Quality Control and
Data Processing

HC PLOTS
Bad pixels
FF profile
FF flux
QC1 database (advanced users): browse | plot

Flat-field exposures are measured during daytime as part of the P2VM calibration sequence. They are recorded with the calibration unit, with one of the grisms inserted (low, medium, or high resolution), and with the Wollaston prism either inserted (split) or not inserted (combined polarimetry). A measurement sequence for one set up consists of four exposures where the four instrument shutters are alternatingly opened.

The example picture below shows a raw flat field of the SC detector for medium resolution and combined polarimetry. Dispersion direction is horizontal. Input 1 was illuminated in this example. This means that light is visible on the detector where inputs 1 and 2, inputs 1 and 3, and inputs 1 and 4 are combined, respectively. Since not the complete fringe is recorded but only four phases of the fringe, there are four spectra visible for each input combination (i.e. baseline).

A flat-field frame for split polarimetry looks similar to the combined case but has eight spectra for each baseline because the input beam is splitted.

Flat-field frame. Medium resolution, combined polarimetry.

QC1_parameters

The number of bad pixels is calculated from dark exposures by the flat-field pipeline recipe or by the P2VM recipe.

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.BADPIX.FT	gravity_p2vm..badpix_ft	number of bad pixels of FT detector	HC		[docuSys coming]
QC.BADPIX.SC	gravity_p2vm..badpix_sc	number of bad pixels of 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

Bad pixels are trended for both the FT and SC detectors, for SPLIT and COMBINED polarimetry, and for LOW, MEDIUM, and HIGH spectral resolution (which have different detector windowing).

Scoring&thresholds Bad pixels

For the FT, the number of bad pixels should be 0 because of the small read-out windows.

For the SC, the number of acceptable bad pixels cannot be determined a priori. The upper limit for scoring has been set relatively close to the actual values. Note that the number of bad pixels is slowly increasing for the SC so that reviewing of the thresholds is needed from time to time. The reason for this behaviour of the SC detector is not understood.

History

Date	event
since 2017-04-15	bad pixel on FT detector, affecting baseline 23, phase A in combined polarimetry

Algorithm Bad pixels

A pixel is regarded as bad if its dark current fulfills the condition: dark_current > meandark + threshold x darkrms. The applied threshold is 10.

QC1_parameters

The master flat is the mean of the four input frames.

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.PROFILE_CENTER.SC1.MED	gravity_p2vm..profile_center_sc1	averaged flux in e/s from calibration unit	CAL		[docuSys coming]
QC.PROFILE_CENTER.SC13.MED	gravity_p2vm..profile_center_sc13	centre of 13th flat-field profile (pixels) in SC data	CAL		[docuSys coming]
QC.PROFILE_CENTER.SC25.MED	gravity_p2vm..profile_center_sc25	centre of 25th flat-field profile (pixels) in SC data	CAL		[docuSys coming]
QC.PROFILE_CENTER.SC37.MED	gravity_p2vm..profile_center_sc37	centre of 37th flat-field profile (pixels) in SC data	CAL		[docuSys coming]
QC.PROFILE_WIDTH.SC1.MED	gravity_p2vm..profile_width_sc1	width of 1st flat-field profile (pixels) in SC data	CAL		[docuSys coming]
QC.PROFILE_WIDTH.SC13.MED	gravity_p2vm..profile_width_sc13	width of 13th flat-field profile (pixels) in SC data	CAL		[docuSys coming]
QC.PROFILE_WIDTH.SC25.MED	gravity_p2vm..profile_width_sc25	width of 25th flat-field profile (pixels) in SC data	CAL		[docuSys coming]
QC.PROFILE_WIDTH.SC37.MED	gravity_p2vm..profile_width_sc37	width of 37th flat-field profile (pixels) in SC data	CAL		[docuSys coming]
QC.FLUX_FT0.AVG	gravity_p2vm..fluxavg_ft1	averaged flux in e/s from calibration unit	CAL		[docuSys coming]
QC.FLUX_FT1.AVG	gravity_p2vm..fluxavg_ft2	averaged flux in e/s from calibration unit	CAL		[docuSys coming]
QC.FLUX_FT2.AVG	gravity_p2vm..fluxavg_ft3	averaged flux in e/s from calibration unit	CAL		[docuSys coming]
QC.FLUX_FT3.AVG	gravity_p2vm..fluxavg_ft4	averaged flux in e/s from calibration unit	CAL		[docuSys coming]
QC.FLUX_SC0.AVG	gravity_p2vm..fluxavg_sc1	averaged flux in e/s from calibration unit	CAL		[docuSys coming]
QC.FLUX_SC1.AVG	gravity_p2vm..fluxavg_sc2	averaged flux in e/s from calibration unit	CAL		[docuSys coming]
QC.FLUX_SC2.AVG	gravity_p2vm..fluxavg_sc3	averaged flux in e/s from calibration unit	CAL		[docuSys coming]
QC.FLUX_SC3.AVG	gravity_p2vm..fluxavg_sc4	averaged flux in e/s from calibration unit	CAL		[docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter **There might be more than one.

Trending

The profile QC1 parameters are available only for the SC detector. Flux is trended for both the FT and SC detectors.

Scoring&thresholds Flat field quality

Scoring thresholds have been determined empirically. The upper limit for the flux in MEDIUM resolution is set slightly below the saturation limit. For low and high resolution, upper limits for the flux are still far away from saturation.

The profile position is an important parameter for extracting spectra. A shift in the position very likely results in a poor P2VM calibration and may also affect extraction of spectra for calibrators and science. Scoring thresholds for profile center and width have been set to median +/- 3sigma in May 2019. Only the first profile is scored since the QC values for the others are highly correlated.

History

Date	event
since start of operations	tilts and shifts of the spectra have been observed a few times

Algorithm Flat field quality

profile_center: Y position of the central part of the 1st (lowest) flat-field profile in SC data. Same for the 13th profile, 25th profile (only for split polarimetry), and 37th profile (only for split polarimetry).
profile_width: Width in pixels of the 1st flat-field cross-dispersion profile in SC data. Same for the 13th profile, 25th profile (only for split polarimetry), and 37th profile (only for split polarimetry).
fluxavg: Flux from calibration unit in e-/s. Separated for FT and SC data, and for inputs 1 to 4. The two polarimetric beams in split polarimetry are added.