VIRCAM: Quality of twilight flats
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HC PLOTS |
RMS of master flat |
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RMS of ratio flat |
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counts (min,max) |
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counts range |
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QC1 database (advanced users):
browse |
plot
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The vircam calibration plan includes twilight flats to calibrate science
exposures. The derived QC parameters measure operational properties,
e.g. if the covered flux range is optimal, or heavily underexposed. No
instrument specific property is monitored.
RMS of master twilight flat
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
QC.FLATRMS | vircam_twilight..qc_twil_rms | rms in master flat | ENG | | [docuSys coming] |
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one. |
Trending
The rms of the master flat is monitored for all filters: NB118, NB980, Y, Z, J, H ,K separately.
The K-band is monitored
here.
Scoring&thresholds RMS of master twilight flat
There is no scoring.
History
No remarks.
Algorithm RMS of master twilight flat
The QC.FLATRMS is a measure of RMS in ADU obtained from master twilight flat products. It is the median of the absolute deviations from the median ( = MAD ) times 1.48 derived from the master twilight flat.
RMS of ratio flat
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
QC.FLATRATIO_RMS | vircam_twilight..qc_ratiotwil_rms | rms of ratio flat | ENG | | [docuSys coming] |
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one. |
The vircam twilight flat pipeline recipe allows to specify a reference flat, which can be any
master flat. The pipeline divides the master flat by the submitted reference flat and extracts some QC parameters
of which the RMS of the ratio flat is the most important. The pixel-to-pixel relative response
differences (= the fixed pattern noise), which dominate the RMS of the master flat, are canceled out in the ratio flat.
The RMS of the ratio flat is therefore sensitive to other changes in the master flat, like:
- changes in the relative gain of the 16 read-out channels (16 vertical stripes each one 128 pixel wide).
Note the comment for the dome flats. Since early 2020 channel gain variations increased. In this period the values of qc_ratiotwil_rms are dominated by
channel gain variations and are no longer sensitive to the
detector characteristics originally intended to be monitored,
in particular for detectors #6 and #10, #12 and #14.
- new dust grains
- new bad pixel (e.g. 2013-03--01 on chip #3)
- controller issues
Trending
The RMS of the ratio twilight flat is monitored here.
Scoring&thresholds RMS of ratio flat
The values are not scored
History
The maintenance of the reference flat means it has to
be updated from time to time, typically twice a year and after interventions. The twilight flat QC1
DB contains the name of the reference twilight flat
(qc_twflat_refname) and the time stamp of the reference
twilight flat (qc_twflat_refdate).
Algorithm RMS of ratio flat
The master flat is divided by the reference flat, the RMS is taken.
Minmax of twilight flat series
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
counts in faintest flat | vircam_twilight..qc_twflat_medmin | minimum counts | ENG | | [docuSys coming] |
( max - min ) / 2 | vircam_twilight..qc_twflat_medavg | average counts | ENG | | [docuSys coming] |
counts in brightest flat | vircam_twilight..qc_twflat_medmax | maximum counts | ENG | | [docuSys coming] |
max - min | vircam_twilight..qc_twflat_medrng | counts range | ENG | | [docuSys coming] |
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one. |
Trending
The counts in the faintest flat is monitored as
red
diamond symbols, the counts
in the brightest flats is monitored as black
square symbols. The qc_twflat_medavg is monitored in the same plot as green circles.
Due to the different responses per detector, the detector with lowest response of the brightest raw flat
can show less counts that the highest response detector of the faintest raw flat.
The counts range of the raw flat sequence is monitored
here.
The counts in the brightest flat can be divided by the DET.SATLEVEL to check how close the counts of brightest flat
are at the saturation level. The values are scored for the individual, but there is not trending plot provided,
as being a pure product quality and not an instrumental property.
In case raw flats are rejected by the pipeline recipe because being too bright or too faint, the master flat of that chip
is composed of a lower number of raw flats, which is expressed by the PRO.DATANCOM key. This value is scored on the product level, but there is no trending plot provided.
Scoring&thresholds Minmax of twilight flat series
The values are not scored.
History
No remarks.
Algorithm Minmax of twilight flat series
min,max,avg,range: The counts of the twilight flat raw frames vary as the
sky brightness varies during dusk or dawn. For each raw
frame the median counts is taken. Out of this sample of
raw frame medians the minimum, the maximum, the average,
the standard deviation and the range is extracted.
fracsat: Among all median flux values obtained from the raw fats,
the highest flux value is multiplied by (MINDIT+DIT) /
DIT to get the flux value on the chip before the detector
reset was subtracted by the acquisition system. The
value is compared with the saturation level. Details
are given on the problems page.
Each detector has its own response. Therefore twilight flats and dome flats show the following relative count rates:
chip13 |
chip 14 |
chip 15 |
chip 16 |
0.79 |
1.04 |
1.17 |
0.91 |
chip 9 |
chip 10 |
chip 11 |
chip 12 |
1.04 |
1.13 |
1.03 |
1.13 |
chip 5 |
chip 6 |
chip 7 |
chip 8 |
1.08 |
1.19 |
1.26 |
1.01 |
chip 1 |
chip 2 |
chip 3 |
chip 4 |
1.24 |
0.79 |
0.86 |
1.04 |
chip #1 and #13 showing the smallest response and chip #1 and #7 showing the largest response. Although the response of chip #5 is close to the array average, it is the first to saturate because of its low saturation level.
