CGS4 is a 1-5um multi-purpose 2D grating spectrometer containing a 256×256 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 1.0×1.5 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 the echelle.