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Quality Control and
Data Processing

CAL | HC | refs | QC
Trending & QC1
QC links:
HAWKI: System efficiency
Zeropints Y
Zeropints J
Zeropints H
Zeropints Ks
QC1 database (advanced users): browse | plot

The zeropoint trending is based on photometric standard star calibration data in J, H, Ks, & Y broad band filters. The data are obtained every photometric or clear night during which HAWK-I SCIENCE observations are executed. Each standard star dedicated observation consists of 4 star images where the target is placed at the center of each detector sequentially. This is repeated for all filters. The computed zeropoints, corrected for atmospheric extinction, are monitored for each detector separately.

As of 2015-10-01 new HAWKI pipeline, prepared by CASU (v.2.+), is used. The zeropoints derived by this pipeline (recipe hawki_standard_process) differ significantly (by about 2 mag) from the previous values. See further for more information.

This is an example of a typical standard star product frame. It contains 4 extensions for each of the 4 chips of HAWK-I. Here shown together.



FITS key QC1 database: table, name definition class* HC_plot** more docu
QC.AXT0 hawki_zp..qc_atx0 Extinction corrected zero-pointHC [docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one.

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 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 witha rough rate of 0.02 mag per year.


The extinction corrected photometric zeropoints (at airmass X=0), derived from photometric standard star calibration frames observed in different filters are trended.

On 2016-11-19 new, CASU pipeline was adopted, which uses revised method for calculating zeropoints. It takes into accout DET.NDIT in the total observing time (DET.DITxDET.NDIT) rather than DET.DIT as was done by the previous pipeline. To be able to compare vales from before and after, the CASU zeropoints are corrected by "+ 2.5 * log10(NDIT)". Only data taken with DET.NDIT=5 are plotted

Scoring&thresholds Zeropoint

The zeropints are not scored.


Date event
September 2009 M1 re-coating; the zeropoints increased by about 0.25 mag in all filters
April 2012 M1 re-coating; the zeropoints increased by about 0.2 mag in all filters
2014-08-05 As the VLTSW pl4 was installed the problem with wrong WCS in the headers of the standard star observations started. The pipeline complained that the detected std coordinates are too far off from those in catalog. The data could not be processed and there were no ZPs produced. The problem continued for until end of October 2014. Fixed in PPRS-058455.
2015-08-30 All zeropoints at the normal level after GRAAL installed in front of the instrument. No degradation in sensitivity
2015-10-01 New, CASU prepared pipeline (v.2.0.2) adopted. Measurements of the zeropoints differ from previous values.

Algorithm Zeropoint

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 2015-10-01, new, CASU written pipeline (v.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=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.

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