Hi Again Graeme
Just be aware that the full aperture Baader ERFs' are not cheap, because they are a specialised filter after all. My 90mm piece of "red glass" cost over $500 AUD and that was when the Aus $ was 10% more favourable than now. The PST has a 40mm objecive lens so even converting a 70 or 80mm refractor to accept the PST black box using a 75mm ERF will increase resolution substantially.
https://www.teleskop-express.de/shop...ium-Light.html
I think that any further discussion about the subject may be better served in the ATM section, as this is the images area.