Thanks both.
The Lunt website talks about the issue, but gives conflicting advice on a solution. At one point says the B600 is fine for imaging with the 50mm Lunt, but then in the next paragraph suggests one should go to the next blocking filter up for imaging. I’ll email them, as they have been helpful in the past. It also sounds like tilt may be an issue, so something else to explore and MUCH cheaper to rectify!
Thanks.
James