top Zeropoint |
QC1 parameters
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:
where
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 2015-10-01, new, CASU written pipeline (v.2.+) is adopted. The zeropoints are calculated with a different formula:
where CASUEXPT [s] is consistent with EXPTIME=DIT*NDIT, APCOR3 [mag] is the pipeline estimated aperture correction (header keyword APCOR3)
and the DRS.EXTINCT [mag] is assumed extintion (header keyword DRS.EXTINCT).
The old pipeline (recipe hawki_cal_zpoint) was assuming EXPTIME=DET.DIT as the integration time returned by the instrument is always averaged over DET.NDIT. Thus, to relate measurements by these two pipelines the CASU values need to be corrected by "+ 2.5 * log10(NDIT)".
To avoid variations among monitored zeropoints, only data taken with DET.NDIT=5 are plotted in the corresponding Health Check plot.
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.