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Interesting article. This problem only started when you changed filters to Astronomik right? What were you using before -Astrodons?
So if this is the case that there are IR reflections inside your scope from the internal painting or the adapters then the earlier filters were handling it better? Better IR blocking?
Getting a good flat I find to be somewhat of a trial and error and I tend to check the workability of the flats before I think I've got all my calibration data.
Some scopes are hard to flat field, others quite easy. Usually an APO is the easiest of all.
These days with 2 mirrors and a corrector system which is popular on almost any compound scope flats are tricky. Too bright. too dim, darks not matching really well, do you subtract a dark flat from the flat or not, do you use a bias to subtract from the flat when making the master flat or when you apply them? All these factors seem to have an influence so I trial and error them to get the best result.
Greg.
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