Correcting the grism image WCS coordinates

For more than 95% of 1500 images tested, the uvotgraspcorr ftool will find a solution to correct and refine the aspect solution provided by the Swift star trackers.

What to do for images where it did not work?

One way is to make an adjustment by hand to the wcs_s ra and dec positions in the header until the star catalog positions line up with the zeroth orders. At the minimum the header must then have the SIP keywords that describe the distortion of the zeroth order positions on the detector. These are put in by uvotgraspcorr, even when not successful in finding a correction.

• At the moment, this needs to be written.

A second method is to make a small star catalog for a selection of the stars in the field that have corresponding zeroth orders, and use that in the call to uvotgraspcorr using the CATSPEC parameter.

• At the moment, this needs to be written.