Bias frames are acquired for the UVB-arm and VIS-arm optical CCD
science detectors only. The pipeline processes five raw frames into a master
bias. Bias frames come in low gain and high
gain read mode and three different types of binning: 1x1, 1x2 and 2x2.
The median of the master bias is trended for two arms:
UVB
and
VIS,
two read-out modi (clocking): 100k/1p/hg and 400k/1p/lg and two types of binning:
1x1, 1x2.
In addition the bias level of the acquisition and guiding camera CCD (
AGC),
is monitored for a unique setup.
The bias level is stable at a level of around 1000 ADU (UVB and VIS, 1500 ADU for the AGC) with +-0.5 ADU fluctuations. The thresholds are at +-2% of the long-term mean.
The read out noise of the master bias is trended for two arms:
UVB
and
VIS,
two read-out modi (clocking): 100k/1p/hg and 400k/1p/lg and three types of binning:
1x1, 1x2 and 2x2.
In addition the read out noise of the master bias of the acquisition and guiding camera CCD (
AGC),
is monitored for a unique setup.
While the read-out noise of the master bias is a property
of the calibration product and describes, how good the read noise has been
reduced in the product, the read-out noise measured in the raw bias frame describes a
detector property.
The read out noise of the raw bias is trended for two arms:
UVB
and
VIS,
two read-out modi (clocking): 100k/1p/hg and 400k/1p/lg and three types of binning:
1x1, 1x2 and 2x2.
The UVB-arm (raw frame) read out noise is stable at 4
ADU with some sporadic outliers in 2015. The VIS-arm
read out noise is less stable and is characterized by
more frequent outliers towards larger values. Thresholds are
set by statistical and operational meaningful arguments to +-1 ADU.
The first two raw bias frames are subtracted. The median
of the statistical noise derived from a random sample
of frame sub regions is obtained and
divided by sqrt(2).
The large scale gradient of the master bias in x-direction is trended for two arms:
UVB
and
VIS,
two read-out modi (clocking): 100k/1p/hg and 400k/1p/lg and two types of binning:
1x1, 1x2.
The gradient in y-direction monitored in a different plots:
UVBVIS.
The gradients of the AGC master bias AGC),
is monitored for a unique setup.
The fluctuations in the AGC x-gradient are related the the ambient temperature.
The gradients of the UVB and VIS are stable, thresholds have
been set, that the long term average and the typical fluctuations are well
within the acceptable region.
Binned wavelength calibration data in the VIS arm tend to be highly saturated in some lines, which may leave remnance in data taken shortly after. The effects decay on the order of 20-60 minutes.
2010
The UVB CCD sometimes suffers from (horizontal) pickup noise, which is visible in low signal data and shows up in the rectified data as tilted lines. Below you see in Fig.A an example of raw data (rotated by 90 degrees) and merged rectified data. The example shown in Fig.B was observed in nodding mode, which creates the bright and dark region in the merged rectified data.
Since_early_March_2010
The VIS CCD shows a time variable pattern and some pickup noise in the bias for the 400k readout mode. The amplitude of the pattern has been strongly reduced after March 20, but it is still present at a low level. Below you see an example in Fig.C of bias VIS raw frame (400k, low gain, 1x1, with cuts of 1025-1035 ADU and rotated clockwise by 90 degrees). Additional work in October 2010 further reduced the pattern, albeit combined with a slight increase in read noise.
2010-03-20
it is significantly reduced. Additional work (improved shielding) in October 2010 reduced it further.
2010-03-06
The VIS bias with readout 400k/lg shows a variable pattern, which increases especially the struct_y parameters, but also the struct_x parameters.
2017-01-01
The UVB arm showed in 2015-07 a period with more structural noise. The VIS arm showed over the instrument lifetime much more often periods with higher structural noise.
2018-12-07
The UVB arm bias frames showed after the maintenance a factor 2 lower struct_y value for the 100k setup.
Figure A, UVB-arm raw frame pickup noise problem in 2010.
Figure B, UVB-arm nodding product pickup noise problem in 2010.
struct_x gives the structure along the x-axis for the left (x1) and right (x2) half of
the CCD. For that purpose the master bias is averaged along the y-axis and the
resulting one-dimensional frame is fit with a linear expression. The slope of
that expression is then stored as struct_x1/2.
mirror sites:
PL (internal link)
HQ
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Bias level
