber selection has been one of the key issues in the designof the fiber system. The fibers must have both excellent transmission across a wide spectral band (380–1260 nm) and low
FRD properties. The project has intensively tested and evaluatedfibers produced by two companies: a Japanese company
Fujikura and a US company Polymicro. Two methods wereused for the FRD measurements (Fig. 4). In the first method, as
shown in Fig. 4(a), monochromatic light (filtered for 550 nm
or He-Ne laser 633 nm) was input at the nominal PFS aperture,F∕2.8. In the second method, shown in Fig. 4(b), a collimatedbeam was input and scanned with the incident angle. Measurements showed that both fibers met system requirements for FRD
and transmission. The Polymicro fibers were found to havea slightly better FRD performance. A Polymicro 50-m fiber
had about 1% loss from FRD at F∕2.5, while a similar Fujikura
50-m fiber had about 3% loss. Fujikura fibers provide slightly
better transmission: the gain depends on wavelength but is
2%–10% higher for a 50-m length.
Based on the above results, we selected Polymicro fibers forcables C and A, and Fujikura fibers for cable B. Cable B is by far
the longest run, so this ensures a high overall transmission.
Cables A and C have tighter geometrical constraints and are,therefore, more susceptible to FRD losses making the Polymicro
fiber the natural choice.
In order to minimize geometrical losses at the connectors, theouter polyimide-buffer diameter of the fibers was specified as
189  3 μm for Fujikura and 190 þ 3∕ − 5 μm for Polymicro.
This ensures better center alignments between fibers at each
USCONEC connector, whose hole diameter specification is
195  1 μm. To further reduce light loss, we specified
core (and clad) diameters as 127 μm  3 μm (165  3 μm),

Fig. 3 Glass-molded microlens will be attached to the entrance aperture of each fiber for F -ratio
transformation.

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128 μm  5 μm (169  2 μm), and 129  3 μm (168  3 μm),
respectively, for cables C, B, and A. This flow from a smaller
core diameter to a larger core diameter (127 < 128 < 129) will
minimize geometrical losses in a statistical way, even with slightmisalignments of core centers between upstream and downstream fibers, as well as with the possibility of a few μm offset
between the core center and buffer center. This intentional slightchange of core diameters at the connectors will slightly affect
the F-ratio, but the effect of geometrical area matching at connectors will be more significant than that of the slight F-ratiochange. Both of Fujikura and Polymicro fiber productions havealready been completed.
To optimize uniformity of fiber image qualities, the mappingof fibers between the fiber positioner and the locations in thefiber slits has been carefully optimized. The focal plane has
its best image quality near the center of the field and has asignificant loss of telecentricity near the edge. Similarly, thespectrograph has better image quality near the center of the
FOV. To balance these two effects, fibers from the edge ofthe telescopic FOV are routed to the inner location of a spectrograph slit, and fibers closer to the center of the telescopic FOV
are routed to the outer location of the spectrograph slit. Such afiber-routing pattern has been successfully designed, maintaining the modularities of fiber positioners and of spectrographs.

(a) First method: Inputting light with actual F-ratio(b) Second method: Inputting collimated light with input angle scanned

He-Ne laser

Fig. 4 Two methods used for focal ratio degradation (FRD) measurements. (a) The F -ratio of 2.8 is input at the fiber entrance indicated bya red arrow and the total effect of FRD is measured. The combinationof diaphragm (DIA) and a camera lens (OBJ) produces the required
F ratio. A camera indicated as CCD2 measures the angular distribution of the output light from a fiber, while another camera is used
for monitoring and alignment purposes. (b) Collimated light is input(shown by a red arrow) and its incident angle is scanned by using
a rotating stage, so that the effects on the specific incident angles
are measured. The angular distribution of the output light from thefiber is relayed onto a charge-coupled device (CCD) camera througha re-imaging lens. Both methods are valid techniques and were used
to improve confidence in the final selection of fiber manufacturer.
The upper figure was taken from de Oliveira et al.’s20 paper. The
measurements made using the latter method were carried out by
Murray, Dunlop, and Allington Smith at Durham University CfAI,
and the lower figure they kindly provided was originally produced
by Ulrike Lemke, who is currently at the Institut für Astrophysik,
Göttingen.

