Introduction to CGS4
General Description of the Instrument
CGS4 is a 1-5um multi-purpose 2D grating spectrometer containing a
256x256 InSb array, installed in a cryostat which is cooled by liquid
nitrogen and closed cycle coolers. Four gratings are available, two of
which are installed in the cryostat at any one time. They are a
40 l/mm grating which provides resolving powers of 300-2000, a 75 l/mm
grating which provides resolving powers of about 600-2000, a 150 l/mm
grating which provides resolving powers of roughly 2000-6000, and a 31
l/mm echelle which provides a resolving power of about 20,000 (15
km/sec). The 75 l/mm grating is rarely used thesedays. These
resolving powers are achieved with the 150 mm focal length camera
optics and a one-pixel-wide slit, which provide a scale of 1.22
arcsec/pixel for the two moderate resolution gratings and roughly
1.0x1.5 arcsec/pixel for the echelle (1.0 arcsec/pixel in the
direction of the dispersion). A 300 mm focal length camera mirror
sometimes is installed in place of the standard camera; with it pixel
scales and wavelength coverages are halved and resolving powers are
doubled (in principle).
CGS4 slit lengths are about 80-90
arcseconds. It is possible to orient a slit at any angle on the
sky. Slit widths of one pixel (~1.23 arcsec with the standard camera,
0.61 arcsec with the long focal length camera), two, and four pixels
are available. Since the resolution is matched to one pixel on the
array, with the one-pixel wide slit fully- (or over-) sampled spectra
must be obtained by mechanically shifting the array in the dispersion
direction, while holding the grating fixed. The array may be "shifted"
over two pixel widths; so that isolated bad pixels do not result in
gaps in the spectrum.
For optimum noise performance and efficiency in short exposures
either multiple non-destructive readout of the array (NDSTARE) or a
single read mode (STARE) can be used. The array can also be read in
synch with the chopper. Chopping often is employed when observing at
medium resolution at thermal wavelengths (beyond the K window).
The instrument has a calibration unit, containing a black-body source
for flat-fielding and argon, krypton, and xenon arc lamps for wavelength
calibration. Six broad band filters continuously cover the wavelength
band 0.95-5.4um. CVFs serve as order-blockers for the echelle.
Spectropolarimetry is available with CGS4 at all wavelengths but not with