Thanks for the advice guys.
I'll give the contacts a clean.
It happend while in the middle of a run of long exposures. Everything was going fine, doing my normal 90 sec subs then suddenly this error appeared.
It was at prime focus on the refractor, that's what was so strange and it was working fine for at least half an hour prior to this. I'll have a go cleaning with IPA.
Bert I'm still keen to give the teleconverter a go, have you tried this by any chance?
LOL Paul, yes I get the message.
