Data Reduction Frequently Asked Questions

Why do I get the message that "no slits could be detected", even though in the raw frames I can clearly see the slits?


This problem usually occurs if you try to reduce FORS2 observations taken before April 1, 2002. At that time a CCD with a different pixel size was used (25 micron instead of 15 micron). You will need to manually override the value for the dispersion [in Å/pixel] from this table by specifying the --dispersion parameter in the fors_calib recipe. The value for the old CCD is 5/3 of the one in the GRISM_TABLE.