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

254 CHAPTER 20. ADVANCED DATA CALIBRATION
5. You can now make the FLAT by clicking on MEAN – the option to use a median is not
available when partial EXPOSUREs are used in the selection.
6. You test the flat by clicking on TEST FLAT.
7. When you are finished click on DONE. The most recently computed FLAT will be returned
by flat builder.
20.11 Background subtraction
The routine bkg builder can be used to manually select IMAGEs from the cube of a PDS, or
indeed any cube, for use in determining a background subtraction frame. Since this requires
that you have to have IMAGES containing no source, then it is most likely that bkg builder
will only be of use to you if you have raster observation data.
To invoke bkg builder:
CIA> background_frame=bkg_builder(raster_pds.cube)
Four windows will appear:
DATA displays all the frames, i.e. IMAGEs, in the input cube.
VIEW displays all the frames in the input cube which you have selected.
BACKGROUND displays the current background frame.
BUILDER is the main window and the user’s interface to bkg builder – see Figure 20.5.
To make a background frame with bkg builder follow this basic procedure:
1. Select some frames by clicking on the panel of buttons under the legend Click on frame
number to select/unselect it!. As you select, your choice of frames will appear in the
window VIEW and the currently selected frame will be displayed in the bottom of the
main window.
2. Now click on the buttons mean or median and the a mean or median frame of the selected
frames will appear in the window background.
3. Some statistical information about your background frame will appear in a subwindow of
the main window. When you are happy with your background frame you can click on done
to exit. Now you chosen background frame will be in the variable background frame.
20.12 Obtaining the best calibration record from a CDS
A CAL-G library file contains many calibration records. (Remember that the CAL-G library
files are stored as CDSs in CIA – see Section 15.3.) These records are comprised of images,
e.g. OFLTs, DFLTs, DARKs etc. . . , and associated CAM parameters, e.g. TINT, FLTRWHL
etc. . . . Obviously, you would wish to choose the best record for your data. CIA provides the
routines find best and find best psf to do just this. In simple terms the algorithm employed
by these routines is: they look at the CAM parameters in each calibration record and try to find
the best match with the parameters of the observation data. However, some extra complexity