GRAVITY: Flat-field exposures
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HC PLOTS |
Bad pixels |
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FF profile |
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FF flux |
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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.
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Flat-field frame. Medium resolution, combined polarimetry.
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Bad pixels
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.
Flat field quality
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.
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