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Alternative construction

For a number of projects where very deep integrations and accurate
flat fields are required, and the targets are point sources or much
smaller than the frame size, the sky background itself can be used to
calculate the flat field. When an object is dithered on the chip, so
that it exposes pixels at different positions in different
integrations, the resulting 'stack' of images may be combined to
produce a very accurate flat field. Since a reference image, such as
the ``LAMP OFF'' frame, is not subtracted, a bias frame of the same
integration time must first be subtracted from all these images. They
should then all be median combined, with suitable scaling introduced
to match the medians of each image to account for the variability of
the sky background level. The resulting image is then normalised to
obtain the flat field.
To subtract the bias from the science images, use `COMPUTE/IMAGE`.
Then use `COMBINE/CCD` to median all the frames
together with multiplicative or additive scaling to match individual
frame medians (you should examine the results of both options to
determine which is working best for your data). Use `STATISTICS/IMAGE`
to calculate the image median, and `COMPUTE/IMAGE` to divide by
this to get the normalised flat field.

** Next:** Sky Subtraction
** Up:** Construction of Flat Fields
** Previous:** Standard construction
*Petra Nass*

*1999-06-15*