Introduction

IRSPEC is a cryogenically cooled grating spectrometer equipped with a relatively small (for optical CCD standards) 2D array of 62x58 pixels. It covers the 1 $\mu$m to 5 $\mu$m wavelength range and the two back-to-back gratings - #2 optimized for observations at $\lambda\le2.5$ $\mu$m and #1 for longer wavelengths - yield a dispersion of $\simeq5$ Å/pixel in H and K (1.4-2.5 $\mu$m), $\simeq10$ Å/pixel in L,M (3-5 $\mu$m) while in J (1-1.3 $\mu$m) one can choose between $\simeq2$ Å/pixel (grating #2 order 2) or $\simeq3.5$ Å/pixel (grating #1 order 3). In all cases the resolving power is quite high and each frame covers a small wavelength range; obtaining complete spectra of an atmospheric window requires $\sim$20 images at different grating positions. Along the slit the pixel size is $\simeq2.2$ arcsec and the total, usable slit length is slightly less than 2 arcmin; small areas at the edges of the slit are vignetted.

Although there is no standard procedure for the observations, it is generally agreed that the most convenient strategy to subtract the strong sky emission - OH airglow up to 2.25 $\mu$m and thermal + molecular bands beyond - is to alternatively measure the `object' and the `sky' and subtract the frames (note that the `sky' frame may also be taken with the object in another position along the slit). On chip subtraction of the sky emission - - the procedure normally used for optical CCD's - gives worse results due to the large intensity of the sky background combined with the relatively bad cosmetic quality of the infrared arrays. The object-sky telescope nodding is typically done every few minutes and one or more frames per telescope positions are produced and stored. At the end of the night, therefore, one has collected several hundred images which may also include data taken at many different wavelengths. This document describes a few procedures (commands) developed to make the handling of the many frames somewhat easier. For each command I did my best to provide you with long and exhaustive help files which are available on line. Here I just briefly describe the steps to follow to take full advantage of the commands available (remember to type SET/CONTEXT IRSPEC to access them).