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 them in PixInsight per “Inside PixInsight” to generate master
flats for each of L, R, G, and B. The R, G, and B versions are very similar in
their dust spot pattern. L has all of the same ones as R, G, and B’s but with a
couple (5-6) larger ones added. I’m guessing the common, small ones are on the
camera itself while the larger L ones are on the L filter. Reasoning that dust
farther from the sensor will be bigger. Make sense?
So then I calibrate my raw L images of M101 from three nights ago using the new
master flat L and my master dark from that night. Inspecting the calibrated L
images reveals that the small common dust spots have been pretty effectively
removed. However, the larger L dust spots remain! ;-( In fact, the large spots
look worse, almost as if they were embossed. Any idea why the flat isn’t
removing the larger spots? Could it be that my EFW isn’t placing the L filter
in the exact same spot each time? That would be bad as I would have to take the
flats for each component at night before moving on to the next component. That
would effectively limit me to shooting a single band per night. (And give our
weather this year, it’ll take months to gather a single set of subframes!)
I’ve put a screenshot of my PI screen at
http://www.mikemac.com/Astrophotos/Flats-and-dust-spots.png
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 (I finally found the control in
StarAlignment that keeps PI from rejecting most of my subframes when
registering them!)
All of the images have had Auto Stretch applied so we can see the details.
So what am I doing wrong?
Mike McDonald
mikemac@xxxxxxxxxxx
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