[Startrek (Martin)]

Hi Peter,
As you probably know I use exactly the same camera and software
Firstly what do you mean by “Light frames were done in APT at 22,000ADU” I’m assuming you mean the Flats ?
Secondly your APT histogram is the roadmap or guide for successful Flats with this camera.
You should adjust the brightness of your flat panel or light box etc..and ADU target value to achieve an ADU range on the histogram of around 8500 to 35,000 ADU or around halfway along the Histogram. Not just select 22,000ADU and hope it’s ok
I found an ADU target of 21,000 to 22,000 was the right value after adjusting the brightness on my light panel and achieving a “ADU range” on the histogram of roughly between 8500 and 35,000 ADU or up to half way

In regard to Stacking with DSS

“RGB Channel Background Calibration” is a definite no no for Startools so set this Calibration option to either “Per Channel Background Calibration”or preferably “No Channel Background Calibration” In “options” leave Calibration Method to Linear and RGB Background Calibration Method to none.

Below are a bunch of settings you should use to ensure your stacked fits file is linear and meets the requirements for loading into Startools

DSS Latest Settings for both RGB and Narrowband data ( Prerequisites for Startools )

1/ Tick “No White Balanced Processing” in Raw Files Digital Development
2/ Leave “Set the Blackpoint to 0” unticked in Bayer Matrix Transformation
3/ Select “Per Channel Background Calibration” in Stacking Parameters even though when using Startools it should be “No Calibration”. I found Per Channel Calibration no issue when using Startools.
4/ Select “Intersection Mode” in Stacking Parameters (helps reduce stacking artefacts
5/ Tick “Align RGB Channels in final image” in Stacking Parameters
6/ Digital Development Process Fits setting and Raw settings select Bilinear Interpolation
7/ In Digital Development Process Settings ensure you tick “Monochrome 16bit Fits files created by a DSLR or a colour CCD camera” when using a DSLR or OSC camera like your 2600MC.
8/ Stacking Parameters, Lights use Kappa Sigma Clipping
9/Stacking Parameters, Calibration Frames use Median
10/ Stacking Parameters, Flats Median Kappa Sigma Clipping
11/ Stacking Parameters, Bias Median Kappa Sigma Clipping
12/ Ensure you save your stacked file as a “32 bit channel integer Fits files with no adjustments applied”
13/ In Digital Development Process Settings under FITS tab , use Generic RGGB for your cameras Bayer matrix ( I have the Latest version of DSS and the 2600MC is not on the camera list)
Leave colour adjustment box as default 1.0000 for all 3 items

I performed exhaustive testing for a month on all types of data earlier this year on calibration frames with the 2600MC and found that Flats and Bias worked fine ( Darks are not needed )
Your Bias frames can be captured at an exposure of 0.000032 sec which is the shortest exposure in the ZWO specs for this camera. I tried 1 sec , 0.5 sec , 0.1 sec as well with no discernible difference. I just use 0.000032 sec

Also when capturing Calibration frames ensure there is no stray light leakage that may be entering your image train.
As I use Newts , I wrap the focuser and camera with a black cloth and fit a shower cap sprayed matt black over the back end of the OTA to eliminate stray light when capturing Flats and Bias frames during the night

The photos you posted look like a calibration issue ( either with colour balance or your background)

Hope in the above info you have picked up something but I’ll keep thinking of things that might have caused this issue

Cheers
Martin