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Astronomical Data Analysis Software and Systems VI
ASP Conference Series, Vol. 125, 1997
Editors: Gareth Hunt and H. E. Payne

Access to Data Sources and the ESO SkyCat Tool

M. A. Albrecht, A. Brighton, T. Herlin, P. Biereichel
European Southern Observatory

D. Durand
Canadian Astronomy Data Center, DAO/NRC



SkyCat is a tool that combines image visualization with access to astronomical catalogs and data archives. SkyCat uses a standardized URL syntax to access a large number of data sources on the net. This paper addresses the issues that were solved in defining and implementing such standardized scheme. More information on SkyCat can be found at the SkyCat Web site.


1. SkyCat Features

The ESO SkyCat Tool includes the following features:

We are working on these additional features: (i) save canvas graphics, possibly as a FITS extension on the image file, (ii) user customizable symbols to ``label'' objects on the canvas, (iii) interact with a WWW browser to access catalog documentation and other documents, (iv) load lists of catalogs from sites (ESO, CADC, CDS, local) and allow user to select a preferred default catalog list, (v) save user preferences for look-up tables and other settings, and (vi) add or edit WCS parameters for a given image.

Figure: SkyCat showing a DSS image of NGC1275. Original PostScript figure (640kB).

Figure: The CatSelect display of the field around NGC1275. Original PostScript figure (208kB).

2. The Software

SkyCat was developed by ESO's Data Management and Very Large Telescope (VLT) Project divisions with contributions from the Canadian Astronomical Data Center (CADC). The tool was originally conceived as a demo of the capabilities of the class library that we are developing for the VLT.

This library consists so far of two major subsystems: (i) The Real-time Display (RTD) image classes with the RTD widget (see The Messenger, 81, 1995), and (ii) the Catalog Interface classes with the CatSelect widget. These constitute the basic building blocks of SkyCat. Early demonstrations of the tool to users were received so enthusiastically that we decided to polish things up, and produce the current version.

2.1. Java vs. Tcl/Tk

Some users might wonder why we did this development in the Tcl/Tk environment rather than in Java. The main reason is the pragmatic need to have the functionality implemented when the VLT comes on-line (mid-1997). However, great care has been taken to develop as much as possible in the short time with object oriented languages (C++, [incr Tcl]), having in mind that the future lies in tele-scripting rather than in the distribution of binary code.

2.2. Distribution and Support

SkyCat is freely available to any users who want to download and use the software at their own risk. SkyCat is available as an executable for these platforms: (i) SunOS 4.1.3, (ii) HP-UX A.09.05, (iii) Solaris 2.5 (SunOS 5.5), and (iv) HP-UX B.10.10; ESO will maintain the latter two in the longer term-they are the platforms on which VLT software will run. ESO does not have the resources to port and maintain SkyCat on any other platforms. We will be glad to redistribute any port that other people or groups may support, but decline any responsibility for them.

The software is also available as source code for research and other non-profit organizations. If you are interested in obtaining the package, please send us a note. Please report problems or send suggestions to or to


We very much appreciated practicing wishful programming at large, i.e., wishing a utility, a function, or just a code fragment that would just do that bit you badly need, then surfing the net, fetching it, and re-using it in our code. Here is an incomplete list of packages that we either partially re-used or used as a source of inspiration: (i) STARCAT contributed many of its internals, (ii) Tcl/Tk, TclX, BLT and [incr tcl] give the glue around the C++ classes and widgets and Tk provides the canvas graphics, (iii) SAOimage lent the WCS lib, (iv) GSC server provided by courtesy of A. Preite-Martinez & F. Ochsenbein, (v) SIMBAD and NED client routines allow name resolving, (vi) the CADC ``press'' library is used to inflate compressed (hcompress, gzip, ...) images, (vii) Midas routines are used to calculate the centroid position, FWHM, and angle of selected stars/objects, and (viii) GNU Emacs routines are used to make a single binary executable from many Tcl/Tk scripts and data files.


Chavan, A. M., & Albrecht, M. A. 1997, this volume

© Copyright 1997 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA

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