Archives of Photographic PLates for Astronomical USE

Digitization of photographic plates at the Dr. Remeis-Sternwarte Bamberg

1. Selection and allocation of the plate (all single glass plates are stored within individual paper envelopes and sorted by their observational time and coordinates in metal cupboards in a separate building).

2. Retrieving the metadata from the WFPDB

3. Low resolution (300 dots per inch, dpi) color scan of the envelope. For this purpose we are using an Epson V750 Pro flatbed scanner and the standard XSane program package on a Linux based personal computer. The resulting files are stored in the Portable Network Graphics (PNG) format.

4. Verification: is the plate in its corresponding envelope? On each plate there are marks denoting the plate number, observation time (year, month, day), and sky coordinates of the center of the observed field. On the paper envelopes all relevant observational data are noted, like, e.g., the plate number, the observation time (local, sidereal, and Julian date), the sky coordinates of the center of the observed field, the observers name, the photoemulsion, observers notes (e.g. weather condition, moon phase, quality of the plate, tracker problems, ...), and sometimes also hints and notes from former analyses.

5. Low resolution (300 dpi) color scan of the plate itself using an Epson Expression 1640XL flatbed scanner, and the standard XSane program package. Again, the files are stored in the lossless PNG format.This scan is meant to document all markings and inscriptions, possibly left over on the plates from former analyses

6. Cleaning of the plate: using a damp microfiber cloth the backside of the glass plates is cleaned from dust which accumulated during the former decades on the plates. All markings on the plate will be removed by this cleaning. This is a prerequisite for automatic computer based analyses, because markings could eventually be treated as false detections, or they can simply mask important discoveries which have been overlooked before. However, such markings are documented by the low resolution scan (see the previous item).

7. High resolution (2,400 dpi = limiting hardware resolution of the scanner) grayscale (16 bit) scan of the cleaned plate. For this purpose we are using an Epson Expression 10,000XL flatbed scanner, and the professional version of the program VueScan on a Linux based personal computer. The resulting files are stored using the lossless Tagged Image File (TIF) format. To save time we always scan two of the 16x16 cm photographic plates at once, side by side perpendicular to the movement of the scanners arm. This, indeed, requires a more careful and much more precise placement of the glass plates on the scanner, but a lot of time is saved by this procedure. The individual scans of the two plates are cut-out afterwards using the computer program ImageMagick and stored using the Flexible Image Transport System (FITS) format, which is commonly used in the astronomy community. In addition to each high resolution scan of the plates, a TG13 gray wedge is also scanned in order to measure possible time variations of the scanner's transparency unit.

8. Storing the plate back to its original location.

To speed up the digitization process the time necessary for the high resolution scan of one plate is used to clean already the next plates to be scanned. Altogether this results in an average turn-around time for one plate of about 6-10 minutes; the more markings, inscriptions and stains are on a plate, the longer it takes.

See also Digitization of photographic plates at AIP