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Standard construction
There are two ways to produce flat fields for use with IR data. The
standard method is to take 'LAMP ON' and 'LAMP OFF' exposures of the
calibration spot on the telescope dome. The 'LAMP ON' exposure
contains light from both the calibration lamp, which should provide
the even illumination needed for the flat field, together with
potentially uneven illumination from the telescope and instrument
environments. To retain only the even calibration illumination, the
``LAMP OFF'' image must be subtracted from the ``LAMP ON'' image. This
subtraction also removes the bias pattern from the flat field image.
The subtracted image should then be normalised for later use as a flat
field.
In MIDAS, you may directly use COMPUTE/IMAGE to perform
the subtraction. The image mean may then be calculated using
STATISTICS/IMAGE or similar. The ``LAMP on - LAMP OFF'' image
can then be divided by this value using COMPUTE/IMAGE for
normalisation.
Alternatively, the procedure MKFLAT/IRAC2 combines these actions.
To improve the signal-to-noise ratio of the flat-field, a number of
flat field images may be used together. These should be combined using
COMBINE/CCD (see later).
Next: Alternative construction
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Petra Nass
1999-06-15