This is something I hadn’t experienced before but is now a problem.
I fitted an OAG onto my scope, to be used in combination with a 2600MC and filter drawer. I had an IDAS NBZ filter in the drawer, main camera in focus and adjusted the OAG into focus. A few days later I removed the NBZ filter and inserted a UV/IR filter into the drawer to keep it parafocal. However the OAG was well out of focus. I since discovered the Astronomic UV/IR filter is only 1mm thick and the IDAS filter is 2.5mm thick.
Given that the 2600MC has an inbuilt UV/IR cut filter should I just buy a clear filter instead of another UV/IR filter? Are there other brands that are 2.5mm thick glass or is it just IDAS. I’m a Bortle 4 but my south is over a large town 5 miles away so maybe a LPF is a better choice? Please help me decide 👍🏼