[pmrid (Peter)]

Thanks Andrew, Theo.
I'll see if I can devise a repeatable process of elimination. I'm not over-confident that bright-star reflections would cause this sort of artifact Andrew. I've seen what you are referring to before when they appear either as great elliptical halos (seen in an old GSO Ritchey-Chretien) or a strong rays coming from off-screen - usually Alnitak when doing a Horsehead etc. But these are unlike that. They also have a distinctly blue colour. That makes me think it is a reflection coming off a coated glass surface somewhere inside the optical train itself rather than from any external starlight-based source.

Theo, for the purpose of this experiment, I will try some out-of-focus images first. If the reflection remains and has the same fairly well-defined linearity and colour while the rest is out-of-focus, it will tell me that the source is probably the inside surface of the the IR filter. I'll then remove that filter and repeat the process with and without a spacer. I note what you have said about the power to the cooler. The thing that worries me at the moment is what to do if the culprit is in fact that nosepiece filter.

Just to add some confusion, here is a third image from the same session - it's a LMC widefield and it shows no sign of that artifact. SO maybe Andrew is right.

Peter