[raymo]

Thanks for all the advice, I shall take it to heart. I might have found the culprit. The moon was high, but behind me, and I forgot to fit the
rubber viewfinder cover. I don't think it was at the other end of the optical train, as I have a very long matt painted dew shield, and the moon was behind the scope.

Thanks Philip, hopefully I'll get there in the end.
raymo