SuMIRe PFS:A Large Multifiber Faint­Object
Spectrograph for SUBARU
Jim Gunn, Princeton University

NAOJ 9 Dec 2010

Why a large, deep spectroscopic survey?
There are on the books a large number of deep imaging surveys,
all so far using a few broad­band filters. The adherents argue thatthe distance and SED information which is crucial to doing ANY
extragalactic science with these surveys can be obtained withphotometric redshifts.
Optimistic accuracies of photoz's place the errors in ln(1+z)
at .03 or a little better for red galaxies. Thus we lose
1. Essentially all dynamical information in populations of galaxies.
errors of even 0.02 are velocities of 6000 km/sec; There are nostructures with velocity dispersions this large, nor do peculiar
velocities get this large.
2. All dynamical information IN galaxies (no velocity dispersions,no rotation information)

Why a large, deep spectroscopic survey? (Cont)
3. All but the very crudest chemical information, and it is yet to
be demonstrated that even that is obtainable in any reliableway.
4. All but the very crudest stellar population information.
5. For BAO, loss of the very powerful H(z) determination,and loss of enough LOS information to make an imaging­only
survey an order of magnitude less sensitive than aspectroscopic one.
Experience with the SDSS indicates that the synergy between
imaging and spectroscopy is profound, and one would like to doa series of SDSS­style surveys at higher redshift to follow theevolution of the galaxy population, as well as emission­line BAO
surveys and very faint stellar surveys. Is this possible??

What do we need?

Suppose we want a spectroscopic survey at high (howhigh? we will consider this later) redshift, to do an investigation on
the galaxy population in several redshift bins on samples
which are comparable to the Sloan main survey, to investigate
the *evolution* of chemical, dynamical, and clustering properties
analogous to those done on the SDSS sample.
The volume of the SDSS main sample to a luminosity limitof L* (z~0.12) is about 0.04 h­3Gpc3, and contains about 300,000
galaxies brighter than L* to the main limit.
So we would like comparable comoving volumes in several
redshift bins out to some highest practicable redshift.

What do we need?
Consider something like this:Boundaries
Mean z0.6­0.8 0.8­1.05 1.05­1.35
0.7
0.9
1.2

1.35­1.65 1.65­1.95
1.5
1.8

2.0
2.0

2.5
2.5

Area/.04Gpc3

110

80

60

50

50

50

50

 3727

0.63

0.71

0.81

0.93

1.04

1.12

1.30

 4000

0.68

0.76

0.88

1.00

1.12

1.20

1.40

rAB(r,L*)

1.07

1.19

1.38

1.57

1.76

1.95

1.64

20.8

21.3

22.0

22.4

22.9

23.4

24.5

So a few tens of sq. deg is a reasonable size scale for such a survey.
Probably at least 4 fields, of variable extent to different limits, somewhat
larger than 100 sq degrees at bright limits aiming at galaxies with
redshifts less than 1, smaller as one goes fainter and more distant. If westep the coverage to keep the volume roughly constant, there will be~3x105 bright galaxies per `cell', 1.5 million in all.
These areas are comparable to those in the proposed HSC deep survey,
and that survey would be used to provide targets.

Some preliminary technological considerationsWavelengths of interest: One needs a handle on the oldstellar populations of galaxies. Even at redshift 1, this certainly
involves the near IR. 6500A ­> 1.3 microns. At redshift 2,6500A ­> 1.8 microns. At the very least, need to reach beyond
the 4000A break, 1.2 microns at z=2.
So ideally, we would like to go to at least 1.8 microns. It becomesvery difficult beyond that because the thermal background becomesvery high. Furthermore, at 1.8 microns the spectrograph must becooled, and will be much more expensive than a room­temperature
instrument. We consider here an instrument which goes from 3800A
to 1.3 microns, which can be constructed with ordinary opticalmaterials and, if the detector cutoff can be tailored sufficiently well,
can be entirely at room temperature except for the detector andthe lenses immediately in front of it.
THERE IS NO REDSHIFT DESERT WITH THIS SPECTROGRAPH:
3727: 0.17 ­> 2.48 Ly­a 2.13 ­> 10.7!
How deep do we need to go?

How deep do we need to go??

SEDs for a galaxy which is L*i today at redshifts between 0.5 and2.2, showing Subaru shallow survey limits, limits for half­hour
exposures with VISTA, and 5 minute exposures with IRAC, fora galaxy with a very short characteristic SFR time, 0.5 Gyr.

SEDs for a galaxy which is L*i today at redshifts between 0.5 and2.2, showing Subaru shallow survey limits, limits for half­hour
exposures, with VISTA, and 5 minute exposures with IRAC, fora galaxy with a short characteristic SFR time, 1.0 Gyr.

SEDs for a galaxy which is L*i today at redshifts between 0.5 and2.2, showing Subaru shallow survey limits, limits for half­hour
exposures, with VISTA, and 5 minute exposures with IRAC, fora galaxy with a moderate characteristic SFR time, 2.0 Gyr.

