5. Mosaics

XIMAGE can be used to take several images from a part of the sky and make a mosaic.

5.1 Without Exposure Maps

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 mosaiced 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 file

Its usually best to start with the central image of the mosaic when 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.

5.2 With Exposure Maps

When making a mosaic, it is best to read in an exposure map for each image. The exposure map must be read in after the image. The following example repeats the mosaic described above, but includes an exposure map.

    read/fits/size=800 file1.fits    ! read the first image into a 800x800 array
    read/fits/exp/size=800 exp1.fits ! read the first exposure map (optional)
    save_image                       ! save the exposure map
    read/fits file2.fits             ! read the second image
    read/fits/exp exp2.fits          ! read the second exposure map
    sum_image                        ! sum the 1st and 2nd images
    save_image                       ! save the summed image
    read/fits file3.fits             ! read the third image
    read/fits/exp exp3.fits          !
    sum_image                        ! sum the 3rd with the saved image
    save_image                       ! save the mosaic
    disp                             ! display the mosaiced 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 file
    write/fits/exp mosaic_exp.fits   ! write the exposure