mDiff - Calculates the difference between a pair of overlapping images

       in1.fits
              First input FITS file.  (Also needs area image in1_area.fits, or use -n flag)

       in2.fits
              Second input FITS file.(Also needs area image in2_area.fits, or use -n flag)

       out.fits
              Difference FITS image to be generated.

       template.hdr
              FITS header template used to generate output image.

       The drizzle algorithm has been implemented but has not been tested in this release.

       If a header template contains carriage returns (i.e., created/modified on a Windows machine), the cfitsio
       library  will  be  unable  to read it properly, resulting in the error: [struct stat="ERROR", status=207,
       msg="illegal character in keyword"]

       It is best for the background correction  algorithms  if  the  area  described  in  the  header  template
       completely encloses all of the input images in their entirety. If parts of input images are "chopped off"
       by  the  header  template,  the  background correction will be affected. We recommend you use an expanded
       header for the reprojection and background modeling steps, returning to  the  originally  desired  header
       size  for  the  final  coaddition.  The  default background matching assumes that there are no non-linear
       background variations in the individual images (and therefore in the overlap differences).  If  there  is
       any  uncertainty  in  this  regard, it is safer to turn on the "level only" background matching (the "-l"
       flag in mBgModel.

       2001-2015 California Institute of Technology, Pasadena, California

       If your research uses Montage, please include the following acknowledgement: "This research made  use  of
       Montage.  It  is  funded  by  the  National  Science  Foundation  under Grant Number ACI-1440620, and was
       previously funded by the National Aeronautics and Space Administration's Earth Science Technology Office,
       Computation Technologies Project, under Cooperative  Agreement  Number  NCC5-626  between  NASA  and  the
       California Institute of Technology."

       The  Montage  distribution includes an adaptation of the MOPEX algorithm developed at the Spitzer Science
       Center.

Montage 5                                           Dec 2016                                            MDIFF(1)

mDiff  calculates  a simple difference between a single pair of overlapping images. This is meant for use
       on reprojected images where the pixels already line up exactly. mDiff analyzes an image metadata table to
       determine a list of overlapping images.  Each image is compared with every other image to  determine  all
       overlapping image pairs.  A pair of images are deemed to overlap if any pixel around the perimeter of one
       image falls within the boundary of the other image.

       $ mDiff proj/in1.fits proj/in2.fits output/diff.fits template.hdr
              [struct stat="OK"]

       OK     [struct stat="OK"]

       ERROR  Cannot open status file: statusfile

       ERROR  Images don't overlap

       ERROR  All pixels are blank

       ERROR  Template file not found

       ERROR  FITSlibraryerror

       ERROR  generalerror

       mDiff - Calculates the difference between a pair of overlapping images

       -d level
              Turns on debugging to the specified level (1-4).

       -n     No-area-images flag. Creates difference image without requiring pixel area FITS image

       -z factor
              Scale second image by factor before subtracting.

       -s statusfile
              Output and errors are sent to statusfile instead of to stdout

       FITS image and area image representing the difference in the overlap region.

       mDiff [-d level] [-n(o-areas)] [-s statusfile] in1.fits in2.fits out.fits template.hdr