Recently, it was discovered that there were significant errors in the distortion corrections implemented for the MAMA imaging modes. An improved correction for the NUV MAMA modes was implemented with the delivery of an improved IDCTAB reference file (nas13578o_idc.fits) on 2003-Oct-29. This new geometric distortion correction is described in STIS ISR 2004-01.
Note also that a bad NUV IDCTAB file was erroneously delivered on 2004-Mar-31, and was replaced with o421844bo_idc.fits (a copy of the correct file) on 2004-Apr-06. Users who requested NUV MAMA imaging data in early April 2004 should check the IDCTAB keyword to insure that their data were not calibrated using the bad IDCTAB, o3p1435io_idc.fits, and, if necessary, re-request their data from the archive.
A similarly improved distortion correction for the FUV MAMA imaging modes is still pending. In the interim, a new IDCTAB reference file (o471334ro_idc.fits) was delivered on 2004-Apr-08, which applies an overall correction to the image by assuming that the FUV MAMA has rectangular pixels with the sizes given in Table 5 of STIS ISR 2001-02, and by resampling them into square pixels of an appropriate intermediate size between the x and y ones. Until a full rederivation of the FUV MAMA distortion solution is performed, this should provide an improvement over the previously implemented IDCTAB corrections.
Please check the STIS Reference File History page for the latest reference file deliveries.