I think I might have found the problem. All of the recent subs were all taken through my GSO RC08, which has a collimation ring mounted just ahead of the focuser. The collimation ring has push-pull screws arranged at 120 degree intervals used to align the focuser precisely with the centre spot on the secondary (much like the technique used to align refractor focusers). The collimation ring therefore has a very small gap between the faces of the two adjusting sections. When the focuser is fully inward there is no way for light to get into the inside of the tube but when the focuser is wound out to the focal point for the camera the inner moveable tube comes back beyond the collimation ring gap and light can then get into the inside of the focuser tube. With the scopes position on the mount and pier in the observatory, the collimation ring is exposed to several leds on pier mounted equipment (heater controller, camera temperature controller, and battery indicators on plugs), potentially also the screen of the guide laptop and my tablet running Sky Safari Pro. So light from these devices, admittedly low level, may make its way into the tube through this collimation ring gap (which would be no more than maybe 0.25mm at the widest point), when the scope is at at high altitude in the eastern and western sky and the camera is nearest the pier (where the test subs were taken). I have verified light leakage with a red headlight, focuser in camera focal point position, and me looking into end of the focuser tube with the scope capped. I need to do some night testing with the camera on the mount, but it looks like the possible source of the light. This also explains why it never showed up in the past when using that camera, as it was always used on my 10" imaging newt or the MN190 (neither of which have collimation rings). I will report back after the test. The simple fix seems to be to wrap a piece of black PVC tape around the collimation ring flange to stop any light from getting in there.
