Problem with calibrating dust donuts

Well, the system is different so that likely produces different data which can result in differences in how well things get calibrated or not. In this case the amount of dust is very high and this is something which is always tricky to perfectly calibrate. Could you post a light and masterflat from your other scope as well, the one that did work?

What is your normal workflow for taking flats, do you make sure you're in the same focus as the lights etc?

I am accurate and extrememly tidy about my equipment: for much much less dirty i would have immediately cleaned everything. 

Keeping your equipment clean and tidy only brings benefits, first of all that image calibration and integration should have become easy and with no problem.

Yes, I normally clean my sensor when I see 3 dust particles. Maybe that's overdoing things, but like you say.. the better that data is, the less needs to be corrected and the better the result will be. I usually see flats as a way to get light gradients out of my data and tiny dust particles to be removed, bigger ones I would clean immediately.

I clean my sensor every time I setup. A simple bulb type blower does the job. Now I have the sensor permanently mounted behind a filter so I don't expect any dust in the near future. Dust on the filter will not be seen because its not at the focal point, same as dust on the main objective.

Anyway, I would check your offset setting, that its the same for all images. APP is supposed to handle the offset but maybe there is something wrong there.

Dust on the filter can also show up, depending on how close it is. Even outside the focal point, although not as prominent as on the sensor.

Posted by: @vincent-mod

Dust on the filter can also show up, depending on how close it is. Even outside the focal point, although not as prominent as on the sensor.

I suppose... good think I clean the filter every time too! 😉

Thank you for all the kind answers. I fully understand, that sensor should be as clean as possible, but the question was not how (or if) to clean it, but if it the excessive dirt can be removed by flats. In this case, I didn't know, if I did something wrong with software. Vincent replied, that "this is something which is always tricky to perfectly calibrate" and this resolved the issue. It looks, that flats, in the subject of removing dirt, are working only to some extent. I didn't know it and I wondered if this is the case, or I made a mistake taking calibration frames or the software works improperly. Now I understand that flats can help removing dirt, but there's no guarantee for it, as amount of dirt can cause problems with perfect calibration. Thank you all.