There is no longer a ghost image present in PFCCD data. As PFCCD users from last fall (and readers of the Newsletter) remember, the deterioration of the Sol-gel anti-reflective coating on the new four-element corrector prompted a certain amount of consternation. A large "ghost image" of the primary was present at the 110% intensity level in both flat-field exposures and program frames; this extraneous light component was additive, and reduction purists were forced to subtract the ghost before flat-fielding their data. (The actual broad-band stellar photometry was unaffected at the sub-1% level.) Although such efforts were largely successful, the extra step had high nuisance value, and the immediate reaction of most observers at the telescope was to comment unfavorably on the aesthetic impact of the ghost image.
Accordingly, we decided to exorcise the ghost permanently and completely by offsetting the PFCCD camera 48mm (840") from the optical axis. As of this writing, we have completed our engineering tests of the camera in its new location, and are happy to report the following: 1) The image quality is excellent in the new location. The best images we've obtained are 2.0 pixels (through an "I" filter), equivalent to 0.84", and limited by undersampling. This is as we expected, since the corrector was designed to work well for the very large field of view (FOV) of the Mosaic camera, and our offset is modest compared to that FOV. 2) The telescope pointing is unaffected, despite the geometrical complications; observers and telescope operators do not have to do anything special to center on either the CCD or offset TV camera. 3) The ghost is most definitely gone, and flat-fields now look normal, with no scattered light component.
When the Mosaic CCD system is used, requiring the full field of the corrector, the ghost still will be seen at the field center. We expect to resolve the problems with the deteriorating coating over the summer, but until then, it will be necessary for Mosaic users to follow the more complex data reduction path.
We are grateful to Gary Muller and Rich Reed for the expert and quick engineering changes that brought the PFCCD camera to its new home, and to Bob Marshall and David Mills for software support.
Phil Massey, George Jacoby