DEEP SKY SURVEYS - A MOTIVATION FOR STACKING DIGITIZED PHOTOGRAPHIC PLATES

With the advent of fast scanning microphotometers and inexpensive digi tal mass storage, there has been a resurgence of interest in performin g deep (B less-than-or-equal-to 25) panoramic surveys by coadding larg e numbers (approximately 10(2)) of digitized photographic plates. Whil e the Kodak IIIa emulsions are highly linear recorders in photographic grain density, we demonstrate that the threshold and saturation level s which restrict the dynamic range of the emulsion can distort the hig her statistical moments of the grain density fluctuations (variance, s kewness, etc.) along the linear part of the characteristic curve. We i llustrate this effect with scanned density step wedges from both IIIa- J and IIIa-F photographic plates. The variance of the grain fluctuatio ns is only additive between digitized plates that preserve the Poisson ian grain noise. In order to correct for the statistical distortion, w e compute the variance of the fluctuations as a function of density fo r five scanning aperture sizes (2, 4, 8, 16, and 32 mum). The statisti cal effect, which reduces the linear density regime of a photographic emulsion, is particularly prominent with small scanning apertures and at high photographic density. We suspect that the statistical distorti on induced by the limited dynamic range is negligible for scanning ape rtures larger than almost-equal-to 12 mum which corresponds to almost- equal-to 0.8'' at the focal plane of major Schmidt telescopes.