icomb will combine and average two or more images.
The images will be pre-processed as required if the appropriate
flags are given. Post-pre processing is done automatically if
input file contains two frames (as FITS extensions).
Dark image subtraction, flat field normalisation, positivity and
cosmic ray/hot pixel removal is done using the same
pre-processing options as for
ipre.
The -r (replace) option can be used to allow output files to be overwritten.
By default icomb will compute a simple average for each pixel,
but median filtering can be invoked by setting the -M flag.
The default is then to reject values that are are more than
3 sigma off the average value for that pixel. The default sigma
clipping factor can be modified with the -s [nsigma] option.
Note that the median filtering algorithm reads all the images
three times; first to calculate the average, then to calculate
the sigma image, and finally to produce the median filtered
image. This is done to avoid allocating arbitrary ammounts of
memory, and allows an unlimited number of frames to be combined.
If pre-processing options are invoked together with median
filtering, all processing will be made three times on every
image! It is therefore reccomended to perform pre-processing
with ipre before combining images with median filtering in order
to save time.
The sigma image can be saved for later use by giving a filename with
the [-S filename] option.
A bad pixel map can be generated from the sigma frame by selecting
pixels with sigma higher than a threshold. icomb invokes
such bad pixel filtering if using eihter the -B or
-b [limit] options. Pixels whose sigma value is higer than
the limit (which defaults to 100 when using the -B option)
will be replaced by the average of the surrounding (good) pixels.
The invocation is:
> icomb [combine options] [ipre options]
input1.fits input2.fits [... inputN.fits] output.fits
Where combine options include:
Last Updated: 14 August 2002 Roy Østensen |