Motivation

 

The plate archive of Hamburg Observatory has a stock of about 45 000 photographic plates taken from December 1st 1911 until the end of the last century. So far this plate archive was not really usable for science due to the lack of searchable electronic meta-data.

The digitization of photographic plates started in 1984 after the installation of a PDS G1010 Microdensitometer and all plates of the Hamburg-Quasar-Survey (HQS) and the Hamburg-ESO-Survey /HES) were digitized for both projects.

Nearly all plates taken before were never digitized or digitally analyzed. Since now a complete digitization seemed to be impossible due the large amount of data ( about 30TB) and unrealistic long scan times.

 Photographic plates are so far the most long living storage media (more than 100 years but with decreasing quality) compared to about 10 years if one includes the life time of the modern reading devices.

 Nevertheless, there are good reasons to start a digitization just now (- degrading, + improving):

 1. - The oldest plates display aging effects of the emulsion at higher densities. Due to inappropriate storage peeling of the emulsion and mold infestation were found and emulsions are sensitive to scratches and dust.

2. - The number of people who used photographic plates is decreasing rapidly. That means that the scientific interest in the plates will decrease, though each plate is a unique observation especially interesting if variability in time or on position in the sky are observable.

3. + Costs for scanners and storage media are decreasing permanently.

4. + The long term storage of the digital data seems possible at acceptable costs.

5. + The provision of data in the Internet becomes possible due to increasing band-withs.

There are 3 groups of people with scientific interests for the data.

 Astronomical research: Series of plates from the same field offer the possibility for a computer based search. Many plates were taken just for one object to analyze. Most other objects are discarded. Drastically changes in stellar objects as outbreaks, or the change of the path of objects in the solar system will immediately increase the interest in former observations.

 History of Science: The collection of plates together with written information in logbooks or plate covers displays a century of astronomical research telling the story of observations, their problems, and the many new ideas to improve their quality. The aim to become UNESCO World Heritage will increase the public interest in accessible data.

Amateur-Astronomy: The free accessible data will be a treasure trove for thousands of amateur astronomers who now can observe the same objects with smaller telescopes but powerful CCDs.

The combination of a data base and a web server which displays all available information as high- and low-resolution-scan plus plate cover, observer notes and logbook pages will enable the search for and the use of plates and objects.

 

Method and Catalog

 

During a pilot project more then 3500 plates were scanned and the automatic creation of the web-pages were developed.

 As scanner the Epson Expression 10000 XL with transmission unit was chosen which can measure densities up to 3.8. The pixel size of 10.6 micron gives a reasonable scan resolution of 2400 dpi which is comparable to PDS scans. The somewhat smaller density range compared to the PDS seems to be sufficient for the historical plates. The only disadvantage is the positional inaccuracy in scan-direction which might be overcome by a second scan after rotating the plate by 90 degrees. Such a second scan is only necessary if the scans should be used for astrometric purposes. Further advantages of the scanner are an autofokus and an automatic calibration. The most important advantage is the short scan-time which allows to scan a length of 10 inch in about 7 minutes. The PDS will need more than 9 hours.

The total time for the scanning of plates, logbooks, and plate envelopes will amount to 15 years for one person. The use of several scanners and more scan-personnel may decrease the time to a couple of years.

One problem in cataloging the plates is the large number of telescopes, cameras and plate sizes. The following directory structure is used:

Scans / direct photography / telescope / plate-size / [ jpegs | x-scans | y-scans ]

Scans / spectra / telescope / plate-size / [ jpegs | x-scans / y-scans ]

where the names of the files are created from 2 characters for telescope initials (except Spiegelteleskop, S) and 5 digits.

Low-resolution-scans (.jpeg) are made in color if there are marks on the emulsion free side of the plate. In the other cases low-resolution JPEGs are calculated from the 16 bit gray-scale data of the high-resolution-scans. The latter one will be stored in FITS-format which is the standard data format used in astronomy.

 A scan-PC will store the data temporarily on a local hard-disk. After customizing the data they are collected on a RAID6-system. Additionally, the data are stored far away on a tape robot at the university to ensure long time storage. The storage costs will shrink with time as every couple of years the double amount of storage capacity is available for the same costs.

Using logbooks, observer notes and plate envelopes the meta-data are created. They will be used for the search function of the data base. Beside the plate number further information is collected in a table as Plate center, Object name, date and time of observation, exposure time, sky conditions, filter or objective prism.

The web pages of the catalog are at first divided in telescopes as the quality of stellar images differs strongly between telescopes(e.g. plates taken with the 1 meter telescope are only usable in the central region due to large coma effects of the mirror).

Behind the telescope-link tables can be found which comprise the meta-data.

Behind the plate number link all scans can be found on a single web-page for this plate.

The JPEG image of the low-resolution-scan enables a visual quality check so that high-resolution-data can be downloaded manually. The size of the FITS-data may be quite large, e.g. at 13cmx18cm 400 MB, at 24cmx24cm 1 GB and at 30cmx30cm 1,5 GB, therefore the size will be given in the download window.

Comfortable search functions using the meta data are available in the archive section. The additive effect of search parameters offers the possibility to define small groups of plates with similar characteristics as e.g. choosing for objective prism plates or direct plates with a special emulsion and used filter types.

 Additionally it will be possible to study the observer notes or the logbooks with a reader. I an interesting entry is found it is possible to directly jump to the web page of the correspondig plates.

 

Linkage and Identification

 

In most cases the plate number is used for identifying plates and envelopes. Nevertheless, there are exceptions as e.g. concerning the  plates of the Great-Refractor. In this case several series exist with plate numbers starting at "1". These plates were ordered by date and small letters are given replacing leading zero(s). Another problem is that not all observer notes are available for this telescope, and so only information given on the plate or the envelope can be used. In some rare cases even the date or the object is missing.

Even more complicated is the identification of spectral plates from the 1-Mirror-Telescope. Here, plate numbers start with letters as 'DS' or 'KS'. As the latter is already used as a telescope code for 'Kleiner Schmidt-Spiegel', these IDs were transformed in the following way: KS --> SK or DS --> SD, respectively. That's not a beautiful but a necessary approach as each identification number must be unique.

In cases were pages or envelopes do not carry any information, they are omitted.

The logbooks are a register for the plates, comparable to  the here used meta data. They were handwritten, mostly not by the observer. In cases preprinted tables were used it was possible to cut the appropriate row automatically and to link it on the web page additionally. In all other cases only the whole pages are linked, if existent.

 

General comments on photographic plates

Photographic plates are non-digital media and will not be linear concerning the density values compared to CCD-frames. At the time they were exposed special plates were exposed with spots ore stripes (in the case of spectra) with known brightness steps, partly already on the plate itself. Without such a calibration a brightness calibration was impossible. Today it is possible to calibrate the plates belatedly due to on-line available catalogs with millions of stars with known brightness and color gradient. Such a calibration may also be possible for objektive prism plates. Single spectra, instead, cannot be calibrated without available calibration spots.


Errors

All meta-data entries might be infected by typos despite careful inspection. By presenting all handwritten information together with the meta-data it is always possible to recheck their correctness. In any case a discrepancy or incorrectness is found, please inform the head of the project by e-mail.