FIGS is described by Bailey et al. (1988 PASP 100, 1178) and in
AAO UM 22: FIGS. It contains 16 InSb detectors widely spaced
(for electrical isolation) across which the grating is scanned
periodically to give complete coverage. The standard
configuration offers a choice of two gratings, which can be
interchanged in about one minute, to give spectral resolutions
around 400 and 1000. FIGS is a Cassegrain instrument, and uses
the f/36 chopping secondary (see § 2.3.4). As in most
spectrographs, the spectral resolutions depends on the aperture.
Square apertures from 1.0" to 5.9" are available, with 3.5" the
most commonly used. In poor seeing the dominant source of noise
on moderately bright objects becomes seeing noise, due to the
changing transmission through a small aperture as the grating is
scanned. The sensitivity limits for the low-dispersion grating
are 3 sigma per channel after 30 minutes integration at J,H,K=
14.5 and L' using the low-dispersion grating. For the high-
dispersion grating the figures are 13.