If you have a hint or tip for processing an image Post here
As opposed to links to complete reference documents,
These can be found here. http://forum.orpington-astronomy.org.uk/index.php?topic=7106.0 (http://forum.orpington-astronomy.org.uk/index.php?topic=7106.0)
I think this needs to be made a Sticky,
Gradient Removal using Iris (MarkS)
I removed the gradient as follows in IRIS:
1) Type the command "pointon"
2) Click 10 points on the background near each corner of the image - 40 points in total. This often works better if you blur the image first (but I didn't in this particular case)
3) Type "poly 1" which creates linear background gradient when followed by the "synthe" command
4) Save this background gradient: "save my_back_gradient"
5) Load the image again, subtract the background gradient image: "sub my_back_gradient 0"
6) Type the command "pointoff" to stop the annoying crosses appearing each time you click the mouse
7) Adjust blackness of background and enhance colour balance.