GIRAFFE: Flat field quality
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HC PLOTS |
Flat field (FF) level: |
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FF saturation: |
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QC1 database (advanced users):
browse |
plot
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In addition to the fibre localization information, the flat field calibrations
are taken in order to register the gain variations of the CCD (pixel to pixel,
or fixed-pattern noise), fringing, and the fibre-to-fibre sensitivity
variations. To properly record these properties, it is important to monitor
the level of the flat field exposures, and avoid underexposure as well as
saturation.
Exposure level (robotic flat field)
The exposure level obtained by the robotic flat-field lamp is routinely
monitored in a pattern involving the L543.1 setup in every slit system ("HC
settings"). A single flat frame is measured. The mean exposure level of the
raw file is measured (including the fibre signal and the inter-fibre signal),
trended and scored. Thresholds are defined to detect underexposure or
overexposure.
While the FFLEVEL plots monitor these standard setups regularly, the mean
exposure level in all fibre flats is evaluated in a similar way in the
FFLEVEL_ALL plots, but with the scoring thresholds being less sensitive since
they cannot be fine-tuned for every single setting.
Finally, the FFLEVEL_WLN plot records the mean exposure level over wavelength.
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
[calculated by QC procedure] | giraffe_flat..mean_raw | mean signal in first raw frame [ADU] | CAL | | |
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one, always check the link 'HC'. |
Trending
Panels of the trending
plot:
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Boxes 1-2: FF exposure level. Data are from HC calibrations in the L543 setup, for Medusa1 and 2. The boxes 4-6 contain similar data for IFU1, IFU2 and Argus.
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Plots of spectrally resolved relative fibre transmission can be found here.
Scoring&thresholds Exposure level (robotic flat field)
The upper thresholds are set such as to indicate the danger of saturated
pixels. They empirically take into account the effective mix of
fibre signal and the inter-fibre signal. Likewise, the lower threshold
indicates the danger of under-exposure. Ideally both thresholds would be the
same, but the lower threshold is based on a compromise between optimal
calibration quality (SNR) and operational constraints (required calibration time).
The thresholds are used for the HC settings. All other settings would also
need individual thresholds but are taken only sporadically (driven by science
observations). Hence a threshold violation in a HC settings is taken as
indication that the lamp flux level needs to be adjusted.
In the trend_report_FFLEVEL_ALL_HC.html plot the HC settings are plotted in
colour, and all other data (any setting) are plotted for comparison.
History
Initially the fluctuations of the QC1 parameter were quite strong but then
stabilized quickly. The actual values are comfortably below the saturation
limit. The loss in quality for an individual raw flat is compensated by taking
at least 3 identical flat exposures for the daytime calibrations.
The HC plots
FFLEVEL and
FFLEVEL_ALL contain only
the read mode 'Normal'.
The non-standard read modes 'Slow' and 'Fast' are
filtered out because of lack of comparison data.
Algorithm Exposure level (robotic flat field)
The mean exposure level of the raw file across the whole file is
measured. Outliers beyond 60.000 ADU are counted as qc_numsat.
For historical
reasons the calculation is done by the QC procedure.
Saturation (robotic flat-field)
A saturated flat field can still be used for localization of the fibre signal (unless
in extreme cases), but the information related to pixel-to-pixel gain variations, to
fringing and to fibre-to-fibre sensitivity is lost in the saturated regions. Therefore
all measured flats are monitored in terms of saturated pixels.
The HC plot monitors all flat field data. The HC flats (L543.1) are displayed in
colour.
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
[calculated by QC procedure] | giraffe_flat..qc_numsat | number of saturated pixels in (first) raw frame | CAL | | |
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one, always check the link 'HC'. |
Trending
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Boxes 1 and 2: number of saturated pixels for Medusa1 and Medusa2. The values are usually 0, a flat with saturation clearly stands out.
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Scoring&thresholds Saturation (robotic flat-field)
This QC1 parameter is more sensitive to saturation than the mean_raw parameter
(above). It is scored more tightly. The upper threshold is set to 1000 ADU which
is more or less arbitrary but experience shows that data with higher values
indeed have an issue.
The upper threshold is set empirically,
based on quality assessments for the few cases with some saturation. The lower
threshold has no particular meaning.
History
No historical issues.
Algorithm Saturation (robotic flat-field)
The mean exposure level of the raw file across the whole file is
measured. Outliers beyond 60.000 ADU are counted as qc_numsat.
For historical
reasons the calculation is done by the QC procedure.
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