(QC1 database table hawki_zp, column qc_atx0, product keyword QC.ATX0)
Description: Extinction corrected zeropoint.
Zeropoints are obtained to monitor the instrument performance. At the observatory, they are also used to make a rough assessment of the quality of the night.
If a good photometry is required by a science program, special calibrations need to be included in the proposal.
The HAWK-I pipeline computes zeropoints using the UKIRT and Persson
catalogues. For many stars the catalog Y filter entries are missing, thus the zeropoints can not be measured.
Following conversion from ADUs to magnitude is used:
zmag = mag + 2.5*log10(flux) - 2.5*log10(DIT)
where mag is the known magnitude from the catalog, flux is the measured flux in ADUs, and DIT is the detector integration time.
The zeropoint is written in the keyword QC.ZPOINT, while the keyword QC.ATX0 contains extinction corrected value:
atx0 = zmag + airmass * extinction
where airmass is taken from the file header and extinction parameters are: J - 0.098, H - 0.039, Ks - 0.065, Y - 0.00.
As of 2016-11-19, new, CASU written pipeline (v.2.0.2) is adopted. The zeropoints are calculated with a different formula:
zmag = mag + 2.5*log10(flux/CASUEXPT) + APCOR3 + DRS.EXTINCT
where CASUEXPT [s] is consistent with EXPTIME=DET*NDIT, APCOR3 [mag] is the pipeline estimated aperture correction (header keyword APCOR3)
and the DRS.EXTINCT [mag] is assumed extintion (header keyword DRS.EXTINCT).
It was found that the old pipeline (recipe hawki_cal_zpoint) did
not account for exposure time correctly (NDIT factor missing),
which resulted in zeropoints higher by about 2 mag. Currently, the
zeropoints are derived correctly.
The HAWK-I detectors show different sensitivity. The Chip3 and Chip1 are most sensitive (highest zeropoints).
Long term monitoring shows that the zeropoints decrease between subsequent M1 (main telescope mirror) recoating events with rough rate of 0.02 mag per year.