The plates were digitized using the ST ScI scanning microdensitometers. A pixel size of 25 um with a 50 um apodized aperture was used throughout. The pixel data are on a photographic density scale, with density 5 being (2^15) - 1, right adjusted in a 16 bit word.
Therefore, to convert data values (DN) to photographic density (gamma) use the expression:
gamma = DN / 6553.4
The nominal 25 micron pixel size is actually 25.284 microm (1.70" at the
Schmidt platescale of 67.2" mm^-1), which is a natural unit in the system
of the HeNe laser used for positional measurement.,
The scans have undergone extensive quality assurance checks.
Two artifacts that can be introduced during digitization are chopping
(misalignment of odd and even pixels within individual columns) and shearing
(misalignment of odd and even scan rows). Algorithms to measure these
effects are based on the symmetries of the appropriate correlations.
Fourier methods are then used toachieve the necessary repairs.
The deshearing is a simple application of the shifting theorem, while
dechopping is a special case of the ``interlace''problem (Bracewell
The keywords DCHOPPED and DSHEARED in the headers of the image files indicates
the image repair status.