«Digitization and Scientific Exploitation of the Italian and Vatican Astronomical Plate Archives Cesare Barbieri1, Carlo Blanco3, Beatrice ...»
Digitization and Scientific Exploitation of the Italian and Vatican
Astronomical Plate Archives
Cesare Barbieri1, Carlo Blanco3, Beatrice Bucciarelli4, Regina Coluzzi5, Andrea Di Paola5, Luciano Lanteri4, Gian
Luca Li Causi5, Ettore Marilli10, Piero Massimino10, Vincenzo Mezzalira1, Stefano Mottola7, Roberto Nesci6,
Alessandro Omizzolo2, Fernando Pedichini5, Francesca Rampazzi1, Corinne Rossi6, Ruggero Stagni1, Milcho
Tsvetkov8, Roberto Viotti9
Department of Astronomy, University of Padova 2Specola Vaticana, Castelgandolfo 3Department of Astronomy, University of Catania 4INAF Torino 5INAF Roma 6Department of Physics, La Sapienza University, Roma 7DLR Berlin, Germany SSADC, Bulgarian Academy of Sciences, Sofia, Bulgaria IASF-CNR, Roma INAF Catania Abstract There is a widespread interest to digitize the precious information contained in the astronomical plate archives, both for the preservation of their content and for its fast distribution to all interested researchers in order to achieve their better scientific exploitation. This paper presents the first results of our large-scale project to digitize the archive of plates of the Italian Astronomical Observatories and of the Specola Vaticana. Similar systems, composed by commercial flat-bed retro-illuminated scanners plus dedicated personal computers and acquisition and analysis software, have been installed in all participating Institutes. Ad-hoc codes have been developed to acquire the data, to test the suitability of the machines to our scientific needs, and to reduce the digital data in order to extract the astrometric, photometric and spectroscopic content. Two more elements complete the overall project: the provision of high quality BVRI CCD sequences in selected fields with the Campo Imperatore telescopes, and the distribution of the digitized information to all interested researchers via the Web. The methods we have derived in the course of this project have been already applied successfully to plates taken by other Observatories, for instance at Byurakan and at Hamburg.
1 Introduction A great amount of highly valuable information is stored in the photographic archives of many Italian Observatories and in the Specola Vaticana, with plates dating back to the end of the XIXth Century. A proper digitization 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 (see e.g. Viotti et al., 1993, Griffin, 2001). Following an encouraging pilot program (see Barbieri et al., 2002), we set up a collaboration among our Institutions on the basis of common scientific and technological interests. Details are provided in Barbieri et al. 2003a, and Barbieri et al. 2003b. Among the many possible scientific aims for exploiting digital
archives, we intend to carry out the following :
• search for past transits of asteroids and comets for a better reconstruction of their orbital and physical evolution,
• discovery and inventory of high proper motion stars,
• time history variable stars in the Milky Way and external galaxies, of AGNs and QSOs,
• inventory of novae and supernovae in external galaxies,
• spectral classification over wide fields.
We present here a report of the activity carried out so far, some results and plans for the future.
2 The photographic archive census The first step of our work was the inventory of the plates contained in the archives of our Institutions and the digitization of the logbooks. As shown in the following Tables, the total number of plates is very large, too large to be digitized in its entirety in a reasonable amount of time. A visual inspection of the material was therefore preliminarily done, in order to select the best material according to the priorities set by our scientific interests.
2.1 Asiago Observatory The Asiago archive comprises the plates obtained by four telescopes: the 1.2 m, the 1.8 m, the 67/92 cm Schmidt and the 40/50 cm Schmidt (see Table 1). The archive is well ordered and the plates conserved in a satisfactory manner. The logbooks of all direct images and objective prism plates have been digitized, and are accessible on-line as PDF or Excel files in the Archive section of each telescope (www.pd.astro.it/Asiago). Their utilization by the international community is already very active. The logbooks of the spectroscopic observations (122 cm and 182 cm telescopes) will come in the near future
2.2 Torino Observatory The photographic material of Torino Observatory consists of approximately five thousands plates (images only, no spectra) obtained with different telescopes during a period of about 70 years, as early as 1923 (see Table 2) in Pino Torinese. This rich material is very heterogeneous in terms of its state of conservation and potential astrophysical impact; at present, it is systematically inspected and suitably archived in chemically inert envelops; an electronic logbook is in preparation.