(a)Convergence to target

4

2.3

We use fiber positioner units developed by JPL in partnershipwith New Scale Technologies. They consist of two-staged
piezo-electric rotary squiggle motors arranged so as to place
an individual fiber onto a target object within a particular patrol
area on the focal plane. Intensive life time tests on such designmodified positioners are being carried out in 300 k cycles
of clockwise plus counterclockwise rotations including hardstop events and some realistic movements at a temperature

(b)

10

Fiber Positioner and Prime Focus Instrument

5 µ m convergence

5 µ m cumulative convergence
100
90

3

Perc entage conv ergenc e (% )Dis t to t arget (µm)

10

2

10

1

10

0

10

-1

10

80

% Converged
95%

70
60
50
40
30
20
10

-2

10

0

5

10

15

0

Iteration

1 2 3 4 5 6 7 8 9101112131415
Iterations

Fig. 5 Convergence to target for the best (among five at the time) engineering model (EM) fiber positioner. The results shown are for 100 randomly selected target points within the 9.5-mm diameter patrol
area. (a) shows how the actual fiber position is converged with iteration onto the target position for each ofthese 100 target positions (cases), (b) shows the fraction of cases where the fiber positions are converged within 5 μm with respect to the target positions.
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of −5°C, i.e., the most severe environment expected on
Mauna Kea.
Since open-loop positioning accuracy is insufficient, we
carry out iterations based on the fiber position measured by ametrology camera, which is described in Sec. 2.5. Figure 5
shows an example of how the fiber position converges onto
the target position for 100 randomly selected target points within
the patrol area of 9.5-mm diameter. Intensive tests using 29 engineering model fiber positioners have been completed and we are
now in the phase of mass production (Fig. 6).
The fiber positioners are built into a module containing 57
positioners each with a microlensed fiber and the Cobra drive
electronics. The total positioner system consists of 42 of thesemodules mounted onto the fiber-positioner optical bench, which
is attached to the PFI structure. As shown in Fig. 1, the PFI is
installed in Subaru’s housing structure, called prime focus unit
POpt2, whose rotator provides PFI with the rotating part and thenonrotating part with respect to the telescope structure. Whilethe nonrotating part is fixed to the telescope, rotation enables
tracking target objects in the sky. A structure called the positioner frame connects the POpt2 rotator and the fiber-positioner
optical bench on which fiber positioners as well as six acquisition and guide (A&G) cameras are mounted. For each of A&G
cameras, we use FLI ML 4720, a 1 K × 1 K frame transfer camera with 13 μm pixels. Its coverage of 5.5 arcmin2 on the sky is
wide enough for providing at least one star for the A&G in areasonable exposure time even in the lowest star-density area.
Ninety-six fiducial fibers are mounted on this optical benchand provide the metrology camera with reference positions
when they are artificially back-illuminated. For this purpose,the fiducial fiber illuminator is also located in the PFI rotating
part. The cable wrapper ensures cables of fibers/electronics/
coolant behave in a well-organized way when the rotator moves.
A glass plate, called the field element, is attached to the fiberpositioner optical bench by three field element mounts. The
thickness of the field element, 54 mm, roughly corresponds tothe total thickness, 52 mm, of a HSC dewar window and a filter
that are removed during the instrument exchange from the HSC
to the PFS. The field element has an array of thin chrome-deposited dots at the surface closest to the image plane to enable better
calibration. It is possible to obtain each pure fiber image or eachpure spectrum on the spectrograph detectors by hiding some offibers from calibration sources, such as a flat-field lamp and awavelength calibration lamp. The dot size has been determined

