Mstraka606 can anyone help explain this 174MM pattern?
Those are interference fringes, also called Newton Rings.
What is happening is there are two glass surfaces that are (too) perfectly parallel with one another. The solar etalon has such a narrow passband that it is very nearly mono-chromatic. That is why you do not see Newton Rings with wider filters.
If you could, try to hold the 174MM with a slight tilt relative to the optical axis. If the rings changes (repeats more often or less often) or disappears, then the solution is easy. Get a tilt adjuster (lots of people sell them, including ZWO) to introduce just a slight tilt that is not noticeable in terms of image plane tilt (entire frame still in critical focus) but introduces just enough tilt to make the interference fringes go away.
Some people use "flats" to get rid of them, but flats don't really work that well without ruining your original image (even Daystar advices against using their own FlatCap diffuser to remove Newton rings). My own experience with flats is not that great.
If you are unlucky, the parallel glass surfaces are inside the ASI174 itself (between its AR window and its sensor, for example), and harder to introduce a tilt. If it is caused by the AR window, you can remove the window and protect the camera using an external clear or IR-cut filter.
If your post processing software allows it, a sufficiently large dither can also remove the Newton rings.
Chen
