Digitization and Distribution of the Large Photographic Surveys
Session 35 -- Digitizing the Sky Part I
Oral presentation, Tuesday, 31, 1994, 8:30-12:30

[35.01] Digitization and Distribution of the Large Photographic Surveys

As the scan archives corresponding to these plate collections occupy terabytes, some form of data compression is an essential element of their use and distribution. The most extreme compression is not truly a compression but the construction of secondary data products: object catalogs. While such catalogs in some form are invariably useful, many programs are best pursued with the image data. Two compression modes, both based primarily on controlled loss of sky-data, are used for this: (a) thresholding, wherein only image sections containing objects are saved, and (b) full-plate compression, wherein an algorithm such as the H-transform provides the basis for isolating and deleting some small fraction of the sky-noise. Both procedures easily produce compressions of order 10$\times$.