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Quality Control and
Data Processing

CAL | HC | refs | QC
Trending & QC1
   dome flats
OmegaCAM ...
QC links:
OMEGACAM: Dome flats stability
dome raw u_SDSS
dome raw g_SDSS
dome raw r_SDSS
dome raw i_SDSS
dome raw z_SDSS
dome raw B_JOHN
dome raw V_JOHN
master dome flat level
number of cold pixels
QC1 database (advanced users): browse | plot

During normal VST/OmegaCAM operations DOME flat frames are taken for each filter for which science data was obtained, and generally have a validity range of 4 days. They are usually obtained during day-time calibration and consist of 5 input, screen flat exposures (each with a flux level of about 20,000 ADU or more). The pipeline applies a bias-correction, trims the images, and then averages them together with a 5 sigma rejection of outliers (intended to reduce photon shot noise and to remove cosmic rays).

The pipeline creates two output frames from its dome flat recipe (omega_mdome): a master dome flat (OC_MFLD_<DATE>_1_1_normal_normal_<FILTER>.fits) and a cold pixel mask (OC_MCPM_<DATE>_1_1_normal_normal_<FILTER>.fits).

An OmegaCAM master dome flat (single detector).

An OmegaCAM master cold pixel map (single detector). A value of 1 indicates a cold pixels; otherwise 0.

DOME Flats


FITS key QC1 database: table, name definition class* HC_plot** more docu
QC.RAW.DOME.MEDIAN omegacam_dome..qc_raw_dome_median median flux level of all input raw dome flats (ADU) HC [docuSys coming]
QC.RAW.DOME.MIN omegacam_dome..qc_raw_dome_min median flux level of input raw dome having the minimum average flux level (ADU)HC [docuSys coming]
QC.RAW.DOME.MAX omegacam_dome..qc_raw_dome_max median flux level of input raw dome having the maximum average flux level (ADU)HC [docuSys coming]
QC.MASTER.DOME.MEDIAN omegacam_dome..qc_dome_median median counts in normalized master dome flat (ADU) HC [docuSys coming]
QC.MASTER.DOME.STDEV omegacam_dome..qc_dome_std standard deviation of normalized master dome flat (ADU)HC [docuSys coming]
QC.NUMBER.COLD.PIXELS omegacam_dome..qc_dome_cold_pixels number of cold pixels HC [docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one.


The QC1 health check plots for the RAW OmegaCAM dome flats are divided into 7 separate linked pages, each of which shows the trending for each of 7 different filters. Since the master dome flat produced by the OmegaCAM pipeline is normalized, it is not easy to detect whether or not one of the input dome flat frames was either saturated or had insufficient flux. For this reason, the raw dome flat parameters are monitored. A further set of QC1 health check plots exist for the OmegaCAM master dome flats monitoring both the level and the number of detected cold pixels.

Following parameters are monitored for each of the filters: u_SDSS, g_SDSS, r_SDSS, i_SDSS, z_SDSS, B_JOHN, and V_JOHN:

  • raw dome flats median level averaged over all 32 detectors
  • raw dome flat median level for each detector
  • raw dome minimum and maximum level averaged over all 32 detectors
  • score outliers

Following parameters are monitored for ALL filters (possible since the master dome flats are normalized):

  • master dome flat median level averaged over all 32 detectors
  • master dome flat median level for each detector
  • master dome flat rms averaged over all 32 detectors
  • score outliers
  • number of cold pixels averaged over all 32 detectors
  • number of cold pixels for each detector
  • score outliers

Since the dome flat lamp is very red, the dome flats made with the u_SDSS, the u-band quadrant of the u_g_r_i_SDSS, and even the B_JOHN filters can have very low flux levels. This has to be tolerated as there is no alternative dome flat lamp.

Scoring&thresholds DOME Flats

The dome flat median, as well as min/max level, averaged over all CCDs, is scored loosely with static thresholds. Also, the averaged number of cold pixels is scored loosely with fixed values.


Date event
OmegaCAM DOME flats tend to be very stable
2018-08-18 CCD_75 has been failing over the last several days, replaced (PPRS-073742)
2019-01-05 Data taken with M1 cover closed
2020-11-15 CCD77 does not work; only displays random signal consistent with noise (PPRS-079377)
2022-05-09 CCD77 started recording some coherent signal again

Algorithm DOME Flats

The median level and the standard deviation are calculated from the raw input frames and master dome flats for each extension (ADU).

The qc_raw_dome_min/max correspond to the median flux level of the input raw dome having the minimum/maximum average flux level (ADU). These QC parameters are used to ensure that the weakest dome flat has enough flux to create a statistically accurate master dome flat and that the brightest does not reach the saturation limits of the detector.

The estimate the number of cold pixels, detected in the master dome flat frame for each extension, master domeflat is first smoothed and divided through this smoothed image -> residual image is flat with a level of about unity. Then a thresholding is applied which is controlled by the following recipe parameter: --low : Low flagging threshold for cold pixels map. [0.93] --high : High flagging threshold for cold pixels map. [1.07] i.e. all pixels above 1.07 and below 0.93 are considered to be cold pixels.

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