OK, I finally got around to creating a set of L, R, G, and B flats using APT’s CCD Flat Aid tool. That was easier than I expected. (Other than trying to see the laptop’s screen in broad daylight!)
I then processed the flats in PixInsight per “Inside PixInsight” (2nd edition) to generate master flats for each of L, R, G, and B. The R, G, and B versions are very similar in their small, sharp dust spot pattern. L has all of the same ones as R, G, and B’s but with a couple (5-6) larger, fainter, fuzzier ones added. I’m guessing the common, small ones are on the camera itself while the larger L ones are on the L filter. My reasoning is that dust farther from the sensor will be bigger and less sharp. Make sense?
So then I calibrate my raw L subframes of M101 from three nights ago using the new master flat L and my master dark from that night. Inspecting the calibrated L subframes reveals that the small common dust spots have been pretty effectively removed. However, the larger L dust spots remain!
Any idea why the flat isn’t removing the larger spots?
Since it's 12MB, I’ve put a screenshot of my PI screen at
From top to bottom, left to right the images are:
1) the raw L subframe
2) the master L flat
3) the calibrated L subframe
4) the stacked L images of all 20 subframes
All of the images have had Auto Stretch applied so we can see the details.
So what am I doing wrong?