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3.2 The Slit, Deckers, and Acquisition TVs

The slit is 25mm long and the entire projected length falls on the CCD. A millimeter micrometer sets the slit width with typical widths varying from 75 to 250 microns at a scale of 79 microns per arcsecond. Reading the micrometer can be tricky - note that the first 5 indicates 50 microns . It is also recommended to set the width going toward smaller numbers, to avoid sticky-slit problems. A Philips ICCD TV camera views the central 3 arcminutes of the slit at all times off the tilted slit jaws. An integrating video memory, "leaky memory", permits integration of the video signal. Under good conditions, stellar objects of V 18.0 can be seen with several seconds of integration. Programs requiring acquisition of objects fainter than 18.5 should come prepared with offsets to ensure success. There is also a wide field TV ( 5') available at an auxiliary port on the automatic guider.

The decker available for controlling the slit length are listed in Table 1.

  
Table 1: Available Decker Sizes

Scattered light may be an important consideration for some long slit problems. The decker will reduce the overall amount of light getting into the spectrograph. Also on the decker is a series of small holes for examining the spatial focus when high dispersion gratings are used.