Annu. Rev. Astron. Astrophys. 1991. 29:
499-541 Copyright © 1991 by Annual Reviews. All rights reserved |
4.1 Multiple-Object Optical Spectroscopy
Surveys demand more efficient use of the corrected field of view of a telescope than traditional single-slit spectroscopy needs. As surveys shift towards the study of fainter galaxies with surface number densities larger than a few per square degree, the multiplexing advantage of multiple-object spectrographs (MOSs) encourages long integrations that would be uneconomical for a single object. MOSs have developed rapidly in the last decade, and hold the promise for an order of magnitude increase in the growth of the extragalactic radial velocity data base in the near future. We discuss separately advantages and limitations of these techniques, including a brief overview of the first effective MOS, the objective-prism Schmidt. Ellis & Parry (1988) have written an excellent comparative review of multiple-object spectroscopy.
OBJECTIVE-PRISM SPECTROSCOPY
OPTICAL FIBER SPECTROGRAPHS
The first fiber MOS was developed at the Steward Observatory. Several
other systems were built at major observatories in the early 1980s, most
notably FOCAP
(Gray 1984)
for the AAT, OPTOPUS
(Lund & Enard 1984)
at ESO, and Nessie at Kitt Peak
(Barden & Massey 1988).
These systems rely
on the preparation of aperture plates, with holes drilled with high
precision at the locations of the target sources. Prior to each
observation, the fiber ends must be attached to the plate. The procedure
has important limitations, for it demands good quality astrometric
information, and therefore considerable advance preparation: The
observing programs are not easily adapted to schedule changes made
necessary by prevailing conditions, aperture plate changes are very
slow, and handling of fibers may produce considerable wear and tear on
the fiber ends. As for all MOSs, the exposure times are those necessary
for the weakest source in the field. In the late 1980s new automatic
fiber positioning devices were built, such as the MX system at Steward
(Hill 1988)
and Autofib at the AAT
(Parry & Sharples
1988).
These two
systems illustrate two different mechanical approaches to the problem.
MX can position 32 fibers over a 45' field, using 32 independent
robotized arms that intrude in the field from its periphery (like
fishing poles in a pond). In the case of Autofib, a single very fast
robot sequentially positions 64 fibers over the 40' Cassegrain field of
the AAT. The fibers enter the field parallel to the focal plane and are
kept magnetically in position. Robotized systems have been built or are
about to enter operation for most major telescopes.
Ingerson (1988)
discusses design trade-offs applicable to fiber MOSs, as they led to the
decisions affecting the construction of the Argus system at the 4-m
telescope at CTIO.
Positioning speed is maximized in the MX design: A given configuration
can be attainable in slightly over a minute. Positioning via a single
robot, as in designs of the Autofib type, typically take several
minutes. On the other hand, many independent robotized arms get in each
other's way, so that the number of fibers can be higher in Autofib-type
systems: MX, Argus, and Decaspec at the 2.4-m Hiltner telescope
(Fabricant & Hertz
1990)
can aim respectively at 32, 24, and 10 sources,
while Autofib and the Norris spectrograph for the 5-m Palomar telescope
(Cohen et al 1988)
can do so at 64 and 100 sources, respectively. The
physical size of the arms in one case, and that of the magnetic buttons
used to keep fibers in position in the other, limits the minimum
distance at which fibers can be set. In both cases, designs have been
obtained where such separation has been reduced to 15" or less. In some
designs, positions of fibers can be seen on the acquisition/guide TV
camera and, with multiple-arm systems, interactive autocentering is
possible after the source configuration has been roughly acquired. The
increasingly good transmission quality of fibers makes it possible to
locate spectrographs and detectors in a remote, immobile environment,
rather than on the telescope, thereby improving instrument stability and
reducing flexure worries. At this time, fiber losses and those related
to focal ratio degradation make an observation through an optical fiber
slower by approximately a factor of 2 than those made with a long slit
(Fabricant & Hertz
1990).
Fibers used in MOSs have wavelength coverage limitations: dry (i.e.
pure) fibers transmit well in the red and infrared, not in the blue; wet
fibers (which are doped with small amounts of OH-1) transmit
well in the
blue, but not in the red. Thus, a choice of the usable spectral region
is set by the fibers installed in the MOS. Typically, the spectrograph
detector is a CCD; spectra from each fiber need to be separated by at
least 6-8 pixels from each other, perpendicular to the dispersion
direction, in order to prevent contamination of light between fibers.
The size of the CCD thus sets an upper limit to the numbers of fibers
in a MOS. Fiber MOS sky subtraction characteristics are restrictive,
especially with very faint objects, unless a large fraction of the
fibers is devoted to acquiring sky photons.
Fiber MOSs have also been implemented for use with Schmidt telescopes,
making the fiber technique attractive for objects with surface-number
densities of less than a few per square degree. The notable example is
the FLAIR system at the UK Schmidt at Siding Spring
(Watson et al 1988),
with 35 fibers.
Parker & Watson (1990)
have reported successful
performance of FLAIR with 116 redshifts to a magnitude limit of
mJ =
16.8. Current limitations in performance will be greatly improved with
the forthcoming 100-fiber system FLAIR-2.
MULTISLIT SPECTROGRAPHS
As in the case with fiber MOSs, technical developments toward having
automatic positioning of slitlets in the field of view have been
pursued. However, this path has not led to a general-purpose instrument
configuration yet, as discussed by
Ellis & Parry (1988),
and the pre-etched aperture plate method is preferred.
Because sky subtraction is generally more effective with slits than
with fibers, slit MOSs can target fainter sources, and they can thus
simultaneously obtain a fair number of targets in spite of their
relatively small fields of view. The EFOSC at the ESO and the Low
Dispersion Survey Spectrograph (LDSS) commissioned in 1988 at the ATT
represent two successful versions of the slit MOS. The LDSS is endowed
with a relatively large field (12'), which, when coupled with a large
CCD camera, such as the Tek 20482, should allow simultaneous
placing of
about 100 spectra, as opposed to 25-30 spectra with 1980s variety CCD
chips.
Colless et al (1990)
describe both the instrument and the first
results of a deep LDSS redshift survey. Many characteristics of
multislit MOSs are better illustrated vis-a-vis those of fiber MOSs.
Several variables determine the relative quality of the performance of
fiber and slit MOSs. Among them, the multiplex gain - or the number of
spectra that can be acquired - can be higher for fiber than for slit MOSs.
The quality of the sky subtraction, on the other hand, tends to be
superior in slit MOSs because sky background is acquired in the
immediate vicinity of the object (however, some fiber systems like the
Norris at Hale and Decaspec at Hiltner, have satisfactory solutions to
this problem). The size of the field of view can be much larger in fiber
MOSs, which can sample the whole field of view of the telescope rather
than just that physically spanned by the detector. Additional concerns
involve sensitivity, ease and the agility of operation, and the need for
elaborate preparation and versatility.
The inroads made in robotized systems with many dozen fibers and the
relaxation of the need for high-quality astrometric data prior to the
observations (allowing interactive adjustments to fiber positioning)
have been important in making fiber MOSs the systems of promise for the
next decade. The lack of agility in the observations and the need for
elaborate and sometimes uneconomical preparation required by systems
with specially manufactured aperture plates impose the strongest
inhibition to their extensive use. To interested outsiders in this
field, such as the present writers, it appears that the latest
generation of robotized fiber systems with large fiber redundance to
guarantee effective probe-to-probe sky subtraction will be the
workhorses of the next decade's survey spectroscopy.