HAWKI: System efficiency
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HC PLOTS |
Zeropints Y |
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Zeropints J |
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Zeropints H |
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Zeropints Ks |
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QC1 database (advanced users):
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plot
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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.
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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.
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Zeropoint
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
QC.AXT0 | hawki_zp..qc_atx0 | Extinction corrected zero-point | HC | | [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.
Trending
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.
History
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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