Scattered light in the UH8k camera at CFHT prime focus.

There is some concern about the incidence of scattered light in in UH8k images when used at CFHT prime focus. This light does not seem to be well behaved and observers should use caution when observing in the vicinity of a bright star.

The following images were obtained on the engineering night of 5 Dec 1996. They are all I-band images and taken near the star alpha Tri (V = 3.4). The images have been flat-fielded using dome flats but no correction has been made for dark current (which explains the occasional anomolously high level in the CCD at bottom right (chip 6)).

The scattered light is not symmetric with alpha Tri placed to the North and to the South. When placed to the South significant scattered light appears with a great deal of structure. When to the North, a broad fan of scattered light appears, with apparently regular fingers, suggesting scattering off a component with regular structure itself.

When the bright star is placed at about 50' away from the mosaic center, its beam begins to cross the edge of the middle element of the wide field corrector. This is a possible source of some of the scattering.

Unfortunately, another bright star appears in the image when alpha Tri is placed away to the North of the mosaic. However, in these images, a primary mirror "donut" appears which may originate from multiple reflections of this spurious bright star between the CCD dewar window and the filter.