HistogramTransformation Stretch - why is Blue graph taller than Red-Green?

[midnightlightning]

Hi,
Just purchased PI after brief trial and am processing M31. I have 300s lights:
Red- 8
Blue - 11
Green -12

When I go to stretch I notice the blue channel appears stronger - see image.


Prior to this I did the following.
Channel Combine of RGB stacks (pre-processed in APP)
Dynamic Crop
DBE
Noise reduction with Multiscale linear transform

I reversing the stretch, splitting the RGB, and applying linear fit but then I got a similar result in HT but with the Red signal spike.

Any thoughts, is the blue spike a problem, if so where did it arise and how do I treat it?

Thanks

Jon

[ngc1535]

I doubt it is a big problem.
The "hump" represents the very faintest  part of your image- basically these are sky values. If you zoom in to your image at the pixel level it is like you will see a slight overabundance of blue pixels over the green or red. (make the image *very* bright perhaps to see). This means there is a minor blue bias to the sky. It is all the same brightness (which is why it is aligned with the other colors channels). This is my take on it.

One easy way to get this result... take data with some moonlight. The sky is quite blue.
-adam

[midnightlightning]

Thanks Adam.
Rightly or wrongly I used the STF/HT combination stretch with the STF unlinked, then backed off the stretch a bit with HT and got a balanced graph. Plus I did a few other things which I will discuss in the earlier thread I posted about colour issues.