Astronomical Plate Archives as Supplementary Data for INTEGRAL 
  René Hudec  

We report on the recent efforts to digitize the sky plate archives and to use these data for various scientific projects including analyses of objects observed by INTEGRAL such as blazars, cataclysmic variables, galactic X-ray sources, and other objects. We also address and discuss the status of the development of related algorithms and software programs. These data may easily provide very long term monitoring over very extended time intervals (up to more than 100 years) with limiting magnitudes between 12 and 23.


There are nearly 3 millions astronomical archival plates located at different observatories. These plates represent a unique database for various scientific projects including INTEGRAL.

These archives can easily provide thousands of exposures for any celestial position, reaching monitoring intervals of up to few years of continuous monitoring - i.e. tens of thousands of hours.

The recent efforts to digitize the plates and the corresponding software development significantly facilitate the extraction of unique scientific data from archival records and related reductions and analyses.

The photographic sky monitoring is available for more than 100 years. However, only the recent development of photographic scanners and powerful computers allows an efficient extraction of scientific data.

Some of the archives have very high quality plates achieving limiting magnitudes of up to 20 … 23 (direct imaging) and /or 17….19 (spectral with objective prism).


The very rare inactive state orbital light curve of the X-ray binary Her X-1/HZ Her based on measurements on Sonneberg Observatory astrograph plates. The behaviour of the object in the inactive state can be studied only on the archival plates, since the object is in active state already for about 50 years. The absorbing structure (disk) around the neutron star is clearly visible in the curve.

The long-term evolution of AM Her on Sonneberg plates. Such long-term light curves can be extracted only from archival plates.

Recent efforts: digitisation and automated evaluation
  • Some of the archives have devices for digitisation of plates
  • Some of the archives have started projects to develop high quality scanners to convert all plates into files/CD ROMs (e.g. The Royal Observatory Brussels).
  • There are efforts to use these data for automated evaluation of objects on the plates and creating their light curves
  • There are attempts to create an European Plate Centre in Brussels, Belgium ( the UDAPAC project, for more details see
  • All interested people can contact us at René Hudec
  • The photographic sky patrols still in operation

    Two of the photographic sky patrols are still in operation, regularly taking photographic patrol images every clear night. The Ondrejov all sky patrol operated for monitoring of bright meteors has a sensitivity of mag 12 in the best case and a very large sky coverage (full visible sky hemisphere). The Sonneberg sky patrol is operated for variable stars simultaneously in two colours with limits 14 ... 15 but a less extended sky coverage. Both programs exhibit suitable scanners to digitize and to evaluate the patrol images.

    Scientific use of archival astronomical plates - a valuable tool in modern astrophysics

    Archival and sky patrol plates represent a valuable tool in investigations of various types of high energy sources such as blazars/quasars, X-ray binaries, X- and gamma-ray transients, etc. It is obvious that the automated evaluation of sky patrol plates has large potential in:

  • providing extended monitoring intervals with good sampling,
  • allowing long-term evolution and changes to be studied
  • searching for optically variable AGNs-QSOs-blazars and other objects
  • providing their light curves with good sampling
  • searching for their flares
  • providing simultaneous and quasisimultaneous optical data for satellite campaigns, even back in time
  • monitoring of objects as base for proposals for ToO (Target of Opportunity) for satellite high energy observations
  • providing extended database for identification and classification of sources
  • The detection and investigation of very large amplitude flares from AGNs may serve as an example. There is increasing evidence that some AGNs may exhibit very large amplitude flares exceeding mag 10 (Hudec et al. 1997). These large flares are however rare so very large fractions of monitoring times (of order of thousands hrs or more) are required to detect them. This can be accessed easily on plates but hardly by other methods.

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    Recognizing the star images on archival plates by innovative flood algorithm
    Identification of high-energy sources

    The recent high-energy satellites, especially those observing in X-rays, provide a large number of detected sources. The identification of them is an important but not easy task. Astronomical plate archives can help essentially.

    The ROSAT catalogues list more than 200 000 X-ray sources detected and 2000 sources with variability factor more than 3. A large fraction of faint X-ray sources is variable.

    The use of deep archival sky patrol plates provides an important tool for identification of especially variable X-ray sources since they provide large coverage in time and, in some cases, colour information. So can archival plates be used to look for peculiar (mostly variable) optical objects at the positions of X-ray sources.

    Some collections (Hamburg, Sonneberg, Edinburgh,...) include also spectral plates taken by objective prism. This can also provide important tool for object identification and classification.

    Some of the X-ray objects detected by recent analyses can be very easily rediscovered and further studied - even back in the history - on high quality direct and spectral archival plates.

    The software development

    So far, the data recorded on archival plates were accessible only by special procedures. The recent wide digitisation of plate collections offers significantly easier access by computers. However, there is still a gap between the digitised archive and the scientific use. Special software is required to fill this gap.

    We have developed new algorithms to access data on digitised plates and have tested these techniques in trial sets of digitised plates from the Sonneberg Observatory sky patrol archive.

    The new algorithm is based on the flood method. This method, applied to the digitised photographic plates, is able to reveal the star images for further analyses. This method has been tested on a set of digitised Sonneberg Sky Patrol plates with very promising results if compared with other methods. The flood method is based on a similar idea as the watershed method and is reasonably simple and quick.

    Plates and INTEGRAL

    The importance of the plate archives for INTEGRAL is that for objects optically brighter than mag 23, there is a chance that the light changes of the object can be analysed over years and often even over decades. This can provide valuable additional scientific data for complex analyses and for better understanding of evolutions of the objects as well as of underlying physical processes. The chance for availability of archival data increases with optical brightness of the object.

    In addition, the photographic sky patrols still in operation (Ondrejov and Sonneberg) can provide real-time coverages and pre-burst images for GRBs detected by Integral.

    The European initiative - the UDAPAC project

    The recent UDAPAC Project represents the first effort to create an European Plate Centre with related facilities, staff, software and expertise to extract scientific information from archival astronomical plates. The UDAPAC plate centre is expected to be located at the Royal Observatory in Brussels, Belgium, with many other scientists involved (

    The North American Initiative at PARI

    A new initiative at PARI (USA) is archival and digitization of astronomical photographic plates. Observatories and universities are space limited and some seek a safe home for the legacy left by so many astronomers. PARI provides space, infrastructure, and Internet access. The goal is to make the archive a resource harnessed by present and generations of astronomers. The proposed solution:

  • Safeguarding the information means minimizing the information loss
  • Purpose-built, rapid scanners are required
  • Laboratories will be equipped at PARI to digitize North American plates
  • PARI will also be developed as a long-term storage facility for North American plate archives


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