WCSTools Programs

Image World Coordinate Setting Program

imwcs (pronounced im-W-C-S) is a utility for setting the world coordinate system (WCS) of a FITS or IRAF image. The WCS is the relationship between sky coordinates and image pixels and can be described in a standard way in the header of an image. The program was originally written at the University of Iowa, but has been adapted and amplified by Doug Mink at the Smithsonian Astrophysical Observatory.

imwcs currently works only on images where the rotation about the center is known to within a few degrees It has been tested extensively only on images with near-zero rotation.

A plane-tangent projection (center right ascension and declination, plate scale, and rotation angle) is fit based on matches between stars in the image and stars in a reference catalog.

As of version 2.8.7, a set of image stars may be hand-matched to sky positions and used to get a WCS or set initial values for a reference catalog-based WCS (See example 6).

Specific catalogs which can be searched include:

  • USNO-A2.0 Catalog using imwua2
  • USNO-B1.0 Catalog using imwub1
  • 2MASS Point Source Catalog using imwtmc
  • GSC II Catalog using imwgsc2
  • USNO-SA2.0 Catalog using imwusa2
  • USNO-A1.0 Catalog using imwua1
  • USNO-SA1.0 Catalog using imwusa1
  • USNO J1.0 Catalog using imwujc
  • HST Guide Star Catalog using imwgsc
  • GSC-ACT Star Catalog using imwgsca
  • Tycho 2 Catalog using imwty2
  • SAO Catalog using imwsao
  • PPM Catalog using imwppm
  • IRAS Point Source Catalog using imwiras
  • USNO ACT Catalog using imwact
  • Portions of imwcs have been turned into stand-alone programs which are useful in their own right. imsize decodes header information to print the size of the image on the sky. imcat finds the reference stars on that portion of the sky. imstar finds star coordinates within an image. immatch matches stars in an image with stars in a reference catalog.

    Command Line Arguments


    1. Find image WCS using HST Guide Star Catalog Stars as references
    2. Find image WCS using Digitized Sky Survey Stars as references
    3. Using SExtractor to find stars in an image
    4. Find image WCS using USNO-B1.0 Stars as references
    5. Find image WCS using USNO-A2.0 Stars as references
    6. Find image WCS using pre-matched stars and USNO-A2.0 Stars

    Last updated 5 February 2003 by Doug Mink dmink@cfa.harvard.edu

    Telescope Data Center