The Skycat tool

SkyCat is a tool that combines visualization of images and access to catalogs and archive data for astronomy.



SkyCat features

  • Display FITS images, with support for the World Coordinate System (WCS), interactive measurement of offsets, and other standard visualization functions (SAOimage-like).
  • Overlay and edit color graphic objects on the image, `tagging' sources with text, arrows, circles, or other graphic elements.
  • Display a compass indicating where north and east are in the image, based on the world coordinates information.
  • PostScript color printing of the display (image + graphics).
  • Access and load images from network image servers, such as the Digitized Sky Survey (DSS).
  • Select previously viewed images from a history menu and catalog. The cut levels, color scale algorithm and colormap settings are remembered for each image.
  • Access and load catalog information from a number of popular astronomical catalogs, such as the HST Guide Star Catalog and many others.
  • Create, search and edit local user catalogs.
  • Save catalog data locally.
  • Overlay catalog sources on an image, taking object size and orientation into account.
  • Interact with Netscape to display more object information when available
  • Access the observations catalog from the ESO, and CFHT Science Archives.
  • Access to SIMBAD as name resolver as well as for information on known objects.
  • Retrieve preview and other compressed images and decompress them on the fly.
  • Retrieve and plot tabular preview data for a selected object as an X/Y graph.
  • Calculate, display and plot the center position, FWHM, angle and other information for a selected star/object.
  • Load and save compressed images in hcompress, gzip or UNIX compress format.
  • Access SkyCat features from a remote process via socket interface or Tk "send".
  • Access image header and data (FITS format) via SysV shared memory or mmap.
  • Interact with WWW browser to access catalog documentation and other documents.
  • Support for dynamically loaded SkyCat plugins that add new features or modify existing ones.
  • Load lists of catalogs from other sites (ESO, CADC, CDS, local) and and allow users to select a preferred default catalog list.


  • Astrometry: SkyCat handles the astrometric positions by translating pixel positions into equatorial coordinates (RA, DEC). This translation is based on WCS (World Coordinate System) FITS keywords that are included in the image header and that give the astrometric solution for the image. The accuracy of the solution varies because, in many cases, an approximation is used.

About 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 was developed for the VLT. This software is however not actively developed any more at ESO and it has no software support allocated. It is not an operationally critical tool, and only limited user support is available.

Distribution and support

The SkyCat binaries are freely available to any users who want to download and use the software at their own risk. ESO can only offer very limited support on Skycat. The tool is available as an executable for several platforms.

Please report problems or make suggestions by submitting a ticket to the ESO Operations Helpdesk.


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 gave a source of inspiration. STARCAT contributed many of its internals.

  • Tcl/Tk, TclX, BLT, Tix, ET and Itcl give the glue around the C++ classes, Tk provides the canvas graphics.
  • SAOimage lent the WCS lib (now a separate package, provided by courtesy of Doug Mink).
  • GSC server provided by courtesy of A. Preite-Martinez & F. Ochsenbein.
  • SIMBAD and NED client routines allow name resolving.
  • The CADC press library is used to automatically compress and decompress (hcompress, gzip, ...) images.
  • MIDAS routines are used to calculate the centroid position, FWHM and angle of selected stars/objects.