DIGITAL STACKING OF PHOTOGRAPHIC PLATES WITH SUPERCOSMOS

Photographic Schmidt plates are among the most effective tools in wide -field astronomy. One of the principal difficulties of photographic pl ates when compared to modern detectors is lack of image depth. We pres ent a technique for stacking plates digitized using the SuperCOSMOS mi crodensitometer, aimed at maximizing the signal-to-noise ratio in fain t images. The efficacy of several image combination algorithms is test ed by stacking plate frames in the presence of spurious images. We fin d that an 'average sigma clipping' type pixel rejection in conjunction with our weighting scheme is most effective in delivering a clean, hi gh signal-to-noise ratio stack. The gain in limiting magnitude obtaine d from stacking is found to be consistent with that expected: Delta M similar to 1.5 for a stack of 16 good-quality plates.