RegiStar Processing by Bill Patterson
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RegiStar is one of my favotite astronomical image processing programs.  It's produced by Auriga Imaging.  Their website is at and it descibes the program as follows:

"RegiStar shifts, rotates, and deforms as needed to precisely align images, even if they are at different scales or orientations or have been produced with different optical systems...."

Simply put, it precisely adjusts two or more images so they may be combined and processed in different programs as long as they have some common data.  I use it to stack multiple exposures of the same object taken with the same optical system on the same night as well as to combine exposures of a common object taken with different optical/camera systems sometimes captured years apart as illustrated below.
This recent image of Messier object #1 (the Crab Nebula) was taken 2/13/04 with a Takahasi FS-152 refractor and a SBIG ST10XME CCD camera.  The focal length is 1215mm and the image scale is 1.15 arc/sec per pixel.  It represents a total of 100 minutes of luminance exposure.  I didn't get any color exposures that night.
This next image was taken on 10/5/02 with a Takahashi FSQ106 refractor and a SBIG ST8E CCD camera.  The shorter 530mm focal length yields a nice wide-field image but lack fine detail in the nebula.  It is presented at an image scale of 3.5 arc/sec per pixel and covers a much wider field of view than the 1st image.  LRGB = 15/15/15/15
The goal is combine the images and that can go two ways.  First you can register and scale one image up to the size of the close-up FS152 image to provide missing color to that larger image (make one image smaller).  A second alternative is to register and scale one image down to the widefield size giving it the benefit of the enhanced nebula detail (make one image smaller).  The result here is a widefield image with better resolution from the larger scope.

In either case you need to "register" the images as shown below.  The "registered" image is tilted to match the orientation of the "reference" image and scaled to match the size of that image as well.  Scaling the widefield image larger to match the size of the close-up yield a 65megabyte image file!
The image is tilted and scaled to match a "reference" image - images are then combined and processed in other programs such as PhotoShop or Maxim.
Check out the results - do you see the differences?
Click on the thumbnails to enlarge
    Original Wide Field                       Revised WideField                         New Close-Up
     Tak FSQ & ST8E                  Inserts FS152 nebula data             Mixes color data w/FS152 close-up
          10/5/02                                10/5/02 & 2/13/04                           10/5/02 & 2/13/04