CGS4 is a spectrometer designed to work in the 1-5
region of the
electromagnetic spectrum. The instrument is cooled by liquid Nitrogen
and by a closed cycle cooler to temperatures below 80 K
[Mountain et al. 1990]. The optical
layout of the instrument is shown in Figure 2.1.
Figure 2.1: CGS4 optical layout (from the UKIRT Home Page,
instruments section and CGS4 subsection).
The spectrometer can be used with a camera of either 150 mm (short
camera) or 300 mm (long camera) focal length. Low and medium resolution
(R - 4000) spectroscopy is achieved by using low order gratings
(150 lines/mm and 75 lines/mm are available) whereas high resolution
(R) is achieved by using an echelle grating 
. A circular variable filter (CVF) acts as an order-block for the
echelle.
The detector array was a 
InSb Santa Barbara Research Center (SBRC)
array until early 1995 and a 
InSb SBRC from then on. The
slit is
90
long and the width can be either 1, 2 or 4 pixels (the latter
is not available if the echelle grating is chosen).
Two readout modes can be used: single readout and non-destructive readout (NDR). Longer exposures benefit from using NDR since read-noise is reduced [Puxley et al. 1992].