Problems may also be related to the pipeline processing or to association errors.

Photometric Accuracy and Flat-Fielding

The twilight images of FORS2, which are used to create master sky flats, show evidence for a feature, whose position depends on the rotator angle (see overview). This limits the achievable photometric accuracy to about 5%. For more information please look at the FORS Secondary Standard and Absolute Photometry Project.

Header keywords for MXU calibration data (until 2006-11-13)

When a sequence of calibration data for different masks is taken, data with a low number of slitlets taken after data with a high number of slitlets would inherit the additional slitlet keywords from the previous observation.

Example: Calibrations for a mask with 20 slitlets are taken after calibrations for a mask with 47 slitlets. In that case the calibrations for the mask with 20 slitlets would inherit the follwoing keywords for slitlets 21 to 47 (33 being the number of the slitlet):

• HIERARCH ESO INS TARG1## NAME
• HIERARCH ESO INS MOS1## RA
• HIERARCH ESO INS MOS1## DEC
• HIERARCH ESO INS MOS1## WID
• HIERARCH ESO INS MOS1## LEN
• HIERARCH ESO INS MOS1## POSANG
• HIERARCH ESO INS MOS1## WIDTH
• HIERARCH ESO INS MOS1## SHAPE
• HIERARCH ESO INS MOS1## XPOS
• HIERARCH ESO INS MOS1## YPOS

The problem has been fixed on November 13, 2006.