Fiber-Multi-Object Spectrograph (FMOS) for Subaru
Telescope
Toshinori Maiharaa , Kouji Ohtaa , Naoyuki Tamuraa , Hiroshi Ohtania , Masayuki Akiyamab ,Jun’ichi Noumarub , Norio Kaifub , Hiroshi Karojic , Masanori Iyec , Gavin B. Daltond ,Ian R. Parrye ,fDavid Robertson , Ray Sharplesf , Deqing Renf , Jeremy Allington-Smithf ,Keith Taylorg , and Peter Gillinghamga Kyoto University, Kyoto 606-8502, Japan,b Subaru Telescope, NAO-J, Hilo, Hawaii, USA,c National Astronomical Observatory, Mitaka 181-8522, Japan,d University of Oxford, UK,e University of Cambridge, UK,f University of Durham, UK,g Anglo-Australian Observatory, Epping, Australia

ABSTRACT
Design concept of the Fiber Multi-Object Spectrograph (FMOS) for Subaru Telescope together with innovativeideas of optical and structural components is presented. Main features are ; i) wide ﬁeld coverage of 30 arcmin indiameter, ii) 400 target multiplicity, iii) 0.9 to 1.8 µm near-infrared wavelengths, and iv) OH-airglow suppression
capability. The instrument is proposed to be built under the Japan-UK-Australia international collaboration
scheme.
Keywords: Fiber Spectrograph, Multi-object Spectrograph, Subaru Telescope, Prime Focus

1. SCIENTIFIC OBJECTIVESThe FMOS with capabilities of 400 multiplicity and OH suppression at near-infrared (NIR) wavelengths is
expected to explore many scientiﬁc frontiers from nearby substellar objects to large scale structure of the highredshift universe. Major scientiﬁc targets to be tackled by spectroscopic surveys with FMOS are brieﬂy describedas follows:
Brown Dwarfs Spatial and mass distribution of the lowest mass stars and brown dwarfs will give clues forthe understanding of initial mass function (IMF) and dark matter in the Galaxy. Nelson et al. (1993)
predicted that there are more than 200 ﬁeld brown dwarfs per square degree to a limiting magnitude of
H = 22.5 mag. NIR spectra of these objects at high resolution (R=1500∼3000) can be used for detailed
spectral classiﬁcation and for accurate determinations of metallicity and temperature.
Protostars and Young Stellar Objects (YSOs) In order to investigate processes and physical mechanismsof star formation, infrared observations of highly obscured objects in molecular clouds are essential. Even
without intracloud absorption, a majority of YSO’s spectra intrinsically peaks in the H band, and has large
Paβ / Brγ line ratios, so that physical information can be recovered through J and H band spectroscopy.
Studies of initial mass function (IMF) in star forming regions is also an important scientiﬁc objective with
FMOS.

Further author information: (Send correspondence to T.M.)
T.M.: E-mail: maihara@kusastro.kyoto-u.ac.jp

Scientiﬁc Area

Spectral Resolution

Brown Dwarfs
Young Stellar Objects
Galactic Center Region
LSS/Galaxy Evolution
AGNs/QSOs surveys

