You don't see many example posts of problems one may encounter when imaging so i thought id throw this up after running into the problem.
I have an issue that i thought was stray light, but after getting the exact same issue out at a dark site, i decided to test a few things i have found that the cause was some form of element pinching on the third optical disk in my ED80.
Examples of what i was seeing are attached.
The first shows the result of stacking RGB channels. The second shows diffraction spikes on bright stars. The third shows what you will see when imaging with a HA filter and the final image is a rather extreme 20min lum frame which clear shows something is wrong. Note the harsh and odd shaped vignetting in the corners as well.
In my i was able to determine the cause of the pinching, or rather compression, was from my tube rings being done up too tight, causing the aluminum tube to bend slightly. Loosen it a bit and all of these issues disappear.
So if you run into problems similar to this with your refractor, look for possible pinching issues. Try some simple steps such as relaxing the grip on the tube rings (within reason) and try imaging different targets on opposite sides of the meridian to see if the issue moves or is static.
