The sequence of commands to do this are:
read/fits/size=800 file1.fits ! read the first image into a 800x800 array save_image ! save the image read/fits/size=800 file2.fits ! read the second image sum_image ! sum the 1st and 2nd images save_image ! save the summed image read/fits/size=800 file3.fits ! read the third image sum_image ! sum the 3rd with the saved image save_image ! save the mosaic disp ! display the mosiaced image grid 0.5 0.5 ! draw a skygrid in 0.5 degree steps disp/exp ! display the exposure map disp/cor ! display the exposure corrected image write/fits mosaic.fits ! write the mosaic to a fits fileTo center the image the X and Y of the pixels of the first image where the image is to be centered should be given. Its usually best to start with the central image, if possible.
The exposure for each image is kept track of within XIMAGE, assuming the entire region has been equally exposed. This may not be accurate, especially for those cases where the active area of the image does not match that of the entire image. Where ever possible an exposure map must be read in.
DETECT and SOSTA will not give the correct results if the summed images come from different pointing directions. They will be unable to make any correction for off-axis vignetting and psf changes, since they reference to the detector coordinates of the last image that was read in.