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The Slit Area

Unless the spectrograph is being used with multi-slits or fibres the slit carriage should be in the long-slit position. If it is not in the correct position it can be driven there with the ICL command LSLIT. The commands SLIT <width(microns)> and DEKKER <position> allow the user to determine the width and length of the slit, in general for setting up one requires DEKKER 6 (long slit) or DEKKER 2 (20 arcsec) for focus and tilt adjustment; and dekker 1 (narrow) for rotation adjustment and determining the s-distortion correction for the IPCS. It is a mistake to make the slit too narrow for determining focus, if it is less that 120 microns then it will project to less than 2 CCD pixels, and the line profiles will be undersampled for the Hartmann test. A value in the range 120 to 180 microns is recommended.