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VIRCAM Quality Control:
Twilight Flats

CAL | HC | refs | QC
Trending & QC1
QC links:
noise parameters | flat flux level | sequence
master noise and struct.
raw twil counts average
raw twil counts range
rms of flat ratio
QC1 database (advanced users): browse | plot
   Click on CURRENT to see the current trending (Health Check).
   Click on HISTORY to see the historical evolution of the trending.

The vircam calibration plan includes twilight flats to calibrate science exposures. The derived QC parameters measure operational properties, e.g. if the covered flux range is optimal, or heavily underexposed. No instrument specific property is monitored.

Each detector has its own response. Therefore twilight flats and dome flats show the following relative count rates:

chip13 chip 14 chip 15 chip 16
0.79 1.04 1.17 0.91
chip 9 chip 10 chip 11 chip 12
1.04 1.13 1.03 1.13
chip 5 chip 6 chip 7 chip 8
1.08 1.19 1.26 1.01
chip 1 chip 2 chip 3 chip 4


0.79 0.86 1.04

chip #1 and #13 showing the smallest response and chip #1 and #7 showing the largest response. Although the response of chip #5 is close to the array average, it is the first to saturate because of its low saturation level.


  • 2017-08-28: twilight flat reference products and dome flat reference products, used for the pipeline, have been refreshed. Consequently, the RMS QC parameter derived from ratio flats becomes lower.
top noise parameters

QC1 parameters

parameter QC1 database: table, name procedure

twilight flat rms

vircam_twilight, qc_twil_rms
- The QC.FLATRMS is a measure of RMS in ADU obtained from master twilight flat products. It is the median of the absolute deviations from the median ( = MAD ) times 1.48 derived from the master twilight flat.

top twil level

QC1 parameters

parameter QC1 database: table, name procedure
qc_twflat_medavg vircam_twilight, qc_twflat_medmin, qc_twflat_medmax, qc_twflat_medavg - The average of the raw flat medians. Derived from QC in Garching. Not derived by pipeline. There are further parameters obtained from the raw frame median values.

RMS of ratio flats

vircam_twilight, qc_ratiotwil_med, qc_ratiotwil_rms - For the QC.FLATRATIO_MED parameter, the master twilight flat is divided by a reference twilight flat product ( PRO.CATG = REFERENCE_TWILIGHT_FLAT ). The median and the robust noise, using MAD, of the ratio frame is given

top secondary parameters

The QC1 analysis scripts triggered after the pipeline processing generate QC reports (graphical information) and measure some additional parameters that describe how the twilight flat was acquired. They so far describe the quality of the master flat product.

QC1 parameters

parameter QC1 database: table, name procedure
sequence property vircam_twilight, qc_twflat_medmin, qc_twflat_medmax, qc_twflat_medavg, qc_twflat_medstd, qc_twflat_medrng

- The fluxes of the twilight flat raw frames vary as the sky brightness varies during dusk or dawn. For each raw frame the median counts is taken. Out of this sample of raw frame medians the minimum, the maximum, the average, the standard deviation and the range is extracted.

  vircam_twilight, qc_photnoise, qc_phot_sn, qc_diff_avg, qc_diff_std - subtract two consecutive raw twilight flat frames (align them by flux) and retrieve a robust estimate of the statistical noise (read out noise) via a histogram fit. qc_diff_avg and qc_diff_std are the Gauss fit parameters when fitting the histogram of the flux-aligned raw twilight flat difference frame. qc_photnoise is the photon noise and qc_phot_sn is the signal to noise ratio.
pre reset-corrected maximum flux in saturation level units vircam_twilight, qc_twflat_fracsat - Among all median flux values obtained from the raw fats, the highest flux value is multiplied by (MINDIT+DIT) / DIT to get the flux value on the chip before the detector reset was subtrcated by the acquisition system. The value is compared with the saturation level. Details are given on the problems page.