Archives of astronomical photographic plates
Through more than a century photographic plates (mostly glass plates with photographic emulsions) were used to record information on celestial objects. They were utilized both as detectors of light collected by astronomical cameras and telescopes as well as data storage devices. Applications range from direct imaging, through photometry using transmission filters to recording spectra of celestial objects.
Wide-field cameras are used to survey sections of the sky for moving objects such as asteroids and comets as well as for light variable and transient source. Objective prisms were used to record low-resolution spectra of many stars in wide fields mostly with Schmidt-cameras and -telescopes. On small scales photographic imaging is important to investigate the structure of extended celestial objects and measure accurate positions. High-resolution spectra of single objects have also been recorded on photographic plates. Long-term monitoring is a necessity to study variability on different time-scales as well as for astrometry (parallaxes and proper motions).
Hudec (2015) estimates that more than seven million astronomical photographic negatives are stored in many collections world-wide. Most of information contained on those plates has never been extracted and analyzed, because of the huge number of objects per plates; some plates contain hundreds of thousands of stars. Compared to modern CCD detectors the photographic plates are very large, typical sizes range from 16 times 16 cm to 30 times 30cm for Schmidt plates (e.g. at the Thüringer Landessternwarte). Because the spatial resolution of the emulsion is 5 to 10 micrometer, such plates contain up to 20000 Megapixel. This enormous amount of information can only be harvested by digitization if metadata from plate envelopes and logbooks are available and incorporated into the database.
Because of the sheer size of the data sets, data handling and processing still is a challenge. The raw data need to be processed, cleaned from artefacts and calibrated both astrometrically as well as photometrically. The multitude of astronomical applications, however, implies that there is no silver bullet for the calibration problem of the data sets.
Internationally strong efforts have been undertaken. For example, in Great Britain, the Royal Observatory Edinburgh (ROE) had been operating a specialized machine to digitize photographic plates since 1967. It was upgraded to become the COSMOS (COordinates, Sizes, Magnitudes, Orientations and Shapes) machine and replaced by a new SuperCOSMOS machine in 1993. These machines were used to digitize the major large Schmidt surveys and an important data archive was developed which proved to be a very important research resource, easily accessibly through the SuperCOSMOS Science Archive maintained at ROE.
The largest plate collection in the world at Harvard Observatory (USA) hosts half a million astronomical plates taken over a time span of more than 100 years. The Harvard Observatory built an expensive scanner to cope with the huge amount of material, started the digitization in 2003 and makes calibrated data available through the archive named Digital Access to a Sky Century @ Harvard (DASH).
In Germany, well-kept archives still exist at many astronomical institutions. Some major ones (Sonneberg, Heidelberg, Tautenburg, Jena) have already been digitized at least partly, but are not easily accessible. A team of German astronomers from the Leibniz Institute for Astrophysics (Potsdam) and the universities of Hamburg (Hamburger Sternwarte) and Erlangen-Nürnberg (Dr. Remeis-Sternwarte, Bamberg) has embarked on a project to digitize their own archives and integrate them into a data base that complies with the Standards of the international Virtual Observatory. Unlike the SuperCosmos and DASH examples, which employed expensive specialized scanners, the German team uses commercial flat-bed scanners, which have been proven to deliver accurate enough resolution for scans to process the digitised images further at much lower costs. Subsequent post processing of the digitzed plates with modern software will deliver useful catalogued information.