I have had a similar problem with a CLS filter that has a strong blue cast.
In my opinion the problem lies in the G and B channels that are highly underexposed.
When you apply the flat it boosts these channels dimming the red one.
I solved taking three different flats for each color channel and than putting together with pixelmath.
Now I'm not at home, this evening I will post the pixelmath expression to merge channels in a bayered image
Hello,
I have used both the Astronomik CLS-LPR and the IDAS-LPS light pollution reductions filters for a few years now. You are totally correct on the blue overcast the CLS-LPR causes during the processing of the image, The IDAS-LPS also casts a strong overcast on the stacked/integrated images as well, but it is a red/orange overcast. This is where things are handle a bit different by me, I don't do any altercation of the flats preprocessing, I have always found in other applications used to preprocess and stack using a color calibration routine after integration always worked best to bring the colors back to true. In using PixInsight this is also accomplishable with even better results for me by using a couple of different methods. First step in both ways is the same, STF linked color channel stretching.... Yikes!!! Look at that overcast! Then I use the Dynamic Crop to trim up any registration overlaps, then on to the removal of background LP gradients, which is where the color correction begins.
I take 3 - 4 preview clips from more neutral areas of the image and aggregate them, then use the aggregation as the sampling reference image in the Background Neutralization; execute the operation. After performing the BG neutralization I use the Color Calibration process to calibrate the colors. If necessary I will move on to the Dynamic Background Extraction process to neutralize the background light pollution gradient as needed, or the Automatic Background Extrication process... either works, but I feel I have more control with the DBE. This routine works, but I have found you can pretty much bypass the BG Neutralization if I set the DBE Process up well, hence the "other way" I use. After DBE if necessary I do a Color Calibration, but the whole process is usually handled with the well setup DBE then on to the rest of my processing routines.
Just thought I'd pass it along to you sorry to kind of hijack things, but thought might be useful to someone!
I've attached an example set of M24:
Clear Skies!
Mark Jordan
Greenfield, IN
www.thestardeckobservatory.com
