Photometric Zeropoints for the KPNO IRIM Observations of the Hubble Deep Field

Most of the time spent observing the HDF with IRIM was photometric. However, a few intervals were not perfectly clear, although usable data was obtained nevertheless. For the version 1 release, we have set the zeropoints for the "grand sum" images by bootstrapping from subsets of the data taken at low airmass and during photometric conditions as established by standard star observations. For version 2, we plan more extensive checks of the photometric calibration, but we believe that the version 1 calibration should be good to 5%.

The zeropoints below are given in the sense that

mag = zeropoint - 2.5 log N

where N is the number of counts measured for the object of interest in the HDF/IRIM v1 release images. Airmass corrections have already been incorporated.

Note that the zeropoints are in the standard system where Vega has m=0 in all bandpasses; these are not AB magnitude zeropoints. The photometric calibration was done using the faint UKIRT standard stars of Casali and Hawarden (1992, JCMT-UKIRT Newsletter, No. 3, p. 33). The K-band observations were made through a "short K" (Ks) filter, but calibrated under the assumption that K=Ks for the standard stars.

The zeropoints are:

Bandpass
Zeropoint
J
25.25
H
25.16
K
24.57

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Copyright © 1997 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.


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