Hi folks, good to have a robust discussion about this technique - having been in touch with Eric himself, with his permission, I thought I'd share Eric's own views on this...
"Let's say you are imaging a target like The Rosette Nebula in NB, one of my favorites. And you start with an AP 130 GT scope and an SBIG STXL 11002 CCD camera. You will get the entire nebula in the FOV. You accumulate the data and after some processing finally adjust/balance the final histogram "globally." That give you one result. Now you image the Rosette with an AP Honders with a 16200 camera, with a smaller FOV. You don't get the entire nebula. Now in the end you adjust the final histogram globally and you get a slightly different color scheme. It's similar, but now the same. Finally you image the nebula with an RCOS 16", you get a much smaller FOV, you adjust the final image globally and you get a noticeably different color scheme. So which one is "correct."
You night say that by using different scopes and cameras you have selected different area of the nebula and have violated some rule of imaging. You should have gotten the same result, but different because balancing the histogram of different areas gives you different results. This is not a theoretically example. Over the years I have done exactly what is described in the previous paragraph with the three scopes. And I kind of wondered why the results were so different, until I realized that they should be different because I was balancing different regions of the nebula with different contributions of HOS emissions. Of course they should be different.
And that is the point of revised processes. In effect you are selecting different areas of the nebula with different contributions of the three emissions. And you absolutely should expect the adjustment process to yield varied results. If they didn't it would be shocking.
One more point. In most cases if we just mapped the HOS filters to SHO=RGB, they would be green. Then isn't that the most "natural" way to view the final mapped image? I see people do that and their image look fine, but they miss some of the variation in the structure of the nebula. So what most people do is try to "balance" the histograms to get the pale green to deep orange images we all enjoy. You have taken a big step away from the natural image into a representation of the nebula and a piece of art. What law says you should not take a more subtle approach like I suggest?
Of course the answer is none. In the end all our images are pieces of art and should please our eyes. And if we can add in a little science along the way, that's just great. But that's just my opinion, I could be wrong.
Stay safe.
Eric Coles, Ph.D."
So there you have it!
Cheers
Andy