Bird! Thanks, mate! Glad to see you have some facts. I'm happy to admit I have learned something. At least you are doing something that demonstrates the effect - it is obviously more significant than I would have thought, and I can't fault your explanation - it fits with the generalised text book data I have available to me.
So radiation is significant... I guess I'm going to have to do some sort of an experiment if I want to determine whether it is dominant - that's a different matter. I would still expect the mechanisms I described to be at least as significant as radiation loss. The experiment I described before may still have some merit, but it will need some refinement. Measurement will be the real issue!
BTW radiation shields are a fairly common device for controlling radiation. If the dew shield is truly primarily intended as a radiation shield, it will perform best if it is polished metal - not a dull matt colour... I guess there's a compromise there to minimise reflections in the OTA!
BTW Bird, have you ever tried to measure the difference in your OTA temperature with and without the dew shield? Perhaps remove the dew shield after the scope has stabilised and see if the temp drops???
Al.