SEDs for a galaxy which is L*i today at redshifts between 0.5 and2.2, showing Subaru shallow survey limits, limits for half­hour
exposures, with VISTA, and 5 minute exposures with IRAC, fora galaxy with a longish characteristic SFR time, 4.0 Gyr.

Magnitude Limits:
We thus need to reach AB limits at 1.2 microns of about
23.5 and at 1 micron of 24.5 for blue galaxies which are theprogenitors of L* galaxies today at z=2 (but this is a little
optimistic because of merging)
Can we do this?

More Technology: The Sky
Working in the IR is incredibly difficult because of the OH airglow,
but the sky is reasonably dark between the lines. At low resolution,
however, the lines overlap and there IS no `between the lines'.
IR detectors are incredibly expensive—current prices on good 4K^2HgCdTe devices, 15 micron pixels, 70+% QE, very low dark currents,~15 electrons read noise per nondestructive read, is ~$700K.
Wavelength coverage vs resolution is thus a crucial tradeoff. Can
we cover, say, the 1.0­1.3 micron band at a resolution which does
not result in a ridiculously expensive instrument?
We can model the cumulative fraction of pixels which are fainter thansome limit as a function of resolving power. Will see that resolving
power is dictated by detector size, camera focal ratio, and fiber size.

Resolving power and the Sky
We need high resolving power to work between the OH lines.
If we build a 3­channel spectrograph , using 1.1 arcsecond fibers and4K x 4K detectors, we could do something like this:wavelength
4500

R
1890

50%
22.4

60%
22.3

70%
22.0

5500

2380

21.6

21.5

21.4

7000

2350

21.36

21.5

21.3

9000

3200

20.7

204

19.8

10500

3800

20.4

19.5

18.6

12500

4800

20.1

18.9

17.4

blue(0.38u­>0.65i)

red(0.65u­>1.0u)

ir(1.0u­> 1.3u)

What would a system with these parameters look like?
For Subaru,
F/2.24 with HSC corrector. Scale 89/arcsec
Need (?) to transform f/ratio with optics or tapered fibers tosomething significantly slower than f/2.4
Best:100u -> 128u : 2.24 -> 2.85 (2.50 w/FRD)
OK:
100u -> 200u : 2.24 -> 4.5 (3.6 w/FRD) (faster is better)
1.1 arcsec -> 9 kpc for z ~ 0.7 or greater.
SDSS has 3 arcsec fibers; z=0.12 -> 7 kpc,
f/1.05 camera, f/2.85 collimator; fiber projects to ~54u
fwhm ~ 47u, 3.13 pixels
1200 resolution elements on 4K pixel detector... sets resolution

Resolving Power vs Wavelength for 3­channel PFS

Sensitivity
Assume throughput of 25%. Normalize to AB=22.5, 1 hr (3x1000s) exposures
In BLUE, (R=2400) get 300 electrons per V pixel,
400 electrons from sky; S/N ~ 14 per pixel
For GALAXIES, degrade to R=1000, S/N ~ 20, 5 at AB=24
In RED, (R=3100) get 300 electrons per V pixel
800 electrons from sky, S/N ~ 8.5 per pixel
For GALAXIES, at R=500, S/N ~ 20 , 5 at AB=24
In IR, (R=4500) get 160 electrons per V pixel
300 electrons from sky, S/N ~ 5 per pixel
For Galaxies, at r=300, S/N ~ 20, 5 at AB=24MUCH shorter times for BAO using 3727

Point­source throughput
with all instrumental losses
and seeing loss at input
in 0.7” seeing (29%)

Per­pixel S/N in 1000s at 22.5
(R=500 is ~30 pixels in IR)

How Long? How Many Fields?
1 hour per field, 2000 objects, 2000 objects/hr. 1.5 million spectra,
700 hours, 90 good nights, 20 fields, 37 visits/field. Field size
does not matter; only number of fibers.

BAO survey?
Shorter; brighter objects, 1 visit per field, 30min exposures,2000 sq degrees, 500 hours, 70 good nights, ~2 million spectra,~4 Gpc^3

SFR=(t/t0)exp(­t/t0)
t0= 4Gyrt0= 2.8Gyr

t0= 1.6Gyr

t0= 0.5Gyr

R=400,300,200 B, R, IRz=2.5; 4000A
break beyond
end of FSR;
must rely onemission
3727 at 1.30u,
Ly­a at 4250

These are simulated
spectra with lines
of constant cgs
flux every 33 A
through the spectrum
At z~2, the spacedensity of 3737
sources withL~1e42 erg/sec
is comparablewith L* galaxies;
the flux is 4e­17

The Spectrographs
Desiderata:
1. High Throughput
2. As wide a wavelength range as is practical
3. Sufficient resolution toa. Do the scienceb. Work on faint objects in the red, *between* the OH lines
4. Simple optics to keep the surface count and costs low.
5. A state­of­the­art faint object instrument. This is our chance to makethe most powerful (and useful) spectrograph in the world. Let ususe it.

