MERGE will merge between 2 and 20 images or spectra into one image or spectrum, with the pixels in the overlapping regions being replaced with the average. Merging is done in relative coordinate space (or "world" coordinates). The FITS cards CDELT1 and CDELT2 are checked to make sure that all of the images are on the same scale/pixel (to within +/- 0.01 pixels across the range of an image). If this condition is met the images are then checked to make sure that the starting coordinates are an integral number of pixels away from each other (again to within 0.01 pixels). If the NOMATCH keyword is used then this last check is not performed and any given pixel is added to the closest adjacent pixel in the "master" image. (This uses the NINT function in FORTRAN, and allows the user to avoid unnecessary interpolations in some cases.)