Chip #16 depression
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
NONE | vircam_twilight..qc_twflat_fluxdep | flux depression (for chip #16) | ENG | | [docuSys coming] |
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one. |
The upper part of detector #16 shows the following effect when being illuminated:
- In J band and NB118 shows about 75% of the flux when compared to the remaining area of the chip. The level of the flux depression was rather constant within the investigated time interval (2009-11 ... 2010-02).
- The H and K band flats are not affected.
- The Z and Y band images show a remaining flux of 60% to 80% . The flux depression is variable in time and the variability of the flux level is not correlated between Y and Z band.
The flux depressed pixel area on chip #16 is not homogeneous, but the area remains constant in time.
- In addition to the large scale fractal-shape area with flux depression there are a considerable number single pixels (isolated pixels) with unreliable response.
- As a consequence source detection as part of the pipeline science recipe is affected for the upper part of chip #16.
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The QC1 parameter qc_twflat_fluxdep, as obtained from detector #16 master twilight flats.
The parameter represents the median flux in the upper part of the detector in the pixel
area: 1500,2048: 2048,2048. The abscissa is the MJD-OBS (modified Julian Date).
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Trending
The depression of the upper part of detector #16 is
monitored for all filters.
Scoring&thresholds Chip #16 depression
The parameter is not scored.
History
No remarks.
Algorithm Chip #16 depression
Using the (normalized) master twilight flat, the median
counts in the following window is given: xmin=1
ymin=1500 xmax=2048 ymax=2048 , which covers almost the
upper part of the detector. Setup: INS.FILT1.NAME = Ks
(2.15), H (1.65), J (1.25), NB118 (1.185), Y (1.02) NB980
(0.98), Z (0.88) As the upper region of detector
#16 shows a rather fractal pattern, the minimum counts
in the analyzed region are much less than the median
value monitored.The purpose of the plot is to monitor
the unpredictable variations in the blue bands.
Twilight cirrus
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
None | vircam_twilight..qc_twflat_expfitrms | rms of skybrightness fit | ENG | | [docuSys coming] |
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one. |
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Quality control report for chip #10 of a twilight flat. Black is reported counts and blue points are
counts corrected for the MINDIT effect. The saturation level, 90% of the saturation level and the pipeline lower
rejection threshold is marked. Right: least square fit of flux(time)= A + B * exp ( - C * (D - time)) to the
data points.
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Trending
The cirrus parameter is monitored
Scoring&thresholds Twilight cirrus
This parameter is not scored
History
In 2019-01 two cases were found where one of the raw
frames of the sequence showed in several but not all
detectors a controller issue (an unillumnated image
with a peculiar fixed patter noise). The inclusion of the
fake raw frame cannot be detected in the QC parameters
extracted from the master twilight flat.
Only the cirrus parameter is sensitive to these events.
Controller issues can show up in cases of high ambient
temperature.
Algorithm Twilight cirrus
Under photometric and clear conditions the sky
brightness is (de)inreases during (dusk)dawn in a
continuous manner. Thin or thick cirrus moving through
the FOV disturb the continuous flux trend. The function
flux(time)= A + B * exp ( - C * (D - time)) is fit to the
raw flux sequence and the RMS of the fit is considered
as a measure of the sky brightness continuity within
the FOV. The CirrusTest parameter is the RMS of the
exponential fit to fluxes retrieved from the twilight
flat raw frames. It is a quality parameter of the
master twilight flat.
2009-10-3 ... 2009-11-03: twilight flat saturation
The acquisition of photons by near infrared detectors consists of an initial reset, a first non-destructive read out followed by one or more further reads. The signal recorded between the reset and the first read is subtracted by the acquisition system before being written to file. The time between the reset and the first read amounts to 1.0011 sec and is called the minimum possible DIT (= MINDIT). The time between the first and the final read is called DIT. The fits frame contains the counts accumulated within DIT seconds, but the pixels on the detector hosted more counts accumulated within DIT + MINDIT seconds and might saturate the chip. This internal saturation can occur for high fluxes and when a DIT of the order of MINDIT is used. This effect occurred for a number of raw twilight flats in some periods during the commissioning (2008-10-01 - 2009-10-25), science verification (2009-10-15 - 2009-11-03) and dry runs (2009-11-04 - 2010-02-15). Assuming linear response the signal on the detector before reset subtraction can be estimated :
counts[detector] = counts[rawframe] * ( MINDIT + DIT ) / DIT
The counts[detector] values are monitored for twilight flats. The maximum values within a raw frame stack is given in units of the individual detector saturation level by the dimensionless QC1 parameter qc_twflat_fracsat. The values are scored.
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Counts of a stack of twilight flat raw frames with DIT=1.0011 sec for detector #5. The black dots show the counts[rawframe] values and are well below the saturation level. The blue dots show the counts[detector] values of which the first few frames show saturation effects. The saturation level and the scoring threshold (= 90% of saturation level) are shown.
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The same stack for detector #11 with a higher saturation level.
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