Optics – Camera Speed is Everything
1. The facts of life:
Subaru is an 8.2m telescope
ddetector = Dtelfcam ”/206265 = 40u fcam ”
So for a 100u fiber, 1.13”, the spot size is 45u f cam
2. Detector manufacturers do not like big pixels, so all
pixels in modern detectors are 15u.
3. So f/1 gives a resolution of 3 pixels. Any faster isprobably too fast, any slower wastes detector real estate AS THE
SQUARE. Fewer fibers, fewer resolution elements
4. R =  c/d = ( c/D ) Npix/Nres; Nres=ddetector/dpixel so R~1/fcam
5. Fiber FRD, lack of telecentricity makes these worse; to get all
the light, focal ratio of collimator must be smaller, need evenfaster camera by ratio of ideal to real collimator f/ratio.

Optics – Simplicity is Almost Everything Else
1. Want few elements to minimize light loss, cost
2. Simplest fast camera is Schmidt. One aspheric, two refractive
elements, simple spherical mirror.
3. Disadvantage: Detector is at prime focus of mirror, IN THE BEAM.
=> Beam must be large.
How large? 4Kx4K 15u detector is 62mm square
250mm beam has 8% obscuration, OK. Camera IS the dewar.
4. Why 250mm??
5. Kaiser can make superb 280mm VPH gratings in one shot, nobigger. Angles such that 250mm beam is OK. Cost ~$40K
Bad news: Simple Schmidt does not make good enough images.
Baker­Nunn lesson: First­order correction with aspherics on both
surfaces of thick corrector is OK. Canon can make and test such
surfaces.

The spectrograph.
1. Need R ~ 3000 or greater in the red (.7­>1u) and 4000 or greater inthe IR (1­>1.3u) to work with the sky OH.
2. Science (?) says R~2000 in the blue (lessons from SDSS).
3. To go from ~4000 to 1.3u requires 3 channels to meet theresolution requirements with an f/1 camera and 1” fibers
with 4K^2 15u pixel detectors.
4. Propose 3 channels:
3800 – 6700A
6500 – 10000A
9700 – 13000A

R=2050 at center
R=2950 at center
R=4350 at centerf/1.05 camera f/2.8 collimator, (best) or f/1.05 camera, f/3.7 collimator. (OK)
Requires f/ratio transformer; more later.

The Spectrograph
These 3 cameras, optically almost identical
Schmidt collimator, f/2.5 from fiber with ideal
f/ratio 2.8. Microlens f/ratio transformer, 100u attelescope focus, 128u in long harness and in spectrograph.
(Use SDSS connectors at PFU)
Two dichroic splitters:
first at 6600A, reflects 3800 – 6600, trasmits longer
second at 9850A, reflects 6600 – 9850, transmits longer
Looks roughly like this:1.

Blue Channel
Room to swing dewar to larger angles. Can replace gratingwith higher density one to reach R~5000 or perhaps even 8000
over blue range 3800­6500.

The Spectrograph

600 fibers per spectrograph with 3 pixel (45u) spacing
So 4 spectrographs.
Simple bolted or welded rectangular aluminum box with internal
baffles/gussets.
Might look like this physically:

The Camera/Dewar
Performance: 25u images with crude hand optimization. Baker­Nunn
gives 12u images over curved field, no reason to expect worse here.
Camera body is cryostat; Double­sided Schmidt corrector isvacuum window. Proven design.
Looks like this:Cost
Spectrograph only:
Box
Slit assy and fiber hdwe
Collimator
Dichroics
Gratings
Grism/wedge/corrector

Cameras:Primary mirror
Corrector
Field Flattener
Pump
CoolerDewar

Detectors:
CCDs
IR

30K
30K
40K
20Kx240K x 3
40K x 3
380K

20K
40K
2K
2K
150K
30K240Kx340K(?)x4
700K
840K
total $2.0M per spectrographScale of Project
Fiber Positioner
Spectrographs + Detectors
FibersSubaruProject + Contingency
Software~$45M;

$10M
$10M
$5M
$5M
$10M
$5M

TheBeginning

BAO and 3727
Number density of 3727 emitters, L>1e42 erg/sec is 3e­4 at z=1, 8e­4 at z=1.5,probably ~2e­3 at z=2. Number density of L* galaxies is 3e­3Detection:z 3727

Rf42

fph42

0.7
1.0
1.4
1.8
2.2
2.5

2700
2600
3300
3800
4500
5200

9.1e­163.6e­161.6e­168.7e­17
5.3e­173.9e­172.9e­41.3e­47.2e­54.6e­53.2e­52.5e­5

6335
7455
8945
10435
11925
13045

ndet/s SkyAB
40
1810
6.4
4.5
2.5

21.5
21.0
20.3
19.7
19.2
18.7

nsky/s
0.7
1.2
1.8
2.7
3.6
3.3