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Throughput Exposure Guide to the R-C Spectrograph (1Sep95)
(from KPNO, NOAO Newsletter No. 43, September 1995)

The R-C spectrograph has been the workhorse for the Mayall 4-m
telescope for 20 years. The spectrograph currently has two observing
modes: the UV-fast camera using the T2KB CCD and the Cryogenic Camera
with its Loral CCD. Using the first mode any of 12 gratings can be
used, providing spectral resolutions from 300-9000. Complete details
can be found in the R-C spectrograph manual, available via anonymous
ftp in the kpno/manuals subdirectory as "rcspx.ps."  Available
apertures include long slits or multi-slit masks that allow the
spectra of approximately a dozen objects (within a 5' field) to be
observed at the same time. The Cryogenic camera mode offers higher
throughput, but lower resolution (150-600). Again, either a long slit
or multi-slit mask may be used.

                           [Figure not included]

We are in the process of combining our spectroscopic manuals for the
R-C spectrograph and the GoldCam spectrograph at the 2.1-m. As part of
that process, we have begun updating our measurements of the
throughput of these instruments, measured under real observing
conditions. We have been utilizing the beginning of "check-out" and
"T&E" nights for this purpose, and although we are still obtaining
data for all of the setups of the R-C, we thought it might be helpful
to prospective observers to see at least some of the data prior to our
final report.We present in the accompanying figure the data for two of
the setups of the R-C.

Our observing procedure has been to observe a spectrophotometric
standard both with a typical (2") observing slit and with a 6" slit.
The latter allows us to determine the overall efficiency of the system
under the limit of "perfect seeing"; the former gives us a measure of
what is likely to be achieved during typical observations. We show the
data for two of the gratings in the accompanying figure. We have
normalized the counts to that of a m_v = 10 mag star observed at an
airmass of 1.0. Using grating 181 in first order (with a GG455
blocking filter) we obtain a peak of 800 e-/s A  at 10th mag; scaling
to other magnitudes is straightforward. (At 18th mag we would expect
about 0.5 e-/s AŹ.) The actual spectral resolution of this grating is
7.2A Ź (2.8AŹ/pixel), so for an 18th mag object and an hour-long
exposure we would expect to achieve 1800 e-//A 5000 e-/pixel, or 13500
e- per spectral resolution element. If sky-noise and read-noise were
inconsequential (as they would be in this example), this would
correspond to signal-to-noise ratios of 40 per AŹ, 70 per pixel, or 115
per spectral resolution element. (We acknowledge Craig Foltz's MMT
spectrograph manual, which gave us the idea for presenting the count
rates in astronomer-friendly units!)

While the primary of the Mayall telescope is always called "4-m," like
all large telescopes the actual collecting area is somewhat smaller
due to several factors: the outer edge of the primary is masked, and
there is a large central obscuration due to the prime focus
cage/secondary mirror supports. As a result the 4-m has an effective
collecting area of an unobstructed 3.42-m telescope. Using the UV
Camera, we are detecting 8-9% of the photons incident on the
telescope's primary with a 2" slit, and 11-16% with a wide-open slit.
Using the Cryogenic camera, grism #770, and a wide-open slit,
approximately 20% of the incident photons are detected. When the
primary mirror cooling system comes on-line this semester, we expect
the improved image quality to increase the total throughput when using
typical observing slits. For the latest available information on any
of our low-to-moderate resolution spectrographs feel free to contact
any of the undersigned.

Daryl Willmarth, Jim De Veny,
Buell Jannuzi, Phil Massey

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