Plates digitisation

A pilot project aimed to digitise part of the photographic archive started in Asiago in 2001; in 2002 further funds were allocated by the Italian Ministry of University and Research to a national collaboration (MIUR/COFIN 2002, P.I. C. Barbieri University of Padova). Details can be found in this paper (PDF).

The Plates

In the '40s and '50s, different Italian emulsions were used; we have not been able to find data sheets on their characteristics. From the '60s, mostly Eastman-Kodak emulsions were used, whose spectral response is as in Fig. 1.

Fig. 1. The spectral sensitivity of the Eastman-Kodak emulsions used in Asiago

Emulsion -E (not shown in the figure) was intermediate between D and F, and was often used to define the R-band in conjunction with the Schott filter RG1.

Regarding filters, Schott material was generally employed. The following combinations defined the Johnson UBVRI broad-band system:

U: emulsion -aO + UG1 or UG2
B: emulsion -aO + GG11
V: emulsion -aD + GG 13
R: emulsion aE or aF + RG1
I: emulsion IN + RG5

During the years, several filters changed name (e.g. Schott GG13 became GG385, GG11 became GG495).

For details, see: The Asiago Data Base on Photometric systems (U. Munari, M. Fiorucci and D. Moro, 2002) ADPS

Scanning Hardware and File Size

Digitisation is performed with 3 commercial scanners: two Epson 1640 XL, A3 format, optical resolution 1600x3200 dpi, 14 bit/px and one Epson Perfection 3200 PHOTO, 3200x6400 dpi, 16 bit/px for an area of 4x9 in. Each scanner is connected to a PC were a dedicated software interfaces with the scanners' driver to produce as output a negative or positive FITS image.

Tests have been performed on different types of plates, both with images and spectra, to determine the effective spatial resolution of the scanners. The result is 16 micron/px at 1600 dpi, which is sufficient for direct image and objective prism plates, while for spectroscopic material and for the Schmidt 40/50 plates, the resolution of 3200 dpi is necessary, with 8 micron/px. The approximate dimensions of the digitised files at 1600 dpi are:

- - - Schmidt 67/92 plates (20x20 cm) 260 MB, 12000x12000 px
    - 182 cm plates (20x12 cm) 150 MB, 12000x8000 px
    - Schmidt 40/50 films (10 cm diameter) 100 MB, 7000x7000 px
    - 122 cm plates (9x12 cm) 70 MB, 8000x6000 px

JPEG sample of a plate of M33 from Copernico 1.82m telescope (662 KB)

JPEG sample of a plate of M42 from Schmidt 67/92 telescope (620KB)

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