GIRAFFE: Bias
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HC PLOTS |
Bias level, RON, structure, bad pixels |
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QC1 database (advanced users):
browse |
plot
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2008-05-24: GIRAFFE CCD "Carreras" operational (often called "new")
(replacing "Bruce").
Both CCDs are identical in format and size.
2011-01-11:
fast read mode added
2016-04-01:
the 'slow' mode is offered in VM,
calibration data are processed and QC parameters are available, but they are
not trended because of their very rare acquisition.
A set of 5 BIAS frames is measured daily as part of the routine calibration
and health check plan. The BIAS level, the read noise and the
structure are monitored.
'Normal' is the standard read mode of the CCD. The
QC1 database has also some data for the 'Fast' and the 'Slow' mode, but (with
the exception of the BIAS plot) there are no HC plots for these rarely used
modes.
Since October 2010, the fast readout mode is offered
in addition to
the normal readout mode. Its trending plots are set up the same way
as the ones for
normal read mode, find them under 'bias_Fast'.
Other CCD parameters are monitored using the darks and
the image
flats.
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BIAS frame, old CCD. There is an upward gradient of the order of several ADU which is monitored by the STRUCTY parameter. The bright vertical feature is a bad column.
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BIAS frame, new CCD. The new CCD has cleaner cosmetics, there is no bad column. The gradients in X and Y are much smaller.
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Read noise (raw and master)
The read noise (RON) is measured as rms in a single raw frame
and in the master bias frame, using in both cases a small central window 100x100
pixels wide. The RON of the master bias frame is about a factor sqrt(N) lower than
the raw file RON, where N typically is 5. RON is given in electrons, calculated from
the ADU value using the nominal transformation factor (CONAD = 1/gain) 2.25
e-/ADU. (The measured CONAD values are close to that value, check out here.)
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
QC.OUT1.RON.RAW | giraffe_bias..sigma_raw | RON value of first raw frame in ADU | KPI | | |
QC.OUT1.RON.MASTER | giraffe_bias..sigma_master | RON value of master bias in 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'. |
Algorithm Read noise (raw and master)
The read noise (RON) is measured as rms in a single raw frame
and in the master bias frame, using in both cases a small central
window 100x100 pixels wide. Outliers below 100 and above 300 ADU are
clipped.
Trending
Box 2 of the trending
plot shows the RON of single raw BIAS frames, box 3 the RON of master
bias frames.
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RON plots. Box 2 shows the RON of single raw BIAS frames, box 3 the RON of master bias frames, typically constructed from 5 input raw frames.
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Scoring&thresholds Read noise (raw and master)
The sigma_raw is tightly scored, with dynamic thresholds following the median average of the current period. The thresholds could be made even tighter but due to the relatively small window for the noise measurement the risk of false red scores due to outlier cosmics would then increase. The window size is chosen as to minimize the larger-scale structure of the bias.
The sigma_raw is a KPI parameter, with its reference value being 4.2 el- (from the commissioning report of the new CCD), slightly higher than the current value.
History
The sigma_raw value is very stable over time.
BIAS level
Box 1 shows the median BIAS level measured across the whole CCD, given in
ADU, for the 'Normal' read mode. It is calculated by the QC procedure
(for historical reasons).
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
[BIAS_level calculated by QC procedure] | giraffe_bias..median_master | median value of master bias in 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
The median bias level is plotted in box 1 of the trending
plot.
Scoring&thresholds BIAS level
Normal read mode:
- The median_master is tightly scored, with dynamic thresholds following the median average of the current period. The only requirement on the bias level is stability on the calibration timescale.
- The sigma_raw (read noise) is scored statistically, it is very stable. The
same is true for sigma_master which however could produce artificial outliers
if less than 5 (or more than 5) input frames get combined.
- The structure
parameters are loosely scored, there is no requirement other than stability on
the calibration timescale.
- For the number of "bad" pixels there is no requirements other than the
detection of sudden outliers.
Fast read mode:
The same applies to the Fast read mode bias plots, with the exception of
median_master which is scored in a less restrictive way then the normal data
because this mode is very rarely used and sufficiently monitored with the
'normal' mode.
History
The mean bias has slowly decreased over the years.
Algorithm BIAS level
The median of the whole master_bias is calculated, values below 100 and above 300 ADU are clipped.
Structure
The GIRAFFE BIAS frames contain some structure which is monitored here.
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
QC.OUT1.STRUCT.X | giraffe_bias..struct_row | structure in X direction in ADU | HC | | |
QC.OUT1.STRUCT.Y | giraffe_bias..struct_col | structure in Y direction in ADU | HC | | |
*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
Box 4 and 5 of the trending
plot show the structure of the BIAS, derived from collapses
of columns (structure in X direction, left) and rows (Y direction, right).
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Structure plots. The structure in X direction is derived
in the master bias, collapsed along the rows (box 4). The
structure in Y is obtained along the columns (box 5). The collapses are shown
below.
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Collapsed master bias.
While the master bias (both for the old and the new CCD) has virtually no structure
in X direction (along the rows), it is quite structured in Y direction, along the
columns.
Old CCD, Y direction: Roughly the first 1000 rows increase from a level
about 4 ADU below average to the average level. The step is due to the bad
columns around column 420 starting at about row 1460.
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New CCD (normal read mode)
Y direction: the increase is much smoother
and has an amplitude of less than 2 ADU. There is no bad column.
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New CCD, fast read mode
Y direction: structure looks very similar to the normal read mode.
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History
Both structure parameters are very stable with time.
Scoring&thresholds Structure
Both structure parameters are scored, with fixed thresholds set such as to be insensitive to normal fluctuations. A red score indicates
a real issue with the detector. There are no reference values available.
Algorithm Structure
The master_bias is collapsed in X or Y direction, the rms of the result frame is measured.
Bad pixels
The number of bad pixels in GIRAFFE BIAS frames is monitored here.
The pipeline delivers proper values only since
2008, for the new CCD.
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
PRO.BPM.NPIX in bpm product | giraffe_bias..bpm_npix | number of bad pixels | HC | | |
*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
Box 6 of the trending
plot show the number of bad pixels which is usually between 0 and 200 in the
new CCD.
History
The pipeline delivered proper values only since
2008, for the new CCD. The number of bad pixels for the old
CCD is much higher than currently, due to the bad columns. It would need to be
corrected for those columns in order to gain significance.
Scoring&thresholds Bad pixels
N_badpix is scored, with a loose upper threshold. If flagging red,
there is a serious detector issue. There are no reference values available.
Algorithm Bad pixels
Pipeline recipe.
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