By default, the mean of each image is calculated (or found in the XVISTA symbol table) and all images are scaled to have the same mean as the first one specified. If nomean is specified, then no scaling is performed; alternatively, if a value is supplied via the pix keyword - say, pix=8 - then only every n'th (in this case 8'th) pixel is used to calculate the mean of each frame. This can speed up program execution times a bit.
The output median image is placed in the file median.fts or whatever name is specified with the outfile keyword. It has a very minimal header, and so it is wise to use the `COMMENT' command to add some additional information about the particulars of the image.
Execution times can reach many minutes for large images, so the verbose option prints out information on exactly how far the program has gotten in producing the output image.
median skyflat1 skyflat2 skyflat3 pix=16Goes through each of the three files and calculates the mean, using every 16'th pixel. Then, scales each file so that the means are the same. Goes through the images pixel-by-pixel, sorting the three values and placing the middle value into the output file, which will be called median.fts.
Note: in generating sky flats you should first remove the bias before using the median.