High
High
High
Low
Low

N
N/ﬁeld
> 200
∼ 100
> 300
> 400
∼ 200

magnituderange
H = 22
H < 19.5
H = 18H = 22
H = 22

Table 1. Parameters for some of the key scientiﬁc goals for FMOS

Galactic Center Region The NIR coverage of FMOS allows the study of the kinematics and metallicity ofstellar populations in the vicinity of the Galactic center, and the wide ﬁeld of view allows current studiesof the Milky Way’s halo to be extended to more distant components at fainter limiting magnitudes. These
data should yield powerful insights for star formation histories and formation process of the Milky Way
galaxy as well as the Local Group galaxies.
Evolution of Galaxies and Large-Scale Structure atHigh Redshifts A next step for studying galaxy evolution and formation with 8-m class telescopes is to probe activities of star formation in a quantitative
way at redshifts z > 1. At the epoch of such large redshifts, many optical lines serving as vital indicatorsof star formation history would be redshifted to NIR bands. In this context, multi-object spectroscopic
observations with enhanced sensitivity through the OH suppression would play an important role, together
with capabilities of medium resolution (a resolving power of about 1000) and of a wide wavelength coverage. The spectroscopy of extremely red objects (EROs) aiming at revealing their intriguing nature is also
suitable for FMOS.
A systematic work of large scale structure as well as the clustering property of galaxies at higher redshift
regions (z > 1) could be conducted by exploiting the expected FMOS performance as described so far.
The wide ﬁeld of view of FMOS (30 arcmin corresponds to about 10 Mpc at z = 2) will allow us to study
large volume (>∼ 107 M pc3 ) at early epochs.
Cosmological Evolution of AGN/QSO Population X-ray surveys for AGNs/QSOs have just started with
Chandra and XMM-Newton and will provide unbiased samples of AGNs/QSOs out to a redshift of about
5 or even larger. Multicolor surveys of high-z QSOs are also in progress or under planning. Spectroscopic
follow-up observations in optical to NIR bands of these samples are indispensable in studying the natureof them, which in turn may give clues of understandings towards an obscured population of AGN/QSO
and their evolution in the cosmological timescale. An expected surface number density of these sources
matches well with the number of ﬁbers of the FMOS. (Mushotzky et al. 2000)
Table 1 summarizes the objects which are supposed to be targets of FMOS.

2. PRIME FOCUS UNIT AND FIBER POSITIONER
2.1. Prime Focus Unit (PFU) Assembly
The central section supported by spiders of the top ring of Subaru Telescope, which we call PFU, comprises thebase ring, the corrector lens system with its housing, the ﬁber positioner, and the focal plane imager for the ﬁberalignment purpose. A hexsapod unit connects the base ring and the inner instrument base ﬂange, from which
the corrector lens system and the positioner with FPI are mounted. The latter portion is ﬁxed on the instrument
rotator ﬂange to compensate ﬁeld rotation. The conceptual design sketch of PFU is shown in Figure 1.

Figure 1. Cross sectional view of PFU. The shape of lenses of the corrector is not realistic.

2.2. Prime Focus Corrector
Since the corrector system currently used in the prime focus of Subaru Telescope is not intended to be used in
the NIR region at longer wavelengths than 1 µm, it is necessary to have another corrector optimized in the 0.9
to 1.8 µm region, and hopefully usable down to about 0.5 µm for optical use. The speciﬁcation of image qualityof 0.5 diameter as an 80 % energy circle is determined basically from the requirement of the ﬁber aperture of1 in the MOS mode.
The result of optical design of the prime focus corrector with an inbuilt atmospheric dispersion correction(ADC) is schematically shown in Figure 2. Since the glass material of S-FPL52 will be diﬃcult to make inlarge pieces, we ”split-oﬀ” a low-powered ”corrector plate” from the middle lens. The plate is positioned justbehind the middle lens in such a way that its leading surface contacts the trailing surface of the middle lens at
a stipulated minimum aperture determined by the marginal ray heights. The layout of the system is shown in
Figure 2 with spot diagrams in Figure 3.

2.3. Positioner and Focal Plane Imager
Since the constraint of available space in the focal plane area of prime focus (about 145 mm in diameter) does notallow a conventional arrangement of positioner employed by AAT, for example, where the 400 ”magnetic buttons”
are deployed in the area of about 300 mm in diameter, we have to invent an innovative ﬁber positioning systemin the smaller area in order to secure observability of the same number of objects. Recently, we have successfully
developed a ﬁber positioning system based on impact-driven tube piezo-actuators. The result is separately
reported in this conference by Gilligham et al. (2000). The positioner components (which we call ”spines”) arebasically maneuvered by electric pulse trains of a saw-tooth shape, and actual positions are monitored by a focal
plane imager CCD system so as to conﬁrm the organized ﬁber positions. The monitor system allows us to giveFigure 2. The schematic diagram of the prime focus corrector optics. This is a 4-element version in which an ADC isincorporated.

Figure 3. The spot diagram of the 4-element prime focus corrector.

