Ah yes. Have never used one of the duo/tri/quad band pass nebula filters and don’t use OSCs. Agree: one of those filters would require properly corrected optics.
I was just looking at how much a Takahashi 106 costs these days and even a Borg 72mm is no change out of £2.5k – and lets not even mention the TB 203.
I’m probably just mean, but I always think “Ouch” if handing that much money over for something that isn’t necessary. Mortgages and such like taking precedence.
I am impressed that you now see refractors down to f/3.9 but a 72mm isn’t really giving that much light to filter. My approach would be a mono CCD/CMOS, single band filters and a SW 80 achromat and a focal reducer/coma corrector – but, yes, that wouldn’t be running at f/3.9. Can see why the 8″ V2 RASAs are so popular.
Taks and fast EDs are great for those who can afford them.