World Coordinates for the KPNO IRIM Observations of the Hubble Deep Field

The "grand sum" coadded HDF/IRIM images were geometrically transformed into registration with the 4096x4096 HDF/WFPC2 mosaics using the IRAF "geotran" task, and then rebinned back down to 1024x1024 format with appropriate pixel weighting from the exposure maps . During the reduction of the WFPC2 data, the geometric distortion of the WFPC2 camera was removed during the drizzling process, so that the x-y pixel grid should be readily translatable into RA and Dec on the sky through a simple rotation matrix encoded into the image header, the World Coordinate System (or WCS).

The zeropoint of the WCS for the HDF v2 images was adjusted using radio positions for two sources detected by the MERLIN array (see Williams et al. 1996, AJ 112, 1335 for details) and should be absolutely accurate to 0.4 arcseconds or better. The corrected astrometric zeropoint was propagated into the headers of the 4096x4096 HDF/WFPC2 mosaics, and the WCS was then transferred to the HDF/IRIM images after registration with the WFPC2 data. Therefore, identical WCS information is encoded into the headers of the HDF/WFPC v2 mosaics and the HDF/IRIM KPNO images. IRAF tasks such as "rimcur" or "imexam", or STSDAS tasks such as "xy2rd", may be used to translate x-y pixel positions into RA and Dec.

The pixel scale of the v1 HDF/IRIM images is exactly four times coarser than that of the HDF/WFPC2 v2 drizzled images, i.e. 0.1594 arcseconds / pixel.

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Mark Dickinson: med@stsci.edu 10/24/96