Now I have managed to produce very accurate flats I had a go a this data to see what I would get.
Large image 15MB
http://d1355990.i49.quadrahosting.co...RhoO_Ant_M.jpg
The reflection 'ring' due to Antares can be eliminated by putting Antares on the optic axis. That way the 'ring' is focused on the position of Antares.
Left panel 8x8 min for each of RGB. Right panel 8x6 min for each of RGB.
All processing with PixInsight.
I produced the flats with a cloudy night sky backlit by a Gibbous Moon. I tracked the scope about 90 deg behind the Moon. The sky looked visually a very even white with no structure. At last I have a use for cloudy skies with near full Moon!
The exposures for the flats were 6 min at -35C which gave an ADU reading of 26,000. To produce a Master Flat I corrected 16 exposures for darks with bias. Used PI to average stack them equalising fluxes.
My system is too fast for twilight flats for RGB as I have an iris type shutter which produces artefacts below about 5 sec exposure.
I tried diffusers which produced artefacts due to too much off axis light reflecting inside the optic. This was a problem even with NB filters.
Bert
Astrograph is an Officina Stellare RH200 which has a focal length of 600mm and is F3. Clear aperture is 200mm.
FLI Atlas Focuser.
FLI ten position filter wheel CFW-3-10 with 50mm square filters.
Astrodon E series LRGB and HA, NII, SII and OIII 3nm NB filters. Also a continuum filter 5nm.
Camera is a FLI PL16803 which has a sensor size 36.8 X 36.8 mm.
The FoV of this system is 3.5 X 3.5 degrees.
Mount is a Software Bisque PMX.