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Real data always comes with some noise, and it is important that the
software does not worsen the noise spectrum (in pixel as well as in
data space) more than necessarily. With the command
CREATE/RANDOM_IMAGE, images with various random-number distributions
can be generated. For most purposes it will be sufficient to just add
such an image (using COMPUTE/IMAGE) to the frame with the test
pattern. This would simulate the case where there is only detector
read-out noise. If for some reason the simulation of photon noise is
more desirable, this case can be approximated by scaling (again using
COMPUTE/IMAGE) the amplitude of a suitable noise distribution (output
of CREATE/RANDOM_IMAGE) with the square root of the pattern before it
is added to the pattern frame. Evidently, all kinds of combinations of
these two cases can be used.
If the STMODEL context is available (activated by SET/CONTEXT
MODEL in a MIDAS session), then the command HPOIS may also be
used for the simulation of noise.
Petra Nass
1999-06-15