On a regular basis, the observatory takes images of every slit through every filter. These images are processed with the eclipse recipe isaacp slitpos which measures the slit position.
The input file can be either a FITS file or an ASCII file containing a list of FITS files. If the input is a FITS file, it is assumed to be a slit image. In the latter case, the first file may be a dark, which would then be subtracted from all subsequent images.
The output consists of two files: a FITS table which records the location of the slit in great detail, and a PAF file that contains the slit angle and slit position. Figure shows the how the FITS table is formatted.