HDF version 2

For the time being, processed data are being stored on the staging disk of the HST archive. They can can be retrieved by anonymous ftp from stdatu.stsci.edu, in the directory /pub/hdf/v2, or via the web links below. The names of the data sets below reflect the directory structure on stdatu. Other portions of the HDF web pages provide details on how these images were constructed. The additional processing for version 2 has resulted in about 10% improvement in S/N for the F450W, F606W, and F814W filters. 10 sigma limiting magnitudes are now roughly as follows, within an aperture of area 0.2 square arcsec:
	Filter		AB Mag limit (10 sigma)
	F300W		26.98
	F450W		27.86
	F606W		28.21
	F814W		27.60
The table below lists the number of frames, total exposure time, and summed sky levels in DN for the version 2 images.
   Filter   Nframes   Texp    Sky1     Sky2     Sky3     Sky4
   F300W     77     153700    39.259   78.642   92.954   89.686
   F450W     58     120600    65.055  310.133  319.222  309.483
   F606W    103     109050   238.507 1146.402 1155.758 1131.676
   F814W     58     123600   158.615  768.667  771.862  754.104
Note that the v2 drizzled images have had sky subtracted and are normalized to 1 sec exposure time. Photometric zeropoints are the same as for version 1.

Much of the improvement in S/N came from the inclusion of data that was rejected from version 1 due to either scattered light or small rotations of the field. The scattered light pattern was carefully subtracted from the affected frames before combining them into the version 2 drizzled images. A number of frames were not used in version2, either because they had high background, or few enough exposures that it was difficult to remove cosmic rays completely.

World Coordinate System (RA and Dec coordinates)

The raw WFPC2 data produced by HST is encoded with a World Coordinate System (WCS) which is used to convert pixel position (x,y) to celestial coordinates (RA, Dec). This WCS is tied to the HST Guide Star reference frame, but for an individual observation it may easily be inaccurate to an arcsecond or more due to errors in the Guide Star Catalog positions of the sets of guide stars used for the actual observation.

The paper by Williams et al. 1996 (AJ 112, 1335) describes how radio positions (obtained with the Merlin and VLA interferometers) have been used to update the celestial coordinate system for the Hubble Deep Field. These corrections have not been incorporated into the images which are available here. Indeed, there is an error in the image headers for the drizzled HDF version 2 images (first noted here on 3/13/96) where the CTYPE1 and CTYPE2 keywords were inadvertantly set to incorrect values. Because of this error, IRAF and STSDAS tasks such as 'imexam', 'rimcur' or 'xy2rd' cannot properly convert pixel positions to RA/Dec or vice versa. The parameters CTYPE1 and CTYPE2 should be set to 'RA---TAN' and 'DEC--TAN', respectively (note the number of dashes in each entry!). Currently, they are incorrectly set to 'PIXEL'.

Here, we provide an IRAF script, hdf_wcs_fix.cl, which can be used to correct the header WCS information for the drizzled HDF version 2 images. This script corrects the CTYPE error described above, and then, if the user wishes, applies an offset to the RA and Dec zeropoints (CRVAL parameters) of the images to bring them into the radio reference frame defined in Williams et al.

The Data

  • calibration. Calibration frames used in the HDF pipeline. This includes the superdark, the superbias, and the flats. The calibration images for version 2 are identical to those for version 1.

  • dither Stacked images at each dither position.

  • drizzled Images at the different dither positions registered to a small fraction of an original pixel, corrected for geometric distortion, and ``drizzled'' onto a final image with a sampling of 0.04 arcsec/pixel. An image with a rough estimate 1/variance at the sky level at each pixel is also available.

  • catalogs of objects.

  • mosaics The separate CCDs of the HDF v2 WFPC2 images have been drizzled together to produce large mosaic images. These are now available in 4096x4096 format, as well as rebinned to 1024x1024 at a scale of 0".16/pixel.

    Harry Ferguson ferguson@stsci.edu 10/24/96
    Andy Fruchter fruchter@stsci.edu 8/1/96
    Mark Dickinson med@stsci.edu 10/30/96