Table 2 – The Torino photographic archive content (~5000 plates) Telescope plate dimensions period No. plates 20 cm Zeiss astrograph 18x24cm 1923 – 1984, 3000 38 cm Morais refractor 20x20cm 1971 – 1980 1000 105 cm REOSC reflector 16x16cm 1971 – 1994 1000 In addition, some 1000 plates are in store, taken with such different telescopes as GPO/ESO, JKT/La Palma, Astrograph telescope/Cape Town for different scientific purposes, from asteroids search to QSO variability. Of particular interest, given their relatively large field size and magnitude limit, we judge the series taken at the Zeiss and GPO/ESO.
2.3 Catania Observatory The Catania photographic archive consists of direct images (no spectra) taken in the original location in town (Piazza Vaccarini) and more recently on the Serra La Nave (Mt. Etna station, now M.G. Fracastoro station) of the Catania Observatory. The old plates were stored in wooden boxes without any protection, and many have been lost to humidity and fungi; now, chemically inert envelops have been procured, and a restoration procedure will be attempted when possible.
The plates considered worth of digitization are detailed in Table 3.
Notes to Table 3:
1. About 1600 plates were made in the frame of the Carte du Ciel and Astrographic Catalogue, from 1897 to 1907 in the declination area from +47° to +54°. Unfortunately, several plates were broken or damaged due to frequent removals of Observatory properties in such a long period, and many are in very bad state of conservation. At least 500 plates are in acceptable condition for digitization. 32 plates of Comet Halley were obtained from January to June 1910. Most plates are in good conditions. A few of these plates were reproduced in the Atlas of Comet Halley 1910 II (Donn et al., 1986), the other plates have been rediscovered thanks to the present program. In addition, plates have been found of the great comet of 1910.
2. From 1956 to 1964, with the same astrographic equatorial, in the framework of an international astrometric program to study high proper motion stars belonging to the astrographic zone of Catania for the Carte du Ciel, 211 plates were obtained. Most plates are in a good state.
3. 1168 plates were obtained with the Schmidt S41/61 telescope, with a scale of 169 arcsec/mm, at the Mt. Etna station in the period from 1968 to 1992. The plates are in good conditions. Only 28 plates of this archive have not been found.
4. 67 plates of comets and stellar fields in open clusters were obtained with the 33 cm Steinheil objective of the equatorial astrograph attached to the S61/41 telescope mounting.
2.4 Vatican Observatory Table 4 gives the content of the Specola Vaticana archive, with plates coming from the astrograph in its historical locations in Rome and from the Schmidt telescope in Castelgandolfo.
The Vatican archive is well preserved and ordered from the very first plate. For every plate of the Carte du Ciel and of the Astrographic Catalogue there is a detailed description of the characteristics, the plate constants to do astrometry, and information concerning exposure time and weather conditions. The digitization of the logbooks of the Schmidt telescope is currently being done by the Sofia Sky Archive Data Centre, Bulgarian Academy of Sciences (http://www.skyarchive.org/SSADC).
Two examples are shown in Fig. 1.
Fig. 1 – A digitized plate of the Halley Comet (1910) and of the Carte du Ciel (plate 615, Nov. 5, 1901). No attempt has been made to clean the emulsions.
3 Digitization and analysis of the plates Several commercial flat-bed retro-illuminated scanners (A3 and A4 formats, optical resolution 1600x3200 dpi), connected to dedicated PCs, have been installed in Asiago, Padova, Catania, Roma, Castelgandolfo. The same model of scanner is used by DLR in Berlin, where a program that convincingly demonstrated the feasibility of the digitization was initially written.
This program (named Scanfits) gradually grew to become a true user-friendly working instrument. It runs in all Windows operating systems and it provides as output a 14 or 16 bit per dot, positive or negative FITS image (including the header, where the main parameters of the scanner configuration are automatically loaded), that can be directly analyzed with IRAF, MIDAS or IDL.
