SAS Watchout

Mosaiced images displayed with wrong coordinates in ds9 v8.1 and later

Mosaiced images of EPIC-pn, generated by the task emosaic, are actually in the FK5 (epoch=2000.0) coordinate system but lack the keywords which describe this system. From version 8.1 onwards, the image display application ds9 assumes a different system in the absence of these keywords. This results in the coordinates of the mosaiced image being wrong by many arcminutes.

How to fix this problem?

This has been fixed in SAS version 19, where emosaic writes the correct keywords into the image header. If the product has been generated by an earlier version of SAS then you can edit the mosaiced image header using the ftool fmodhead or fv to add the following keywords into the PRIMARY header of the mosaiced image.

  • RADESYS = "FK5"
  • EQUINOX = 2000.0
  • the image will then display correctly in all versions of DS9.