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5.1 Registration Fiducial

Although the reseaux have traditionally been used for CROSS-CORR purposes, using the fixed pattern as the registration fiducial has several advantages over the reseaux; one being that fiducials can be found anywhere on the exposed part of the image, so that many more such fiducials are available than reseaux. Also, the fixed pattern can be detected at even the lowest exposure levels, which eliminates the need for predicted information. Finally, the presence of the exposed spectrum can be excluded from the cross-correlation region, and therefore, does not inhibit the use of the fixed pattern in the surrounding background (De La Peña et al., 1989, 1990). A consistent fixed pattern has been shown to be present in the literally thousands of raw IUE images tested; however, it is known that the character of the fixed pattern is modulated to some degree by observation date and camera temperature (Bushouse 1991b).

Karen Levay