jerrysykes wrote: ↑Thu Jan 23, 2020 4:46 pm
Hi Tom. Great looking M81 & M82.
With regard to the large circles in your images, I don't have a ZWO camera for deep sky imaging. I use an Atik 383 OSC but if I don't clean the window in front of the sensor before every use I get the same exact circles.
Don't know if you have the same problem as I have but you could have convinced me it was taken with my camera.
Thanks, I actually just got through chasing down where the pesky dust or whatever were. I did the rotate the entire imaging chain Field Flattener, filter, camera, and the defect stayed in the same place. Then I tried leaving the
FF in place and rotating the camera & filter, still in the same place. Then I took the filter out of the chain and tried again. Still in the same place. That only leaves one surface. The protective glass in front of the sensor, just as you say!
So I took things apart and cleaned the protective window and it was much better. There was still one doughnut near the edge now. A little more cleaning and now it appears to be defect free... knock on wood.
BTW here is a stretched but otherwise unprocessed frame that clearly shows the weird defect. Almost looks like a crater! I was convinced due to it's size it was somewhere else than the protective glass but I was wrong!
Pre Proc 1 by
Tom Whit, on Flickr
I don't think it was dust as it did not make a doughnut, rather it was some sort of dew drop or some such. But it appears to be all clean now. Wont really know until this weekend when it is allegedly supposed to clear up.
Oh and here is a stretched master flat that shows the circle that would not go away
Stretched Flat 1 by
Tom Whit, on Flickr