I have been struggling with flats on my RC8 (with Moonlite focuser) since getting the scope. This was quite a surprise coming from ED80 where at least the flats were easy. I have tried sky flats, LED panel, diffused LED panel, diffused light sources but the flats were never right. Sometimes over-correcting (similar to
this) and mostly not fully correcting the vignetting.
Eventually I have found that the issue was caused by:
- taking darks with the camera still on the scope (to save time and preserve alignment for imaging over multiple nights)
- making the dark library also during daytime (in a darkened room with blinds down)
- focuser leaking light - I definitely did not expect this.
Basically the resulting flat darks (and darks) were not darks and, of course, the flats never worked properly. And thinking more about it, this probably also explains why sometimes the amp glow would not properly calibrate out.
The 2" Moonlite focuser leaks light where the drawtube enters the focuser body and also where the 2" tube enters the drawtube. Google indicates that this is common with most of focusers and that this is normally not of concern for nighttime imaging.
By the way, the easiest way to test for light tightness of the imaging train is to have dust on the sensor. Any light leak will be quite obvious
The moral of the story is that focusers may not be light tight. Take darks in darkness with camera only, i.e. do not try to cut corners.
Hopefully my experience helps someone else with similar issues.