Introduction

The Cerro Tololo Infrared Spectometer (IRS) uses a 256$\times$256 InSb array by Santa Barbara Research. The array is operated under the WILDFIRE controller system built at Kitt Peak. A liquid nitrogen cryostat contains the spectrometer optics and the detector which is cooled to 35K using a closed cycle cooler. The optical layout of the IRS is explained in DePoy et al. (1990, PASP, 102, 1433). It has a 66mm diameter cassegrain collimator and a 300mm focal length parabola for a camera. The IRS has 3 stepper motors to configure the instrument (slit, filter and grating tilt). They are computer controlled but are also encoded by mechanical Durant counters so the mechanism position can be read from outside of the dewar. The IRS is currently only available for use on the CTIO 4m telescope at f/14.5 with the new tip-tilt secondary mirror. To accomodate the f/14.5 beam from the telescope, the foreoptics of the instrument were completely modified. The new foreoptics include a cold Lyot stop as well as f-ratio changing optics. The design leaves the scale at the slit and at the detector unchanged, however.

Telescope Scale

Telescope	Slit	Detector	Slit length (50pixel)	Focus
	arcsec mm-1	arcsec pix-1	arcsec	Approx
4m	2.01 ''/mm	0.32''	16''	456000

Light from a star is directed into the IRS by a warm dichroic mirror mounted in the ``tip-tilt box". The blue light that is not passed into the IRS is sent to a modifed, high-speed CTIO CCD TV camera. See Section 1.13 for a discussion of modes of acquisition and guiding including tip-tilt.