Excessive auto stretch in RGB combine

No idea but you might try "none" for normalize in the RGB combine settings. I read that it would not be necessary after APP's preprocessing. And also maybe not necessary to have neutralize-bg in the stretch control - just that it doesn't seem to do anything much after preprocessing as well.

Thanks @astrogee Using 'none' just seems to produce a dark screen with barely any stretch but I think that you are right in so much as the problem is something to do with normalisation. The blown-out images occur mainly with the recommended 'Add-scale' and 'Multiple-scale' options. If I just use 'Add' or 'Multiple' I can get one or two of the SHO options to look reasonable (hadn't tried this before as they are not recommended). This does seem to be an 'interesting' new problem that I have inflicted on myself though... 🙄

Not sure I can help much and it is difficult to see clearly in the screen shot but to me the Red channel in the histogram looks to have a very unusual distribution in that in that the data all looks to be piled up on the RHS.  I would take a careful look at your Red channel subs for anything abnormal.  I am thinking that to get that data pushed mostly to the Right then there are possibly a small number of abnormally very dark (clipped) pixels in the Red integration.  Not sure what might cause this though apart from a camera fault or darks calibration problem.

Regards

Mike

Speaking of channels, I just noticed that your OIII image is very strong. Usually this is faint compared to Ha. So it looks like your channels are not normalized properly. When you then do RGB combine with one of the formulas, it will probably not look right because I think the channels need to be at the correct relative levels.

thanks everyone. I agree that this looks like a normalisation/stretch issue but it does seem to be happening within the RGB combine tool. If I take the very same FITS stacks and save out a TIF from each one then use the TIFs in the RGB combine tool (rather than the FITS files) then everything looks fine with no overexposure. Very strange...but I will go back and looks at the subs..

Hi @mabula-admin

I wonder if you might be able to comment on possible things which could be going wrong here as I am running out of ideas?

In short, after years of no problems,  I no longer seem to be able to use the RGB combine tool. The issue is that none of the normalisation methods produces a reasonable scaling of the fits stack files, whether they are three RGB, or three HSO stacks etc. There is always a crazily different scaling between the channels which results in terrible combined images.

I had thought that maybe there was something wrong will the filter assignments but I have now confirmed the same issue present with data from two different scopes and filter systems, both RGB and HSO.

Strangely though, if I export a tiff. file from each R,G,B,H,S,O stack...then use the tif files in the RGB combine tool, rather than the fits files, then everything works just fine.  It is something about the fits scaling...but I have no idea what's going wrong.

I have also tried a freshly downloaded version of APP on a different computer...but with the same result.

As always any advice most welcome 🙂 

thanks, Jon