next previous contents
Next: 2.4 The 2.1-m GoldCam Spectrometer
Previous: 2.2 4-m RC Spectrograph

2.3 4-m Cryogenic Camera

The ``CryoCam" is a high-throughput analog of the RC Spectrograph. The instrument actually consists of the RC Spectrograph but with the grating replaced by any of 6 ``grisms" as the dispersive elements and with the collimator mirror replaced by a collimating lens. The detector is a dedicated Loral (Ford) 800 X 1200 device with relatively good cosmetics sitting in a fast (f/1) camera-dewar combination. The result of these all-transmission optics is a total system throughput (telescope + spectrograph + CCD) that is typically 20% (29% peak under excellent seeing conditions). Glass optics curtail the throughput below 4000Å. CryoCam is often used with the multi-slit mechanism (see below); in this mode the acquisition is done by interrupting the light path with a pick-off mirror below the mask assembly. The entire 5 field can be viewed with this camera when the masks are in their out position. Of course, during the actual exposure the TV pickoff mirror must be out of the beam and so one is observing ``blind", but with the excellent guiding provided by the auto-guider this is, in practice, no problem. The instrument rotator at the 4-m allows remote control of the position angle of the slit on the sky, although such rotations need to be done with the telescope within 30 of zenith. Thus the spectrograph can be rotated to the parallactic angle (to keep all of the light down the slit), or, if you need to use a particular position angle, you can use the atmospheric dispersion corrector (``Risley prisms"); see Sec. 4.4.1.

The table below shows the six grisms, the resulting wavelength coverage, dispersion, and resolutions. A table of the various offset slits can be found in Sec. A.1.2.

Notes: (a) The central wavelength changes by up to 7% depending upon location in the field (perpendicular to the slit). (b) Resolution is based upon a FWHM of 3.5 pixels, corresponding to a 2.5-3.0 slit. (c) Grism 810 covers an entire octave; choice of wavelength coverage from 4300->10000Å is dictated by blocking filter (i.e., a BG-39 will provide <4000-6000Å coverage without overlapping; using an OG530 filter would allow coverage from approximately 5500Å to >10000Å. Note, however, that Grism 730 would be more appropriate in the latter case, providing the useful wavelength coverage but at higher spectral resolution. (d) Glass optics curtail the throughput below 4000Å.

2.3.1 Blocking Filters for the 4-m RCSpec and CryoCam

The RC and CryoCam spectrographs take 3.5-in square filters used for order separation (see Sec. B). The following table gives the available filters; transmission curves can be found in Fig. 4.

Blocking Filters for RCSpec & CryoCam
Long cut-off Short cut-off
Filter Thickness Filter Thickness Filter Thickness
CuSO4a 8mm WG-345 1mm OG-515b 3mm
BG-38 2mm WG-360 2mm OG-530 3mm
BG-39 2mm GG-375 1mm OG-550b 3mm
4-96 5mm GG-385 3mm OG-570 3mm
KG-2 2mm GG-400b 3mm OG-590b 3mm
KG-3 2mm GG-420 3mm BG-610 3mm

GG-455 3mm GG-645 3mm

GG-475 3mm RG-695 3mm

GG-495 3mm RG-830 3mm

Notes: (a) CuSO is too narrow for multi-slit work, but does cover the entire length of the slit. (b) New filter on-order; should be available in early 1997.

Figure 4:   The transmission curves for the various RC and CryoCam blocking filters. You may examine these electronically at

next previous contents
Next: 2.4 The 2.1-m GoldCam Spectrometer
Previous: 2.2 4-m RC Spectrograph
Updated: 02Sep1996