On GMOS-S MOS data reduction: fixing MDFs

After the GMOS-S detector troubleshooting earlier this semester, the physical location of the array has shifted by a small amount in the -direction. While this did not have harmful consequences, the Y-shift has the effect of the MOS mask slits as defined on the MDF appearing with a small (~20 pix) yet systematic offset with respect to the actual location of the spectra. This may cause some difficulties upon extraction of the slitlets: as a result, when reducing MOS data (for data taken after July 2022), the individual extensions extractions (corresponding to individual slitlets) will look truncated. This brief document describes how to get round this. Don't hesitate to get in touch with your Contact Scientist, or submit a Helpdesk ticket, if you have questions.