Quality Control and
Data Processing

HC PLOTS
twilight raw u_SDSS
twilight raw g_SDSS
twilight raw r_SDSS
twilight raw i_SDSS
twilight raw z_SDSS
twilight raw B_JOHN
twilight raw V_JOHN
master twilight flat level
number of bad pixels
QC1 database (advanced users): browse | plot

During normal VST/OmegaCAM operations TWILIGHT flat frames are taken for each filter for which science data was obtained, and generally have a validity range of 7 days. They usually consist of 4 input, screen flat exposures (each with a flux level of about 20,000 ADU or more). Although the twilight template generally consists of 5 sky flats, the first frame is made to determine a good count-to-exposure time ratio and therefore it is excluded from the master twilght recipe. The input frames are bias-corrected, trimmed, corrected for bad pixels (hot and cold pixels), and averaged together with a 5 sigma rejection of outliers (intended to reduce photon shot noise and to remove cosmic rays).

The pipeline creates three output frames from its flat recipe (omega_mflat): a master twilight flat (OC_MFLT_<DATE>_1_1_normal_normal_<FILTER>.fits), a master flat (OC_MFLA_<DATE>_1_1_normal_normal_<FILTER>.fits) which is a combination of the master dome flat (for the pixel-to-pixel variations) and the master twilight flat (to account for the global shape of the flat field), and a bad pixel map combined from the hot and cold pixel maps (OC_MBPM_<DATE>_1_1_normal_normal_<FILTER>.fits).

An OmegaCAM master flat combined from twilight and dome master flats (single detector).

An OmegaCAM master bad pixel map combined from the hot and cold pixel maps (single detector). A value of 1 indicates a cold pixels; otherwise 0.

QC1_parameters

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.RAW.TWILIGHT.MEDIAN	omegacam_twilight..qc_raw_twilight_median	median flux level of all input raw twilight flats (ADU)	HC		[docuSys coming]
QC.RAW.TWILIGHT.MIN	omegacam_twilight..qc_raw_twilight_min	median flux level of input raw twilight flat having the minimum average flux level (ADU)	HC		[docuSys coming]
QC.RAW.TWILIGHT.MAX	omegacam_twilight..qc_raw_twilight_max	median flux level of input raw twilight flat having the maximum average flux level (ADU)	HC		[docuSys coming]
QC.MASTER.TWILIGHT.MEDIAN	omegacam_twilight..qc_twilight_median	median counts in normalized master twilight flat (ADU)	HC		[docuSys coming]
QC.MASTER.TWILIGHT.STDEV	omegacam_twilight..qc_twilight_std	standard deviation of normalized master twilight flat (ADU)	HC		[docuSys coming]
QC.NUMBER.BAD.PIXELS	omegacam_twilight..qc_bad_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.

Trending

The QC1 health check plots for the RAW OmegaCAM twilight flats are divided into 7 separate linked pages, each of which shows the trending for each of 7 different filters. Since the master twilight flat produced by the OmegaCAM pipeline is normalized, it is not easy to detect whether or not one of the input twilight flat frames was either saturated or had insufficient flux. For this reason, the raw twilight flat parameters are monitored. A further set of QC1 health check plots exist for the OmegaCAM master flats monitoring both the level and the number of detected bad (hot + 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 twilight flats median level averaged over all 32 detectors
• raw twilight flat median level for each detector
• raw twilight minimum and maximum level averaged over all 32 detectors
• score outliers

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

• master twilight flat median level averaged over all 32 detectors
• master twilight flat median level for each detector
• master twilight 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 master twilight flat produced by the OmegaCAM pipeline is normalized, it is not easy to detect whether or not one of the input sky flat frames was either saturated or had insufficient flux. For this reason, the raw twilight flat statistics are monitored.

Scoring&thresholds TWILIGHT Flats

The twilight 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. The upper and lower thresholds ensure that the flux is not too week and that it does not reach the saturation limits of the detector.

History

Date	event
General	OmegaCAM TWILIGHT flats tend to be very stable.
2018-08-18	CCD_75 has been failing over the last several days, replaced (PPRS-073742)
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 TWILIGHT 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 twilight flat having the minimum/maximum average flux level (ADU). These QC parameters are used to ensure that the weakest flat has enough flux to create a statistically accurate master twilight flat and that the brightest does not reach the saturation limits of the detector.

The number of bad (hot + cold) pixels is combined from input master_BIAS (hot pixels map) and master_DOMEFLAT (cold pixels map) for each extension. The recipe does not calculate or add new bad pixels.