Quality Control and
Data Processing

HC PLOTS
Dark level
Dark RMS
Acq camera
QC1 database (advanced users): browse | plot

Dark exposures are measured during daytime as part of the P2VM calibration sequence and also independently for each DIT that has been used during the night for science observations. Raw fits files from dark exposures have, as all GRAVITY raw fits, twelve extensions (see also description of file format). The actual pixel data are stored in extensions 4, 5, and 10; they contain data from the acquisition, SC (Science Camera), and FT (Fringe Tracker) cameras, respectively. The SC detector features a windowed read-out in order to minimize file sizes: only the actually needed areas of the detector are read. Sizes of the images from the SC detector depend, therefore, on the resolution mode (low, medium, or high) and on the polarimetry (combined or split). Windowing is also applied for dark exposures. FT data is always stored as a binary table with 24x5 (combined polarimetry) or 24x10 (split mode) entries.

In March 2017, a dedicated check for non-responsive pixels in the acquisition camera detector has been introduced. All other QC parameters are measured from the SC and FT detectors.

QC1_parameters

In case of the SC detector, the pipeline subtracts the average level (so-called "bias") of each input frame before any further processing is done. The master dark is then the median stack of the NDIT bias-subtracted layers of the input raw dark frame. For the SC detector, NDIT is typically 100 for dark exposures. In case of the FT detector, no bias subtraction is performed.

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.MEANDARK.FT	gravity_dark..darklevel_ft	mean dark level of fringe tracker (FT) detector [ADU]	HC		[docuSys coming]
QC.MEDIANDARK.SC	gravity_dark..darklevel_sc	median dark level of science (SC) detector [ADU]	HC		[docuSys coming]
QC.PIXBIAS.AVG	gravity_dark..biasavg_sc	average detector bias that is subtracted by pipeline (SC only)	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 parameters darklevel_ft and biasavg_sc are trended for the different read-outs but only for darks that are measured within the P2VM calibration sequence. These darks can be identified by the INS.FDDL.WINDOW header key which is 0.1 for the P2VM darks. This distinction is particularly important for the SC because the biasavg_sc parameter is slightly higher (about 1 ADU) for the non-P2VM darks. Please note that the FDDL window has only been introduced in the QC1 database table as of May 2019. Earlier entries have fddl_window=0.1 as default.

Scoring&thresholds Dark level

Scoring thresholds have been set to median +/- 3sigma in May 2019.

History

In early operations until December 2016, dark levels have been significantly higher.

Algorithm Dark level

darklevel_ft: The master is determined as the median stack of the input data; the QC parameter is the average of all pixel values in the master.
darklevel_sc: The master is determined as the median stack of the input data after bias subtraction; the QC parameter is the median of all pixel values in the master.
biasavg_sc: The average bias that is subtracted from the input frames in case of the SC detector. The actual dark level of the input frames is then darklevel_sc + biasavg_sc.

QC1_parameters

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.DARKRMS.FT	gravity_dark..darkrms_ft	standard deviation of master dark for FT detector	HC		[docuSys coming]
QC.DARKRMS.SC	gravity_dark..darkrms_sc	standard deviation of master dark for SC detector	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 parameters are trended for the different read-outs but only for DITs that are used in the P2VM calibration sequence (i.e. SC DITs of 0.3 sec and FT DITs of 0.00085 sec).

Scoring&thresholds Dark RMS

Scoring thresholds have been set to median +/- 3sigma in May 2019.

History

None.

Algorithm Dark RMS

The RMS of the master dark of the FT or SC detector.

QC1_parameters

The images from the acquisition camera are checked for pixels that do not respond. In a dark exposure, all pixels that have 0 ADU are counted. These are potentially non-responsive. An increase in the number of these pixels indicates an issue with the detector that could affect the acquisition procedure or the usage of the acquisition images during science analysis. Typically, about 6% of all pixels of the acquisition camera detector have zero value in a 0.7s dark exposure.

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
none	gravity_dark..numpix_zero_acq	number of pixels in acquisition camera with 0 value	HC		[docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter **There might be more than one.

Trending

The number of zero level pixels in the acquisition camera is monitored over time.

Scoring&thresholds Acquisition camera

About 6% of all pixels of the acquisition camera detector have zero value in a 0.7s dark exposure. This corresponds to 60000 pixels for a 1000x1000 pixels detector window. The scoring threshold has, therefore, been set to 65000 pixels in order to allow some statistical variation.

History

2017-03-24: change of format for acquisition images, 1000x1000 pixels are saved instead of 2048x1536.

Algorithm Acquisition camera

The parameter is calculated by a QC script. The number of pixels with 0 ADU in the first layer (i.e. first exposure) of the 3D acquisition camera image is counted.