(a)
100
80
60
40
20
0
0

(b)

50

100

150

200

(c)

Figure 4. The ﬁber-ﬁber connector: (a) Optical layout. The collimating (left) and focusing (right) lenses are Selfoc
SLW1.0 and Selfoc SLS2.0 gradient index microlenses, respectively; (b) Simulated Point Spread Function (PSF) of the
Selfoc SLW1.0 collimating microlens. The PSF is diﬀraction limited in the 0.9-1.8 mm wavelength range; (c) A measurement of encircled energy for the Selfoc SLW1.0 collimating microlens. The horizontal axis is the radius in microns andthe vertical scale is the fractional enclosed energy in percent.

information of positional errors, which can be corrected by one or two iterative operations of the actuator. (Seethe details presented by Gilligham et al. 2000).

2.4. Fibers and Fiber Connectors
The input focal ratio at the prime focus of Subaru Telescope is F/2 to which an NA of ﬁbers has to be matched.
Currently we assume that the ﬁber core-diameter of MOS mode is 100µm or 1.2 on the sky. (85µm ﬁbers werealso be considered, but it is found that available ﬁbers made of silica-glass type favor a 100µm ﬁber in termsof the NA constraint.) Because the PFU must be removed from the telescope when FMOS is not in use, an
optical ﬁber-ﬁber connector is used to provide a break in the longﬁber feed at the top-end ring. Fiber-ﬁberconnectors which convert from F/2 to F/5 are necessary to match the input focal ratio for the NIR spectrograph.
In the present design we plan to build simple F-conversion couplers connecting the output F/2 beam to the
input of second F/5 ﬁbers. Dichroic couplers are envisaged as a future option for the simultaneous optical
spectroscopy mode of FMOS. Each converting coupler comprises a pair of matched SELFOC gradient index
lenses as a collimator and a focusing lens. The expected eﬃciency of coupling is estimated as 95% due to theaberration (about 3 − 4µm) of the lens system over the wavelength range from 0.9 to 1.8µm.

3. OH-AIRGLOW SUPPRESSOR AND INFRARED SPECTROGRAPH
3.1. OH-Airglow Suppressor Optics
Since the wavelength bands of FMOS suﬀer badly from OH-airglow lines, it is essential to eliminate as much
OH lines as possible, while the advantage of the OH suppression eﬀect being proven by the ﬁrst-generation
OH suppressor (e.g. Iwamuro et al.1996), and the second-generations are also planned to be built for 8m-classtelescopes (Maihara et al. 2000, Parry et al. 2000). In the present FMOS design we have adopted the opticallayout for OH suppression similar to CIRPASS (Cambridge IR Panoramic Survey Spectrograph) which has
been proposed by Parry et al.(2000) as a ﬁber-based, integral ﬁeld and multi-object spectroscopic instrument
at Gemini’s F/16 Cassegrain focus. It has a fast F-ratio, and is fairly compact. Nevertheless, the number ofobjects to be measured simultaneously of about 400 can only be handled by two identical spectrographs. The

Figure 5. A 3D optical layout of the NIR spectrographconceptual layout of an NIR spectrograph optics is shown in Figure 5, and the speciﬁcations of each component
are presented in Table 2.
The beam train of the OH-airglow suppressor optics is schematically shown in Figure 6 (a) and (b), in which
optical components are multiple times put on along the real optical path.

3.2. Mask Mirror
Masking of OH airglow lines is secured by line absorbers put on the so-called ”Mask Mirror” where the ﬁrst
spectroscopic images are focused. The mirror is of concave sphere, concentric with the Oﬀner mirror. In thecenter of the mirror of the size of 140mm x 460mm, a thin rectangular through-hole is to be excavated for putting
a slit assembly. The masking plate is made of a thin metal by the photo-chemical etching method, so that about
200 absorbing lines are placed throughout the wavelength range of FMOS. A reduction of background of a factor
of about 20 is expected in the H band, while the factor being somewhat lower in shorter spectral bands,

