ISOCAM Interactive Analysis User's Manual Version 5.0 - ISO - ESA

238 CHAPTER 20. ADVANCED DATA CALIBRATION
20.2.5 Flat-fielding
Currently several methods of flat-fielding, all of which are handled by corr flat, are available.
In general corr flat performs flat-fielding as follows:
1. Looks at the keywords method and inflat to determine how the FLAT is to be selected.
2. Divides each EXPOSURE of .IMAGE with the selected FLAT, or, if the corr flat keyword
/cube is set then the IMAGEs in .CUBE are divided by the FLAT.
3. Finally, places the FLAT in the PDS field .FLAT.
For all the different methods of flat-fielding the impact on the PDS is the same. The field
.FLAT is filled with the .FLAT used for flat-fielding. Either the reduced EXPOSUREs are flatfielded
(.IMAGE is modified) as is the default, or if the keyword /cube is set then the IMAGEs
are flat-fielded (.CUBE is modified).
1. method=‘library’ or method=‘calg’
method: CAL-G or library FLAT correction. In a similar manner as for the CAL-G
DARK, get sscdstruct uses find best (see Section 20.12) to find the most suitable
DFLT and OFLT images from the CIA CDSs and places the product of these images,
i.e. the FLAT, in the PDS field .CALG.FLAT. When the the keyword method is set
to method=‘library’, corr flat uses the FLAT in .CALG.FLAT.
If the corr flat keyword /cube is set then the flat-fielding is performed on the IM-
AGEs, otherwise it is performed on the EXPOSUREs.
called routine: flat library
2. method=‘oflat’
method: CAL-G or library OFLAT correction. This is exactly the same method as for
method=‘library’ except that only an optical flat-field correction is performed.
called routine: flat library
3. method=‘oflat’
method: CAL-G or library DFLAT correction. This is exactly the same method as for
method=‘library’ except that only a detector flat-field correction is performed.
called routine: flat library
4. method=‘auto’
method: Automatic creation of the FLAT. In the case of a raster observation, a very
good FLAT can be determined from the actual observation data. To use this ‘auto’
FLAT set the keyword method to method=‘auto’.
The algorithm used to create the ‘auto’ FLAT is as follows:
(a) A median image is derived from the EXPOSUREs in .IMAGE, or if the keyword
/cube is set, a median image is derived from the IMAGEs in .CUBE.
(b) The resulting median image is then normalized by dividing by its mean.
(c) This FLAT is placed in the PDS .FLAT and flat-fielding proceeds in the usual
way.