WFPC2 Associations
About the Associations

The Canadian Astronomical Data Center (CADC), the Space Telescope European Coordination Facility (ST-ECF) and the Multimission Archive at STScI (MAST) are pleased to make available combined images from the Wide Field Planetary Camera 2 of the Hubble Space Telescope. These combined images are the products of the basic registration and averaging of related sets of WFPC2 images, referred to as associations, that is usually performed by archival researchers after the retrieval of individual images. As of November 2002, over 15,000 combined images have been created from associations of nearly 50,000 individual WFPC2 images. The product of many person-years of development at CADC and ST-ECF, these combined images form an important new set of high-quality astronomical data that should be useful for a wide range of investigations. They also provide better previews of a given field than the individual WFPC2 images.

Association Example: The Chandra Deep Field

Single Exposure


Further details about the association definitions and image registration process can be found here. Documentation of the WFPC2 Associations Science Products Pipeline (WASPP) used to create the combined images can be found here.

A representative sub-sample of the combined images has been found to be photometrically accurate. Details of the photometry tests are available here. The images have been found to be astrometrically accurate to within ~ 0.3 - 0.4 arcseconds of the USNO2 reference frame, accounting for proper motions. Larger shifts (up to several arcseconds) have been discovered in images containing extended sources, so users interested in astrometric accuracy from such images are advised to perform independent checks. The registration of the combined images has been found to be accurate to within a few tenths of a pixel.

Available Products

Two products from the image combination process are currently retrievable from this database:

  • The combined image, e.g. u2he0701b_c0.fits. The root name of the image is derived from the name of the leader image of the association, where the last character in the name has been replaced by the letter "b" to denote a combined image.
  • Variance images for each WFPC2 chip resulting from the combination process, e.g., u2he0701b_variance_n.fits, where n goes from 1 to 4.

Preview files for each combined image are also available. Files listing the individual members of the associations will also eventually be made available, after conversion to a format compatible with existing image drizzling software.

Contact Information

Questions and comments about the WFPC2 associations can be directed to the CADC, ST-ECF or MAST helpdesks.