to be 1.5 mm, considering not only the geometrical obscuration
at the distance of 500 μm between the field element and tips of
microlenses but also the open-loop accuracy of last fiber positioning. The production of the field element with dots has been
completed.
Calibration lamps are mounted on top of the PFI structure. To
ensure the uniformity of lamp illumination, we will have a single
flat-field lamp as well as wavelength calibration lamps (one forblue and another for red/near-infrared). These lamps illuminatea Lambertian flat-field screen attached at the bottom of thedome shutter. The desired irradiance pattern is obtained byusing strongly aspheric plastic lenses in the illumination optics.
Figure 7 summarizes all the above functions contained in the
PFI.

2.4

Spectrograph

The optical design of a double Schmidt-type spectrograph has
been completed, with the 380 nm − 1.26 μm wavelength range
divided into three arms by two dichroic mirrors. Figure 8 shows
one of the four such identical spectrograph modules. The slit
structure at the entrance of each spectrograph module uses holes
in front- and rear- thin electroformed nickel masks to define thefiber–fiber distances as well as their alignment. The arrangement
is complicated by the requirement to allow for a nontelecentric
slit plane. This requirement is driven by the 280-mm diameter
limit on the grating fabrication. After developing prototype slit
assemblies and measuring thermal effects on their stabilities, weare now completing the first final slit assembly. The productionof the spectrograph optical elements is also in progress. All
silica glass blanks are ready and being polished by Winlight
System. Figure 9 shows an example of the precision optics
required for the spectrographs. This 305 mm × 600 mm ×
60 mm thick optic meets our stringent requirements: radiusof curvature ¼ 1389 mm, a measured surface error <130 nm
peak-to-valley (P-V) for each beam footprint, and root meansquare (RMS) surface roughness <1.3 nm. Figure 10(a) shows
one of the three prototype volume phase holographic (VPH) gratings produced by Kaiser. Although these are prototypes, they
have qualities close to our specification: e.g., the RMS wavefront error for the blue VPH grating prototype was 250 nm
for the 280-mm clear aperture, with a trefoil pattern as the dominant error source. We have recently identified the main cause ofthis trefoil pattern as a slight misalignment of optics for VPH
production. This misalignment is now fixed and we are starting

Fig. 6 EM fiber positioners arranged in a module. The final module will contain 57 positioners in two
adjacent rows. Many aspects of operating a number of positioners have simultaneously been tested,
including collisional avoidance between fiber arms and a crosstalk check within an electronic board
and/or adjacent electronic boards. For scientific operations, the PFI control software will provide target
and current positions of each fiber positioner to the Cobra movement planning software, which generates
parallel sets of commands to move individual positioners to their targets after computing a safe path foreach positioner. The baseline approach to avoid collisions between positioners is to run each positioner
radially when transiting common patrol areas.

Journal of Astronomical Telescopes, Instruments, and Systems

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Jul–Sep 2015 • Vol. 1(3)Power/control/data

Positioner
controlelectronics
Glycol coolant

Science fibers (cable C)

Rotationg lower assembly(positioner frame/link structure)

Optical ethernet

Glycol coolant

Opticalethernet

Subaru facility

Positioner elec.
power supplies

Discrete signal lines TBD

Strainreliefboxes (48)Towerconnectors

1 Gbpsethernet

Opticalethernet

Calibration
lamps

Auxiliary
power/control
electronics
Fiducial fiber
illuminator

Cable wrapper

Positioner driver electronics (84 boards)

Fiber positioner/bench
A&G
camera(6 total)

Field element

Focal plane mask (dots)

A&G
camera(6 total)

Power/control/data

Bulkhead connectorbox

Sugai et al.: Prime Focus Spectrograph for the Subaru telescope. . .

