Stellar Positions from CCD Images

Abstract

The CIDA visual refractor of 65-cm aperture and focal length 10.5 m, has been used extensively for position determinations on photographic plates. The combination of Kodak D plates and a yellow filter permit an almost perfect adaptation to the focal curve of the telescope. It appeared of interest to test whether the telescope could be used for astrometric purposes with a CCD detector. As is well known, the spectral sensitivity of these detectors extends well into the infrared where the images formed by the telescope optics will be far out of focus. The blue spectral region where this would also be the case can easily be cut off by a yellow filter. There are no filters which would produce a similarly sharp cut-off towards the red region. On the other hand, given the small field covered by a CCD, the displacement of the red out-of-focus image with respect to the center of the visual image might be negligible. Recently obtained accurate positions in the area of the Perseus Double Cluster made this field suitable for the test of this possibility.