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Calibration Access and Data Handbook
Next: PSF1DRB
Up: PIXTOPIXSENS
Previous: Generation from Ground Calibration
Contents
Ideally the in-orbit PixToPixSens file will be generated from internal
calibration measurements using the fullframe high resolution engineering
mode. However due to their large data volume the fullframe high resolution
data won't be available frequently enough
to base the in-flight generation of the PixToPixSens file
on the high resolution data.
Therefore a different strategy has to be adopted.
The SAS task omflatgen will be used to compute the PixToPixSens
files from low resolution internal calibration exposures (tracking off,
BPE on), which are predominately acquired during slews.
The analysis can be outlined as
- Multiple exposures may be co-added to achieve
high enough statistics.
- removal/flagging of bad pixels
- calculation of average CCD frametime in the used exposures
- calculation of average CFRR,
i.e. the average counts per pixel per CCD-frame
- fpn?
- check dark counts and exclude bad pixel?
- calculation of the local normalization mask(?)/value for each
8x8area/pixel
e.g. by calculating the average within a sliding box of TBD width.
- local image normalization
by dividing the pixel content by the local average value
- eventual exclusion of large excursion regions and recalculation of
step 4 and 5
- global normalization
e.g. by division with the average of the local normalization values
or by normalization to values of a reference area/pixel.
If the non uniformity of the calibration lamp illumination is known with
sufficient accuracy,
the inverse of the illumination pattern can be be used for uniformity
correction and the normalization steps can be omitted in the data
analysis. The non-uniformity of the calibration lamp illumination can
be derived from a comparison of the smoothed high resolution flatfield
with the large scale flatfield.
In case not sufficient high resolution flatfield data are available
the low resolution data must be expanded into high resolution
PixToPixSens data.
This can be achieved in three different ways, depending on the availability
of data.
- If high resolution data are available and provide
sufficient statistics, these data are used to calculate the PixToPixSens
file. A pixel expansion is not required.
- If high resolution data are available, but the data do not provide
sufficient statistics or they are out of date,
each pixel of the low resolution data is expanded into four subpixels
by applying a weighting factor to each quadrant of the old pixel.
The weighting factor is derived from full resolution images.
- If no high resolution data are available or for the initial calibration
procedure
low resolution data are expanded into full resolution data applying
a weighting factor of 1.0 to each quadrant of the old pixel.
Next: PSF1DRB
Up: PIXTOPIXSENS
Previous: Generation from Ground Calibration
Contents
Michael Smith
2011-09-20