Fixed upper assembly(fixed interface frame)

Fig. 7 Functional block diagram for PFI. The upper assembly is fixed with respect to the telescope structure, while the rotating lower assembly through the cable wrapper provides tracking target objects in thesky. The calibration lamps are in the fixed assembly, while the field element with dots, fiber positioners,
fiducial fibers, and A&G cameras are in the rotating assembly.

mass production of VPH gratings. Figure 10(b) shows an Ohara
high-refractive-index prism blank made of S-LAH53 for a
medium resolution grism in the red channel. Such a blankwith high refractive index of nd ¼ 1.80610 and with a large
size of 320-mm diameter has been successfully produced, realizing the homogeneity of 4 × 10–6 P-V for the whole clear aperture when the spherical component is removed. This blank has
been cut into two prisms, each of which will be glued to eitherside of a medium resolution VPH grating to form a medium resolution grism. The exchange between low resolution mode andmedium resolution mode can be done remotely by using a slidetype exchanger of red gratings/grisms.
For the red channel in each spectrograph module, we use apair of 2 K × 4 K Hamamatsu fully depleted charge-coupleddevices (CCDs) with a pixel size of 15 μm × 15 μm. This isexactly the same CCD used for the HSC. To simplify the control

detector

Fig. 8 One of the four identical spectrograph modules. Schmidt-type optics are used both on the F ∕2.5
collimator and the F ∕1.09 camera. The entrance slit is vertical to the page and the distance from the slit to
the collimator is 693.5 mm. The three arms have almost identical optical elements, so only those forthe red arm are labeled. Corrector 1 for each color arm works also as the dewar window. The medium
resolution mode for the red channel will be realized by using a slide structure (not shown) with a low
resolution grating and a medium resolution grism. The scale shown at the bottom left is only for theleft drawing.

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Fig. 9 A polished collimator mirror. The size is 305 mm × 600 mm ×
60 mm thickness at its center. The measured surface shape error was
147 nm peak-to-valley (P-V) for the whole effective area and was lessthan 130-nm peak-to-valley per each fiber pupil.

electronics system, mechanical interfaces, as well as operation
on optical channels, a pair of 15 μm × 15 μm-pixel 2 K × 4 K
Hamamatsu fully depleted CCDs is also used for the blue channel but with a thinner fully depleted region and a blue optimized
coating. The 100 − μm depletion layer provides a smaller charge
diffusion of about 4 μm (1 sigma) compared with 7.5 μm for astandard thickness of 200 μm. The blue optimized coating provides a detector quantum efficiency of 57%, 74%, and 91% at
380, 400, and 500 nm, respectively, a significantly improvement
over a standard coating. All the CCDs have been successfully
produced. In collaboration with NAOJ Advanced Technology
Center (ATC), AlN pin bases were prepared for individual optical CCDs by placing Ti pins with a positional accuracy sigma ¼
1.4 μm in each of the x, y directions and a pin-diameter accuracyof 0.4 μm. These pins allow accurate alignment of a pair of
CCDs. For the NIR channel in each spectrograph module,
a 4 K × 4 K Teledyne H4RG-15 mercury-cadmium telluride
device is used with the same pixel size of 15 μm × 15 μm to theoptical CCD case. The special cutoff wavelength of 1.7 μm, as
short as is feasible, suppresses the thermal background.
The expected location for installing the spectrograph modules is the fourth floor of the infrared side (IR4) in the Subaru
enclosure. The four spectrograph modules will lie within a single light-weighted spectrograph clean room, with thermal insulator walls to improve both temperature control and cleanliness.
The temperature inside of the spectrograph clean room will be
kept within 1°C. This thermal control stabilizes image quality
and limits drift of image position to less than 0.2 pixel over 1 h.
The optical bench for each spectrograph module is made of carbon fiber reinforced plastic (CFRP), which has a low coefficientof thermal expansion, further stabilizing the spectrograph optics

