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A great amount of highly valuable information is stored in the Asiago Observatory photographic archive, with plates dating back to 1942 and reaching 1994, when the use of photographic material was discontinued at all telescopes (with few exceptions in the following years until 1998).
The total amount of plates and films, obtained with four telescopes (122-cm since 1942, Schmidt 40/50cm since 1959, Schmidt 67/92cm since 1965, 182-cm since 1973) exceeds 70.000. A proper digitation of this veritable treasury is therefore of paramount importance, both for the preservation of their volatile support and for the fuller exploitation of the scientific content.

The University of Padova started in 2001 a pilot project to digitize the plates; in 2002 further funds were allocated by the Italian Ministry of University and Research to a collaboration involving also the Observatories of Torino, Monte Porzio (Rome) and Catania, the University of Rome I and the Specola Vaticana at Catelgandolfo (MIUR/COFIN 2002 to C.Barbieri, Dpt. of Astronomy, University of Padova).
Work is proceeding regularly at all sites. Details can be found in Poster SAIt April 2002 and Jenam 2003 (PDF files). 

The Plates

The original log-books of the Asiago photographs have been well kept and ordered. Their content has been almost entirely transferred to electronic files available in PDF and ASCII format. Note that in the 122-cm spectroscopic logs and in all the 182-cm logs, U.T. is given at mid exposure.

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

At present, digitation 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 with 1.5 GB of RAM and at least 120 GB of hard disk. A dedicated software, working in the Windows operating system, interfaces with the scanners' driver to produce as output a negative or positive FITS image (with customizable header), that can be directly analyzed with the most common astronomical software (e.g. MIDAS, IRAF, IDL).
Two scanners are in Asiago, and one in Padova. A fourth scanner, Epson 1680 Pro, A4 format, 1600x3200 dpi, is available in Asiago; it belongs to the University of Rome, and it is dedicated to objective prism plates.

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 digitized 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

File Distribution

The size of the files poses a serious problem, both for storage and for distribution. At the moment, we are storing the files to a Network Attached Storage (NAS) unit, using as backups DVDs of 4.7 GB each. The NAS unit accepts NFS, FTP and Windows protocols. The present capability is 540 GB, in configuration RAID 5 without hot spare, and it can be expanded to 0.8 TB.
This unit will be available on-line by mid 2004. For the time being, to obtain the digitized FITS file of particular plates via FTP, please contact the Digitation Project.

An Excel file with a list of already digitized plates is updated regularly.

JPEG sample of a 182-cm telescope plate of M33 (662 KB)

  JPEG sample of a Schmidt 67/92-cm plate of M42 (620KB)

FITS sample detail (right-click to download, 3.6 MB)