As promised, we have recently completed a manual covering low-to-moderate resolution (R < 5000) optical spectroscopy at Kitt Peak. The manual covers the R-C Spectrograph and Cryogenic Camera at the 4-m telescope, and the GoldCam Spectrometer at the 2.1-m. The manual is available both by anonymous ftp (to ftp.noao.edu, cd kpno/manuals, binary, get spectroscopy.ps.Z) or via the Web under documentation on the Kitt Peak home page http://www.noao.edu.
In preparing this guide, we obtained data on spectrophotometric standard stars to serve as an exposure guide. We show in the accompanying figure the actual measured count rates (per Angstrom, not per pixel) normalized to an mv = 10.0 mag star. We show both the case where the slit is essentially wide-open (8") and when the slit has a nominal size in modest seeing conditions. Although various gratings and grisms are shown, the basis for comparing different gratings really should be the efficiency plots included in the manual. But this figure should provide a good guide as to what to expect at the telescope in terms of count rates near the grating blaze in medium and excellent seeing.
Thus, we might expect to obtain about 800 e s
Å
at mv = 10th mag with the 4-m R-C Spectrograph under moderate
seeing conditions. If the seeing were spectacular (and we're working
on that; see the article elsewhere in this Newsletter), it might be as
much as 1000 or 1300. Thus at 18th magnitude we would expect to obtain
800 x 10
= 0.5 e s
Å
. To obtain a
SNR of 50 per 3 Angstroms resolution element would require 2500 e
per 3 Å, or 830 e
per Å, which we would achieve in 1630s
at 0.5 e
s
Å
. We assume in this example that photon
noise from the sky is immaterial; a more complex example is given in
the manual.
What does this translate to in terms of actual throughput? For the 4-m (which has a clear aperture of 3.42 m), the total system throughput (telescope + spectrograph + CCD) is 15% for the R-C Spectrograph and 21% for CryoCam under excellent seeing conditions. At the 2.1-m (which has a clear aperture of 1.94 m), the total system throughput is 19 GoldCam under excellent seeing conditions.
Phil Massey, Jim DeVeny,
Buell Jannuzi, Daryl Willmarth