Very interesting and strange that you typically get higher resolution at 396 than 393, since the wavelengths are so close. Moreover 393 being much narrower, should be less sensitive to seeing.
Do you have an explanation for this?
Could you please share the TOA 396nm spot diagrams? I only found for 435nm.
Yes, I agree a newton is the more cost-effective solution. It would have its own issues as obstruction and mirror scatter…
Another cost-effective solution could be to change spacing in an cheap achromat. I remember Valery doing this many years ago with a cheap 102 achro, to optimize for Ha. However I think, not sure, that in CaK case it would require spacing reduction, which may not be possible.
Yes, the D-ERFs are excellent. My 180 D-ERF was measured as transparent with the 96% Ha 175 :
And to come back 100% on-topic, unfortunately a Baader CaK ERF will not happen. I e-mailed with Mr. Baader directly about this, and the development is too expensive, not possible for a such a small market as solar CaK.
It would have the big advantage of preventing heat from entering the tube, but is out of reality.
Edited by pbsastro, 13 October 2015 - 09:16 AM.