Next: Running REDUCE/CCD
Up: Processing the Data
Previous: Processing the Data
How the Data is Processed
Before any processing on the input frame is done, the REDUCE/CCD
command will first collect all resources needed for the calibration of the
science frame(s). These include the master calibration frames, the overscan
offset, and scaling parameters. So, at this point no operations are
done yet. This is done for efficiency reasons: all standard calibration
arithmetic on the input frame is done in one go. As an example,
suppose the science frame is supposed to be corrected for dark current
and to be flat fielded. From the Association Table the command first
identifies the names of the master dark and flat frames, and checks for
their existence. If they are not present they will be created.
Next, the master dark and flat field will be checked on their
processing status. If they have not been processed yet, that will first be
do by another (recursive) run of the REDUCE/CCD command. In this
second run also the scaling factors (i.e. exposure times and
the mean of the flat field frame) will be determined.
The standard calibration operation is done by a large COMPUTE/IMAGE
with the following input:
out = (in-scan-biasfrm-darkscale*darkfrm) * flatscale/flatfrm
|
(3.11) |
Here, out is the output frame, in is the input frame, scan is
the overscan bias value or frame, biasfrm is the master bias,
darkfrm and darkscale are the master dark frame and scaling factor, and
flatfrm and flatscale are the master flat field and the mean value of
the flat field. In the COMPUTE biasfrm and darkfrm can also be
constants.
Next: Running REDUCE/CCD
Up: Processing the Data
Previous: Processing the Data
Petra Nass
1999-06-15