Using Scanfits, tests have been performed on many different types of plates, both with images and spectra, to determine the spatial resolution of the scanners, and their astrometric and photometric precision, as detailed in the following paragraphs. The effective pixel size of 16 micron provides a spatial resolution sufficient for a large part of the direct image plates of our archives, but not so for the fine grained spectroscopic material nor for the very short focal length S40/50 films. Another limitation we identified is the amount of internally scattered light, and the different optical quality of the three RGB channels.
Therefore the initial activity of digitization was concentrated on images and on objective prism plates, leaving aside the spectra and the S40/50 material for which a small area 3200x6400 dpi scanner has been recently acquired.
In addition to Scanfits, another software tool has been developed at the Astrophysical Observatory of Catania, namely AstroPlates, a program that takes advantage of MS Visual Basic
6.0. It runs in any 32 bit Microsoft environment. AstroPlates acquires the images in TIFF format, either in B/W (14-16 bit/pixel) or RGB (14-16 bit/pixel for each channel). In the latter case the program is able to isolate a single channel. In the next step the images are transformed in FITS (with appropriate header), JPG or Bitmap format. It is also possible to record plate information in MDB database. An important characteristic of the program is the use of the powerful development environment IDL and its libraries by using the OCX component of IDL to communicate with it and to take advantage of its graphical and numerical capabilities. All functions of AstroPlates use the IDL libraries without activating the IDL environment.
Therefore, it is not necessary that the user knows this language and its functions. During the acquisition phase it is possible to optimize the scanner parameters by a procedure taking into account the analysis of frequency histograms of grey levels in the plate.
Two different acquisition programs are therefore available, and the generic user can choose the one most suited to his taste or environment.
The approximate dimensions of the digitized files at 1600 dpi, 14 bit per dot, are: S67/92 cm plates (20x20 cm) 260 MB, 122 cm plates (9x12 cm) 70 MB, 182 cm plates (20x12 cm) 160 MB, S40/50 cm plates (10 cm diameter) 100 MB. The size of the files obviously poses a serious problem, both for storage and for distribution. After the initial period of test, when the files were saved to DVDs of 4.7 GB each, Network Attached Storage (NAS) units, capable to grow up to 1 TeraByte per unit, have already been implemented in Asiago and Roma.
3.1 Astrometry The astrometric precision of the Asiago S67/92 digitized plates was tested in subfields of 1500x1500 pixels (1”.55/px, 0.65x0.65 sq deg), with approximately 150 stars from the USNO Catalog (Monet et al. 2003) per field, finding a standard deviation in both coordinates of about 0”35. This precision is quite compatible with that obtained for instance by Barbieri et al. (1988) for the astrometry of Pluto using conventional measuring machines.
A further check was done in Torino as part of a research program dealing with the realization of the optical link to the quasi-inertial radio reference frame: approximately 400 plates taken around 1990, covering about 80 radio sources, were digitized using TO.CA.M.M, an original ASCORECORD device which was modified to be fully automated and housed at Cagliari Observatory in 1994.
As part of the standard calibration procedures, the geometrical/temporal stability of the measuring machine was monitored using rotated/repeated scans; tests have proved that TO.CA.M.M. routinely performes to better than 1 micron stability (Lattanzi et al. 2001, Bucciarelli et al. 2003).
In order to have an independent check of the astrometric properties of our scanners, we made a direct comparison between the x,y measurements on a particular plate digitized with TO.CA.M.M, our "reference catalog" in this context, and those obtained with the scanner Microtek Artixscan 1100, currently available at Torino. Setting the scanner at its maximum resolution - 1000 dpi - the effective pixel size of the image is ~ 25 microns, which we have reproduced with TO.CA.M.M. to maintain a comparable PSF sampling.
A 20x20 cm photographic plate of the series taken at the 38cm F/18 MORAIS refractor for the QSO program mentioned above was selected for this test.
An automatic match of the two lists of x,y measurements found 370 common objects over the entire plate; the vectors of differences in the two coordinates were minimized via a linear least squares fit, which gave an average positional error of 23 microns, corresponding to a disappointing 0.7 arcseconds. However, inspection of the individual residuals revealed the presence of systematic patterns in the residuals' behaviour as function of both x and y axes.