configuration against residual temperature variation. The selection of CFRP has also been essential for reducing weight, as
compared with stainless steel.
The effects of possible vibrations have also been thoroughly
investigated. For cooling the dewars and their enclosed detectors, we use Sunpower compact free-piston stirling cryocoolers.
These have been selected because of cost and reliability; the meantime between failures (MTBF: the predicted elapsed time between
inherent failures of a system during operation) is about 14 years.
The vibration amplitude produced by the cryocooler is reduced,
e.g., down to −40 dBG RMS level at the 60 Hz fundamental withan active damper. The project, however, has carefully designed
the mechanical and optical components so that their resonancefrequencies are larger than 85 Hz for all components when
attached to the optical bench and are larger than the second harmonic (120 Hz) for internal modes except for the heaviest component cases, such as a grating/grism exchange mechanism.

2.5

Metrology Camera

As described in subsection 2.3, iteration is necessary to accurately position each fiber. To facilitate this iteration, the spectrograph is equipped with light emitting diodes that provide
back-illumination when the spectrograph shutter is closed.
These back-illuminated fibers are imaged by a metrology camera located at Cassegrain. The metrology camera is set in thecommon-use Cassegrain container,21,22 which has been developed in collaboration between the Kyoto tridimensional spectrograph II23,24 team and NAOJ. The container is installed witha robotic instrument exchanger Cassegrain instrument auto
eXchanger for the Subaru telescope.25,26 Given the large distance between the fibers and the metrology camera, this configuration permits all fiber images to be viewed in one exposure. To
validate the precision of this measurement, the “dome seeing”
was measured at the Subaru telescope by using a similar configuration to back-illuminate Fiber Multi-Object Spectrograph27
fibers imaging with a commercial-based CCD camera Atik 450
set at the common-use Cassegrain container. To average out
short-timescale variations in the perceived fiber positions due
to “dome seeing,” an exposure time of around 1 s is optimal.
Exposure times of 0.5 and 1.0 s lead to RMS image position
variations of 2.5 μm and 2 μm, respectively. These variations
are acceptably small compared to the final fiber positioning
accuracy requirements of ∼10 μm.

Fig. 10 (a) Prototype VPH grating for the blue channel. The outer part of the grating is masked withchrome, which provides us the only real light stop in the optical system. The size of the grating is
340 mm × 340 mm × 40 mm thickness. (b) A blank for a pair of prisms that will be attached to a volume
phase holographic (VPH) grating to form a red-channel medium-resolution grism. The diameter of theblank is 320 mm.

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Jul–Sep 2015 • Vol. 1(3)Sugai et al.: Prime Focus Spectrograph for the Subaru telescope. . .

Fig. 11 (a) Schematic diagram for the light path (coming from the left side of this figure) of a target object
through the Subaru telescope. (b) Light path from the artificially backilluminated fibers (at the right side ofthis figure) to the metrology camera located at Cassegrain. The metrology camera uses a tiny portion of
WFC lenses.

The diameter of the metrology camera optics was chosen tominimize the effects of small surface deformations within the
WFC optics. The optical path from the fiber plus microlens to
the metrology camera is much narrower than the path from thetelescope primary mirror to fiber plus microlens since the metrology camera aperture is much smaller than the primary mirror
aperture (Fig. 11). This difference does not matter much if the
WFC lens surface shapes are perfect, because the different paths
share the same chief ray. The WFC lenses, however, have surface shape errors including high spatial frequencies around
6 mm with the P–V amplitude of 10–30 nm; these small errors
are inherent to the manufacturing process that is required forthe aspheric surfaces in the WFC. Such errors are allowed
for the HSC imaging since the amplitude is much less than
the wavelengths used. However, because the PFS metrology
camera uses only a tiny portion of the WFC lenses for eachfiber, the local slopes of the lenses produce significant deformation of coordinates of fibers in the focal plane when they aremeasured in the metrology camera coordinates. To suppress
this effect down to the level of standard deviation of 1.7 μm,
an aperture diameter of 380 mm is required for the metrology
camera; this diameter corresponds to the unobscured maximum
aperture at Cassegrain. This large entrance pupil diameter would
cause diffraction-limited images of the fibers to be undersampled, preventing accurate centroiding. To prevent this condition, the metrology camera optics were designed with a lot of
spherical aberration, ensuring the point spread functions are uniform across the field and are several pixels in diameter.

