Yes Bill, I see that your Variable 'airgap' extender is looking rather close to being fully retracted - good piece of kit too. This distance set is achieving your desired flat field I assume ? If so, then agree, would find it near impossible to fit a filter in there without altering the air-gap, thus impinging on the flat field .
Only other option is at the scope end of the fucus tube, ??
Not sure what the go is there tho, concerning if thread is available and/or what size - and only providing you have few mm of in-focus travel to work with to compensate for the filter in situ ?
I was just reading & watching vids about these Variable extensions last night 'need one myself' and there are a few types out there, some have a smaller spacing when set to minimum than others ?
Not sure if may be an option tho
