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Cryogenic Camera News (1Mar95)
(from KPNO, NOAO Newsletter No. 41, 1 March 1995)

The Cryogenic Camera (aka "CryoCam") is a low-to-moderate resolution
(8-15 AŹ) CCD spectrometer on the 4-m telescope.  It was designed for high
efficiency, with transmission gratings acting as the dispersive element.The
detector is a dedicated Loral 800 Xť 1200 CCD with 6-8 e- read noise and
good cosmetics. The thinned instrument is used either with a 5' slit, or with
"multi-slit" masks, which allow a dozen or so objects within a 5' field to be
observed simultaneously.

During the early fall 1994 observing season, we noticed that the actual
read-noise achieved on the telescope was typically much worse than the
laboratory value.  Subsequent tests have revealed a grounding problem with
the electronics, and we are in the process of correcting the problem.  Users
of CryoCam during the current scheduling period will be pleased to find the
read-noise is as advertised.

Grism    Resolution(AŹ)    (lambda)(A)   A/pixel     Filter     CoveragA)
 650          12            4950           3.2        BG-38    3800-6800
 770          15            5970           4.3        GG420    4300-8500
 730          15            8010           4.3        OG530    5500-10000
 780-II        8            4850           2.2        BG-38    3800-6100

Note that in the case of multislit masks, the central wavelength may differ by
up to 15% of the full wavelength coverage, depending upon the location of
the object in the field.

We have also completed measuring the overall (telescope + instrument)
quantum efficiency of the system. Without a slit, the overall throughput
peaks at 20%.  We show these efficiency curves in the accompanying
figures.  For planning purposes, we also show the actual number of
photons/sec-Ź detected for the standard star G191B2B with one of the
grisms, along with the magnitude of the standard star as a function of
wavelength.  This figure can be used with the former to predict the expected
count rate for any grism combination.  For instance, at 6000 Ź grism 770 has
an efficiency roughly twice that of grism 650.  We obtained 130
photons/sec-Ź with grism 650 of the standard star (m = 12.0 at 6000 AŹ) and
thus we would expect to get about 260 photons/sec-Ź with grism 770 of
m = 12.0 object.  At a resolution of 15 AŹ we are actually then obtaining 3900
photons per sec per spectral - resolution - element at m = 12.0.  At m = 17.0,
we thus expect to obtain a SNR of 50 (2500 photons) per spectral resolution
element in about a minute.  (We have assumed the sky to be negligible in
this example!)

                         [Figures not included]

Phil Massey, Jim DeVeny,
Taft Armandroff, Todd Boroson


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