IMERG SECRETS

IMERG is a program that combines single dish and interferometer data (see Stanimirovic (2002) for details on the algorithm). In short, this algorithm (also referred to as "feathering") fourier transforms both the single dish and the interferometer data, uses the overlap region in uv space between the interferometer and single dish data to determine a scaling factor for the single dish data, adds the interferometer image and the scaled single dish image in fourier space, and then fourier transforms it back.

IMERG has a couple of extra caveats that the user should be aware of before running:

• Both images need to be square and the dimensions a power of two.
  
• Make very sure there are no blanks in either image. Use REMAG to replace any blanks with zero. Note that OHGEO will blank any region in the transformed image that is outside the boundaries of the original image. One of the signs that you may have some blanks in your image is really small scaling factors (~1e-8).
  
• Make sure that the single dish image is big enough, i.e., make sure the galaxy is much smaller than the entire image.

Edit (11/13/09): As far as I can tell, IMERG does absolutely no checking that the two images have the same axes, so beware and double check that both images are the same. Also the beam that gets put in the resulting image is the single dish beam, not the interferometer beam. Use ADDBEAM to change.