For each pixel of an image in the combiner,
sigma_clipping masks the pixel if is more than a
user-specified number of deviations from the central value of that pixel in
the list of images.

The sigma_clipping method is very flexible: you can
specify both the function for calculating the central value and the function
for calculating the deviation. The default is to use the mean (ignoring any
masked pixels) for the central value and the standard deviation (again
ignoring any masked values) for the deviation.

You can mask pixels more than 5 standard deviations above or 2 standard
deviations below the median with

Numpy masked median can be very slow in exactly the situation typically
encountered in reducing ccd data: a cube of data in which one dimension
(in the case the number of frames in the combiner) is much smaller than
the number of pixels.

For each pixel position in the input arrays, the algorithm will mask the
highest nhigh and lowest nlow pixel values. The resulting image will be
a combination of Nimages-nlow-nhigh pixel values instead of the combination
of Nimages worth of pixel values.

You can mask the lowest pixel value and the highest two pixel values with:

Flux conservation Whether flux is conserved in performing the
reprojection depends on the method you use for reprojecting and the
extent to which pixel area varies across the image.
wcs_project rescales counts by the ratio of pixel area
of the pixel indicated by the keywordsCRPIX of the input and
output images.

The reprojection methods available are described in detail in the
documentation for the reproject project; consult those
documents for details.

You should carefully check whether flux conservation provided in CCDPROC
is adequate for your needs. Suggestions for improvement are welcome!

Align and then combine images based on World Coordinate System (WCS)
information in the image headers in two steps.

First, reproject each image onto the same footprint using
wcs_project. The example below assumes you have an image with WCS
information and another image (or WCS) onto which you want to project your
images: