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Since the number of parameters involved in a complete CCD reduction
is quite large, most command parameters have default values. These
defaults are taken from the CCD keyword structure. So, after the data
have been read in and organized, you can set the CCD keywords to
control the CCD reduction process. Basically, one can divide the
CCD keywords is three categories:
- general keywords for like telescope, instrument/detector
specifications (which CCD has been used; what is the useful area on
the chip; what is the read-out noise, its gain; what is the overscan
area) and the various entries, needed for the various steps in the
calibration sequence. Most of the keywords are filled by the
LOAD/CCD command (see below).
- calibration keywords related to the handling of calibration
data, like fitting of the overscan area, the creation of
master calibration frames, the method of bad pixel correction,
the creation of illumination and fringe frames. For combining
the calibration frames in master calibration frames the
systems is provided with a set of keywords for each exposure
type (see below);
- reduction keywords for controlling the reduction of the science
frame(s). Examples are: should the overscan area be used to
determine to bias; should we correct for bad pixels; is dark current
to be subtracted.
Although the CCD commands mostly get their input parameters
from the CCD keywords, most commands accept a limited number of input
parameters on the command line. These parameters will supersede the
corresponding keywords. However, apart from a few cases the keyword
setting itself is not modified: principally, keywords can only be
changed by the SET/CCD command. For handling all these keywords the
CCD package in equipped with five commands: HELP/CCD, SHOW/CCD,
SET/CCD, SAVE/CCD and INIT/CCD.
HELP/CCD without parameter gives an overview of all CCD commands
available. With an existing CCD keyword as parameter the command will
show the present values of that keyword and a short explanation of it use.
SHOW/CCD gives you the current CCD keyword contents. Given the
number of keywords, to display the information the keywords are grouped
and displayed according to the functionality:
- GE display keywords concerning general information;
- BS keywords for combining bias frames into a master bias frame;
- DK keywords for combining the dark frames;
- FF keywords for combining the flat fields;
- SK keywords for combining the sky frames;
- OT keywords for combining other exposure types;
- OV keywords for fitting the overscan region;
- FX keywords for the bad pixel correction;
- IL keywords for the creation of an illumination frame;
- FR keywords for the creation of a fringe frame;
- SC keyword for control of the actual reduction sequence.
With SET/CCD up to a maximum of 8 keywords in the CCD context can
be changed in one go.
After (partially) having finished a CCD reduction the user may want to
store the current keyword setting. This can be done using the command
SAVE/CCD. The command will store the keywords in descriptors
of the CCD reduction table, or in case that isn't available, will create
an empty table with descriptor information only. The keyword setting
can be restored by the command INIT/CCD red_table, where
red_table is the reduction table containing the CCD keyword
setting, previously saved.
Next: Calibration Frames and
Up: CCD Reductions
Previous: General Overview of
Pascal Ballester
Tue Mar 28 16:52:29 MET DST 1995