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The Fabry-Perot infrared grating spectrometer, FIGS

 

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 13.5 and 7.0 respectively. This assumes full spectral sampling at 13 positions of the grating. The limit through the L' window is a steep function of wavelength.

An attractive and powerful feature of FIGS is the imaging mode. The focal plane is scanned across the FIGS entrance aperture, using both the telescope and the chopping secondary. The grating may be scanned at each point in the image, to give a full spectrum, or it may be held fixed so that 16 different wavelengths are imaged at the same time. Obviously in the former case the procedure is very slow, and one should not get ambitious ideas about the sky coverage. An estimate of the sensitivity can be gained from the figures for conventional spectroscopy. Allowance should be made for a minimum of four complete scans of the field being observed; more is better to smooth out irregularities due to seeing and sky variations.

Combining FIGS and IRPS

FIGS attaches to the telescope via the Figsmount, which also contains various mirrors and dichroics, a viewing TV, and the xenon comparison lamp. FIGSMOUNT, a routine run from the FIGS or IRPS accounts, controls the necessary motors, etc.

There are three output ports on Figsmount. FIGS occupies the uppermost one and the f/36 IRPS dewar can be fitted at the second. It is therefore possible to run IRPS and FIGS on the same night. Changing between the two instruments involves some recabling in the control room and usually takes about 5 minutes.



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