Is the astrometric calibration the same quality from image to image?
The main limitation on the astrometric repeatability is our knowledge of the fine-grained distortion map for each detector. As fields are observed at different roll angles, errors in the distortion map, especially at the field edges, may creep into the astrometric solution. Flight data is currently being used to refine the ground-data-generated distortion map, and several special observations may be planned for the purpose of observing a large number of FUV stars in order to complete this task. Currently, the astrometric precision is of order 1″, but there may be isolated areas at the edge of the field where the error is significantly larger. More details are given in Section 6 of the GALEX Observer’s Guide.
