|Catalogs||Sources detected in the images.|
|mosaics||Merged images of the separate CCD fields.|
|drizzled||Final subsampled images and weight maps|
|dither||Stacked images at each dither position.|
|Calibration||Calibration frames used for making version 2.|
|Flanking Fields||1 & 2 orbit exposures of 8 adjacent fields.|
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/hst/hdf/v2, or via the web links above. The names of the data sets 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 27.60The 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 158.615 768.667 771.862 754.104Note 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.
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.
Copyright © 1997 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.