2.6

Software and Execution Processes ofObservations

3

Summary

Following the successful project conceptual design review andpreliminary design review and further subsystem critical design
reviews, PFS has now entered the construction phase. Mass production of the microlenses and fibers is complete. These twocomponents are now being integrated, which includes gluing
them in the fiber arm structure. Later, they will be integrated
with each fiber positioner, shortly to enter the mass production
phase, by gluing the fiber arm to the positioner motor shaft. The
positioner modules will be integrated onto the positioner opticalbench and then into the PFI structure. This integrated PFI system
will be tested together with the metrology camera. For the
spectrograph module side, all optical elements, including gratings, are in production. These optical components will be integrated with cryostats and dewars on four spectrograph opticalbenches. After acceptance tests of each of the PFI and spectrograph components, they will be transported to Hawaii, where thewhole system finally will be tested all together: the PFI systemin the POpt2 at the telescope top, spectrograph modules in asingle clean room on the fourth floor IR side, and the metrology
camera in the Cassegrain container. The technical first light isplanned in early 2018.

Acknowledgments

For science operations, the observation preparation softwareparses large surveys into a series of individual observations,matches fibers to designated targets, selects appropriate guidestars, predicts observing times, and provides sequence files ina Subaru format. The PFS observation control computer(OBCP) controls the entire instrument under the Subaru observation management software Gen2. Execution of an observation
begins with moving the telescope to the desired field center.
While the telescope is slewing, the fiber positioning system executes commands from the OBCP to simultaneously move allfibers to their required positions with the real positions verified
by the metrology system. With several iterations, the position
error of each science fiber can be effectively minimized. In parallel with this process, the A&G system refines the pointing of
the telescope and angle of the instrument rotator. The hexapod iscommanded to move to the appropriate position to maintain the
WFC alignment in the presence of flexure in the POpt2 structure. Once positioning is complete, autoguiding commences.
Journal of Astronomical Telescopes, Instruments, and Systems

The onsite data reduction system produces quick-look information on the completed observation so that data quality and survey progress can be monitored. The collected raw data will betransferred to and archived in the Subaru telescope archive system data archive system, which then provides data retrieval forfurther scientific data reduction and analysis.

We gratefully acknowledge support from the Funding Program
for World-Leading Innovative R&D on Science and Technology(FIRST) program “Subaru Measurements of Images andRedshifts (SuMIRe),” CSTP, Japan, and Fundação de Amparoa Pesquisa do Estado de São Paulo (FAPESP), Brazil. We appreciate staff members at Subaru Telescope for continuously
supporting our activities. We thank NAOJ ATC staff members,
particularly Tetsuo Nishino, Norio Okada, and Yukiko Kamata,for preparing AlN pin bases, and Durham University staff members for their consultancy to IPMU on the fiber system. We also
acknowledge the WFMOS-B team whose accumulated efforts
of many years have inspired us. This paper has been revisedand updated, for the purpose of journal publication, from the
PFS overview paper28 presented in an SPIE conference.

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Jul–Sep 2015 • Vol. 1(3)Sugai et al.: Prime Focus Spectrograph for the Subaru telescope. . .
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Journal of Astronomical Telescopes, Instruments, and Systems

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Hajime Sugai received his doctor degree (astronomy) from the
University of Tokyo in 1993. From 1996, as an assistant professorof Kyoto University, he researched active phenomena in galaxies
and also developed Kyoto tridimensional spectrograph II, which has
integral field spectrograph and Fabry-Perot modes. He has been
an associate professor of Kavli IPMU (WPI), the University of Tokyo,
since 2011. He has worked as the PFS project manager.
Biographies for the other authors are not available.

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