3.3. Volume Phase Holographic Grating
Considering recent research and development eﬀorts of Volume Phase Holographic (VPH) grating (e.g., see reportof NOAO: http://www.noao.edu/ets/vpgratings/), we have decided to utilize the VPH grating as an inversedispersive element, with which the outgoing light transmitting through the grating becomes a spectroscopicallycombined beam of a wider spectral range with a lowered resolution. This functional requirement of the FMOSNIR spectrograph meets with expected performance of VPH grating, although the fabrication technology has
still been in progress.
Speciﬁcations of the VPH grating in the NIR spectrograph is summarized as, i) groove number of 340
lines/mm, ii) deﬂection angle of 27.9◦ , iii) ruled area of 210 x 320 mm2 , and the thickness of 20 mm. With thisconﬁguration, the optical throughput is expected to be about 60% at the peak and about 40% in the shorter

Figure 6. The beam train presentation of the OH suppressor optics. Upper: Optical elements from the input slit to the
big Oﬀner mirror are schematically shown. Lower: The similar beam train presentation of the re-focusing big mirror to
the detector, including the achromatic NIR camera is schematically shown.

SphereFlat/Evenb
Flat

Curvaturea
(mm)
1940
−
−

Optics Size
(mm)
1260φ
800
240×310Beam Size(mm × mm)−
209×209
209×223

Flat/Even
SphereSphereSphereFlat/Even
Flat

−
1940
995
1940
−
−

800
1260φ
164×460
1260φ
800
323φ

209×262−−−209×262209×317

No.

Component

Surface123Primary Mirror
Schmidt Plate
Grating(500/mm, θb = 20.74◦)
Schmidt Plate
Primary Mirror
Mask Mirror
Primary Mirror
Schmidt Plate
VPH grating(θdeflection = 27.9◦ )c

4
5
6
7
8
9
a

An radius of curvature for each component is indicated.
“Even” means an even asphere.
c
The angle by which the light axis deﬂects after passing VPH.
b

Table 2. IRS Optical Components

Figure 7. The cross sectional view of the achromatic NIR camera. The speciﬁcations of each lens are tablated in Table3.

and longer edges of the band. The resultant spectrum obtained at the focus of the F/1.2 spectrograph camera
has a spectral resolving power of about 900 in average over the entire wavelength range.

3.4. Infrared Camera and Detector
The optical layout of an achromatic camera design is presented in Figure 7, and the components of optics aretabulated in Table 3. The incoming beam through the VPH grating of F/5 is re-imaged onto a MCT 2K × 2K
(HAWAII II) array, thereby realizing full-wavelength covered spectra of about 220 target objects. Since the sizesof optical elements are fairly large, feasibility of lens fabrication is not fully explored, especially large uniform
crystals of BaF2 may be hard to obtain, not impossible though.

4. SUMMARY
The instrument concept of the planned ﬁber-based multiobject spectrograph (FMOS), a wide ﬁeld-covered multiobject spectrometer utilizing the prime focus of Subaru Telescope is presented in connection with major scientiﬁc
objectives together with the present status of developments. Design studies under the multinational collaboration

a

No.a

Component

SurfaceGlass

10

Lens 1CAF2

11Lens 2

12

Lens 313

Lens 414

Lens 515

Lens 6

SphereSphereSphereSphereSphereSphereSphereSphereSphereSphereSphere
Even

BAF51
CAF2
BAF2
CAF2
SF56A

Curvature
(mm)
479
444
342
685
408
569
293
814
148
134
114
96

Diameter
(mm)
329
328
316
327
347
348
306
289
253
106
101
58

Beam Size(mm × mm)
209×324202×323
191×311
191×322
195×341
192×343
164×302147×285
141×251
41×10037×94
10×48

This number is continued from Table 1.
Table 3. IRS Optical Components

scheme show that unique features of FMOS, such as the multiplicity of measurements of about 400 targets within
a 30 arcmin ﬁeld, the simultaneous, full wavelength coverage from 0.9 to 1.8 µm, and the OH-airglow suppression
capability to gain high sensitivity, can be realized, although speciﬁcations of some critical components are still
to be improved.

ACKNOWLEDGMENTS
Authors would like to acknowledge contributions of Richard Ellis, Alan Holmes, Ian Lewis, and Craig Mackay inpromoting the FMOS project. Thanks are also due to Prof. Higuchi of University of Tokyo for precious advice
in designing impact driven piezo actuators. The FMOS project in the deﬁnition phase of conceptual design has
jointly been sponsored by NAO (Japan), PPARC (UK) and AAO.

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