THE SCANS

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.,
However, there are ten POSS E plates (XE203, XE206, XE284, XE509, XE515, XE524, XE541, XE543, XE566,XE574) that were instead scanned with pixel size of 24.996 microm using the refurbished ST ScI microdesitometer, and two SERC J plates (S208, S512) that were scanned with a rectangular pixel 25.284 microm x 25.000 microm in size.

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 1965).
Of the 645 scans of northern plates , 3 required deshering and 133 required dechopping.
Of the 896 scans of southern plates, 14 required deshearing and 34 required dechopping.

The keywords DCHOPPED and DSHEARED in the headers of the image files indicates the image repair status.
In general, repaired scans are indistinguishable from good scans with small or negligible digitization artifacts.


Return