Figure 1: Client-Server architecture of the ALADIN system.
Figure 1: PS 11 Kb
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PS reprint
James G. Bartlett, François Bonnarel, Daniel Egret,
Françoise Genova, Houri Ziaeepour, Olivier Bienaymé, François
Ochsenbein, Michel Crézé, Joseph Florsch
CDS, Observatoire de Strasbourg, 11 rue de l'Université, 67 000 Strasbourg, France
Mireille Louys
ENSPS, Bd. Sébastien Brandt, 67 400 Illkirch, France
Philippe Paillou
Institut Géodynamique, Université de Bordeaux 3, 33 405 Talence
Cedex, France
Presently, a reduced version of the client is already available for the CDS and its co-workers. Full realization of the project is planned for the middle of 1996.
The development of muliwavelength astronomy heightens the need for data reduction packages capable of addressing the following question: Given a set of objects detected in a certain waveband, how does one integrate the new sources with previous data? Usually, this involves the painstaking examination of film copies of photographic survey plates, such as the Palomar surveys, whether it be for detailed work on a small number of objects or the training of more automatic algorithms for object cross-identifications (see, e.g., Egret 1995). The ALADIN system (Paillou et al. 1994) of the CDS (Observatoire de Strasbourg) is a software package designed to tackle this problem. It provides simultaneous access to digitized sky photographs, catalogs and databases to facilitate direct, visual comparison of user data with previously classified data. Specifically, ALADIN will provide the interactive environment in which a user can overlay the positions of cataloged objects on optical images, digitized Schmidt plates, and query relevant catalogs and databases for supplemental information. The system will manage user data, public domain data, either stored at CDS or elsewhere, the CDS database SIMBAD, and an archive of optical images covering the whole sky. In addition, one will have the ability to recalibrate the astrometry and photometry of the images with up-dated catalogs of standards (e.g., HIPPARCOS/Tycho) (Bartlett et al. 1994).
Figure 1 shows the basic client--server architecture of ALADIN in which the client communicates with the CDS ALADIN server and manages local data, such as user catalogs. The CDS ALADIN server in turn directs a host of servers dedicated to each of the individual CDS information services. Other, non-CDS services may be connected to the ALADIN server if they conform to the prescribed communication protocol. This architecture, combined with a set of image analysis tools provided, among other things, for recalibration of the astronomical images, determines the capabilities of the system.
Figure 1: Client-Server architecture of the ALADIN system.
Figure 1: PS 11 Kb
The Digital Sky Survey (Lasker 1994; Lasker 1992) of the Space Telescope Science Institute (STScI) forms the core of the image archive. To create these images, the STScI scanned the first epoch Palomar E and United Kingdom (UK) Schmidt J plates with a pixel size of 1.8 arcsecs (25 microns). The low resolution and a light data compression (factor of 10) permit storage of images covering the full sky on a reasonable number of CD-ROMS. These CD-ROMS are stored at the CDS in a juke-box from which the ALADIN image server obtains the requested pixel data. Because one sometimes requires higher resolution images, for example, when working in crowded regions of the sky or in areas with deep observations, the CDS is also building an archive of images digitized at the full resolution of 0.7 arcsec pixels (10 microns). This archive consists of an optical disk juke-box with a capacity of 500 GB which we will stock with images of the Galactic Plane and Poles, the Ecliptic Poles, the Magellanic Clouds and other, small regions of specific interest. The full resolution images are being provided by the MAMA at the Observatoire de Paris, who will supply scans of ESO red plates, and in the near future by SuperCOSMOS of the the Royal Observatory in Edinburgh, providing scans of the UK Schmidt J plates. The user can ask for images by plate name, by the coordinates of the region of interest, by the name of an important, well known central object, or by the specific image filename. The latter option will in particular allow access to local images with a format compatible with ALADIN .
ALADIN communicates with the SIMBAD database via the dedicated server, which permits the user to search for SIMBAD objects using the SIMBAD filters. The client displays on the current image objects falling within the studied region along with their associated names and error boxes. Clicking with the mouse on a displayed object produces a window containing detailed information on the object. The user can save information on a selected subset of objects in a local file in either ASCII or FITS format.
