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

20.2. CORE CALIBRATION 239
called routine: flat auto
5. method=‘manu’
method: A FLAT may be manually from observation data. In this case corr flat calls
the routine flat builder to interactively aid you in obtaining the perfect manual
FLAT – see Section 20.10. After exiting flat builder your custom FLAT is placed
in the PDS field .FLAT and flat-fielding proceeds as normal.
called routines: flat builder
6. inflat=my flat
method: Flat-fielding with your own FLAT. If you happen to have your own FLAT, e.g.
say my flat, then you can pass this to corr flat by setting inflat=my flat. Again,
my flat will be placed in .FLAT and the flat-fielding procedure will continue as usual.
called routine: N/A
20.2.6 Flat-fielding and wheel jitter
Due to wheel jitter CAM images can become considerable shifted in the horizontal or instrument
y axis direction. This shift is caused by a misalignment between the lens and Fabry mirror and
the optical axis. The extent of this shift can be up to 2 pixels. This shift also has the effect of
making flat-field correction with the library or CAL-G optical flat-field images invalid. These
flats expect a well aligned lens wheel. As an alternative, data from suitable raster observations
may be flat-fielded with the ‘auto’ method. However, this is not an option for data from raster
observations with few raster points and certainly not an option for data from other AOTs. A
further alternative is suggested here:
1. Create a rough ‘auto’ flat from the IMAGEs in a PDS. This will be used to measure the
shift caused by the lens wheel:
CIA> flatauto = flat_auto(raster_pds)
2. Use xdisp to measure the shift in the ‘auto’ flat image.
CIA> xdisp, flatauto
The shift will be apparent in two ways:
• You may see dark columns at the edge of the image. The number of such columns
helps to indicate the shift in the image.
• Features in the rough ‘auto’ flat should correspond to features in the library flat. Any
misalignment will indicate the amount of shift. You can check the library flat with:
CIA> xdisp, raster_pds.calg.flat
You might also want to try correlating flatauto with the library flat raster pds.calg.flat
using the IDL Astronomy User’s Library routine correl optimize. However, this routine
does not usually work well with images of this nature.
Whichever method you choose you need to measure the (x, y) positions of the limits of the
image containing good quality data.