Deep sky surveys: A motivation for stacking digitized photographic plates

With the advent of fast scanning microphotometers and inexpensive digital mass storage, there has been a resurgence of interest in performing deep (B less than or equal 25) panoramic surveys by coadding large numbers (approximately 10²) of digitized photographic plates. While the Kodak IIIa emulsions are highly linear recorders in photographic grain density, we demonstrate that the threshold and saturation levels which restrict the dynamic range of the emulson can distort the higher statistical moments of the grain density fluctuations (variance, skewness, etc.) along the linear part of the characteristic curve. We illustrate this effect with scanned density step wedges from both IIIa-J and IIIa-F photographic plates. The variance of the grain fluctuations is only additive between digitized plates that preserve the Poissonian grain noise. In order to correct for the statistical distortion, we compute the variance of the fluctuations as a function of density for five scanning aperture sizes (2, 4, 8, 16, and 32 micrometers). The statistical 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 distortion induced by the limited dynamic range is negligible for scanning apertures larger than approximately equal 12 micrometers which corresponds to approximately equal 0.8 sec at the focal plane of major Schmidt telescopes.