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Digitization process

Digitization of the FBS plates started in 2002. After several test scans on a set of plates, all the FBS plates were carefully cleaned and digitized (the emulsion in contact with the scanner glass plate) with the scanner Epson Expression 1680 Pro. in transparency (positive) mode.

An `ad hoc" program, SCANFITS, written by Stefano Mottola (DLR - Institute of Planetary Research, Berlin;) allowed the resulting image to be written directly in FITS format with corresponding information about the plate in the header.

In order to obtain the zero for a transparency scale, a black paper sheet was used to cover one of the unexposed plate corners; the higher limit of the transparency scale was obtained by utilizing one of the clean unexposed corners. The data numbers (DN) range from 0 (dark) to 16383 (transparent); a 16-bit transparency range was used. In practice, the black corner counts were around 600 DN and the clear plate counts 14000 DN, thus corresponding to an effective 4.1 density range of scanning machines. The scanning resolution was 1600 dpi: so the pixel size is about 16 micron. The typical length of an FBS spectrum is 1.7 mm, corresponding to 107 pixels along the wavelength scale.