Quality Control and
Data Processing

HC PLOTS
bias level
bias level for selected CCDs
hot pixels
QC1 database (advanced users): browse | plot

BIAS frames are taken on a daily basis during normal VST/OmegaCAM operations. They are usually obtained during day-time calibration and consist of 10, zero second, closed shutter, raw bias exposures.

From this, the pipeline creates two output frames from its bias recipe (omega_mbias): a master bias (OC_MBIA_<date>_1_1_normal_normal.fits) and a hot pixel mask (OC_MHPM_<date>_1_1_normal_normal.fits). The master bias frame is created from the 10 input raw bias frames averaged together with a 5sigma rejection of outliers (where sigma is defined from the dispersion of each pixel through the 10 input frames). The master bias is used for all subsequent processing and the hot pixel map is used in the flat field recipe in order to create a master bad pixel map consisting of both the hot and cold pixels.

An OmegaCAM master bias (single detector) as computed from 10 raw bias frames (zero exposure time, opaque filter).

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

A schematic of the geometry of an individual OmegaCAM CCD..

The master bias frames from detectors ESO_CCD_68 and 90 have a pronounced central ridge. This is stable and can be well subtracted. Therefore, these features are tolerated, but can create a larger number of hot pixels when compared to other detectors.

QC1_parameters

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.MASTER.BIAS.MEDIAN	omegacam_bias..qc_bias_median	median value of master bias frame (ADU)	CAL		[docuSys coming]
QC.MASTER.BIAS.STDEV	omegacam_bias..qc_bias_std	standard deviation of master bias frame (ADU)	CAL		[docuSys coming]
QC.RAW.BIAS1.MEAN	omegacam_bias..qc_raw_bias1_mean		CAL		[docuSys coming]
QC.RAW.BIAS2.MEAN	omegacam_bias..qc_raw_bias2_mean		CAL		[docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter **There might be more than one.

Trending

The median bias level averaged over all CCDs is plotted in box 1 and of individual CCDs in box2 of the trending plot. Box 3 shows scores for each CCD.

The mean level of the first and second raw bias frame for selected detectors 82, 81, and 83 (in ADU) is plotted here .

Scoring&thresholds BIAS level

The median bias level is tightly scored, with static thresholds following the median average of the current period. The only requirement on the bias level is stability on the calibration timescale.

History

Date	event
2012-09-13	the bias level of det.96 changed from 380 to 650 ADU
2015 March/April	the bias level of det.73 occasionally drops to almost 0 ADU; identified as problem with cabling; fixed
2016 October	occassional increase of RON of CCD_69 and CCD_77 caused jumps in bias std values
2018-08-20	CCD_75 has been failing over the last several days; after closer investigation the faulty FIERA videoboard 77 was replaced with spare 82 (PPRS-073742); since then, the CCDs_73/74/75/76 controlled by the videoboard, show increased bias_std values
2020-11-15	CCD77 does not work; only displays random signal consistent with noise (PPRS-079377)
2020-11-15	CCD69 shows blob in upper border; visible in bias but more evident in darks (PPRS-079620)
2022-05-09	CCD77 started recording some coherent signal again

Algorithm BIAS level

The median level and the standard deviation of the master bias frame are calculated for each extension (ADU).

QC1_parameters

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.NUMBER.HOT.PIXELS	omegacam_bias..qc_bias_hotpixels	number of hot pixels in master bias frame	CAL		[docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter **There might be more than one.

Algorithm Hot pixels

The number of hot pixels is detected in the master bias frame for each extension. The hot pixels are derived by thresholding. First the mean (Mean) and the standard deviation (Stdev) is derived on the master_BIAS. Then all pixels exceeding "Mean" + "rej-sigma" * "Stdev" are marked as hot pixels. "rej-sigma" is a recipe parameter with the default values: --rej-sigma : Rejection threshold for outlying pixels in Hot Pixels Map. [5.0]

Trending

The number of hot pixels detected in the bias pipeline product frame HOT_PIXELS_MAP is trended for individual CCDs, as well as averaged over all 32 detectors.

Scoring&thresholds Hot pixels

The number of hot pixels averaged over all CCDs is loosely scored, with static thresholds following the median average of the current period. The only requirement is stability of the number on the calibration timescale.

History

The number of hot pixels found in each individual detector (detectors: ESO_CCD_68, ESO_CCD_82, ESO_CCD_84, and ESO_CCD_90 consistently show very high values).