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Hi Clive,
The halos are of course caused by a double reflection (1st towards the sky then back onto the sensor). The 1st reflection is almost always caused by the CCD/CMOS sensor itself. They actually have a lot of shiny metal bits on them that are very good reflectors. (If you ever get the chance to shine a low power laser onto a sensor you would be amazed at the checker board pattern coming back!) The second reflection is the killer, but you might be able to do something about it.
Measure the diameter of the halo in mm. Find out your f/ratio, distance from offending surface to ccd is the fratio times the diameter divided by two (since the light travels the distance twice).
If the offending surface is curved (e.g. the back of a Petzval) the formula won't work. You would need to know the surface curvature and od more math in that case. Better to swap the imaging train to another scope to diagnose that case.
Halo problems in recent years with LRGB filters have been due to filters with leaky IR spectrums. They didn't block the NIR wavelength up past 1000nm very well and/or their anti reflection coating wasn't very AR. Many CCDs are still sensitive at those wavelengths and recorded up the NIR reflections along with green or red or whatever. In these cases the reflection originates off the filter itself. I think this is not your case due to the OSC camera.
HTH,
EB
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