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Spectroscopic Multiplexing at the 4-m (1Mar94)
(from KPNO, NOAO Newsletter No. 37, 1 March 1994)

Greedy observers (aren't we all?) have been able to obtain spectra of
as many as 100 objects at a time at the 4-m using Hydra.  However, now
that Hydra is moving to WIYN we would like to remind users that there
exists a complementary mode of multiple object observation at the 4-m
using "multi-slit" masks.  These masks can be used with either the
Cryogenic camera (for low dispersion work) or with the regular R-C
spectrograph (for moderate dispersion work).

Multi-slit masks typically contain perhaps a dozen slits distributed
throughout the 5 arcmin field.  In this mode, a pattern of slits is
created on high contrast photographic film corresponding to the
positions of program objects, say a cluster of faint galaxies or stars
within a galactic cluster.  This film is then used at the focal plane
of the telescopes to serve as an entrance mask for the spectrograph.
There is a small light loss (approximately 10%) in the film base but
under normal conditions this loss is comparable to seeing variation
losses.  The slit allows precise determination of the sky signal
immediately adjacent to the object and thus enables excellent sky
subtraction.

The advantages of multislits over Hydra are:  (a) closer spacing of
objects, (b) better sky subtraction in fields with variable background,
and (c) probably higher throughput under many seeing conditions.  The
disadvantages are: (a) considerably smaller field, (b) more limited
number of objects, and (c) the fact that the wavelength range covered
is a function of position within the focal plane.  The overhead time is
probably similar, about half an hour.  Clearly, whether the multislit
mode has advantages or not for your program depends on your
requirements.

If your program would benefit from multi-slits be sure to specify this
in your observing proposal.  Further details can be extracted from the
cryogenic camera instrument manual or by contacting us.

Phil Massey, Jim DeVeny

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