Except for differences arising from the greater variety of formats, the catalog server works on the same principles as the SIMBAD server. The user may select a subset of CDS catalogs to search for relevant objects, display these objects on the current image, obtain and then save locally the information of interest in an ASCII or FITS file. This applies equally well to user catalogs and other public domain catalogs accessible to the ALADIN server.
The images displayed by ALADIN will not simply represent a reference map, but also quantitative data: it will be possible to perform astrometric and photometric measurements, search and extract objects, measure object parameters, and classify objects as either stars or non-stars (e.g., galaxies), all of which requires astrometric and photometric calibrations and image analysis routines. We divide the image calibrations into 2 levels. At the first level, ALADIN provides both the astrometric calibration, as given by the digitizing machine, and the photometric calibration; the user need do nothing at this level. The next level allows a recalibration of the image. The idea here is that the user wishes to recalibrate the image with a new set of standards, either his own or standards available in one of the CDS catalogs, or elsewhere. At this level, ALADIN supplies the software routines necessary for the recalibration once the user specifies the catalog of standards. For an astrometric recalibration, he may choose, for example, the HIPPARCOS/Tycho standards (when they become available) to solve for the astrometric plate constants, either for the entire plate or for a subregion. In particular, a photometric recalibration will greatly help to reduce systematic errors in regions away from a plate center. This ability to recalibrate the optical images is one of the unique characteristics of ALADIN .
While the first level astrometric calibrations are given by the digitizing machines, the photometric calibrations (surface and stellar) will be performed in Strasbourg by using the Guide Star Photometric Catalogs (GSPC) I and II (Ferrari et al. 1994; Lasker et al. 1988). With GSPC-II data for a small number of fields (kindly provided by B. Lasker), we will determine the relation between stellar image size and magnitude down to an equivalent V magnitude of 20 (the goal of the GSPC-II program). ALADIN will normalize this relation for each Schmidt plate to the GSPC-I stars found at the center of all survey plates. For the surface photometric calibration, we apply the internal calibration technique (Agnelli et al. 1979) to the GSPC data to find the form of the relation between surface optical density and true flux. Once again, this relation will subsequently be normalized to each plate by using the GSPC-I standards.
A prototype of the ALADIN system has existed since late 1993 and has already been used by several researchers at the Observatoire de Strasbourg. The prototype has now been replaced by a more extended system capable of managing a set of about 70 full resolution Schmidt plate images, stored in the optical disk juke-box, and the STScI CD-ROMS. This extended, or intermediate, version of the client also uses the SIMBAD and CDS catalog servers, and has been installed at several astronomical institutions in Paris. We are currently finishing the definitive version of the ALADIN client, incorporating all of the functionalities of the proposed system, which we hope to distribute for testing by the summer of 1996.
Bartlett, J. G., Bonnarel, F., Crézé, M., & Paillou, Ph. 1994, in Newsletter 6 of the IAU working group on Wide-Field Imaging, 8
Egret, D. 1995, in Proceedings of the Euroconference on Near Infrared Sky Surveys, S. Miniato, Italy, in press
Ferrari, A., et al. 1994, in IAU symposium 161, Astronomy from Wide-Field Imaging, eds. H. T. MacGillivray & E. B. Thomson (Kluwer Academic Publ), 301
Lasker, B. M., et al. 1988, ApJS, 68, 1
Lasker, B. M. 1992, in Digitised Optical Sky Surveys, eds. H. T. MacGillivray & E. B. Thomson (Kluwer Academic Publ), 87
Lasker, B. M. 1994, in IAU symposium 161, Astronomy from Wide-Field Imaging, eds. H. T. MacGillivray & E. B. Thomson (Kluwer Academic Publ), 167
Paillou, Ph., Bonnarel, F., Ochsenbein, & F., Crézé, M. 1994, in IAU symposium 161, Astronomy from Wide-Field Imaging, eds. H. T. MacGillivray & E. B. Thomson (Kluwer Academic Publ), 347