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