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HAWKI Quality Control:
zeropoints

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top Introduction

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 2016-11-19 new HAWKI pipeline, prepared by CASU (v.2.0.2), 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.

top Zeropoint

QC1 parameters

  • Extinction corrected zeropoint (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.

 

top History
  • 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.
  • 2016-11-19 New, CASU prepared pipeline (v.2.0.2) adopted. Estimates of the zeropoints differ from previous values by about 2 mag. This is because the old pipeline did not account for exposure time correctly (NDIT ignored). Current zeropoints are calculated correctly.

 

 


 
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