In general, I do no suggest a newtonian for Ha/Cak/etc solar. It's good for photosphere imaging. Fantastic for that and a very inexpensive way to get into high res photosphere imaging in solar. But it's not a good choice for HA or CaK, or any other ultranarrowband imaging. The mirrors of the longer focus newtonian are superior to F2 SCT mirrors, but the design of the Newtonian has issues with thermal handling and this is why I cannot suggest it for Ha/Cak, even with an appropriate D-ERF that is full aperture.
I have that setup that people are talking about. And using the Quark or something else doesn't matter. It has nothing to do with Daystar nor another etalon product being used there. It has everything to do with that secondary mirror design instead and the issue is thermal handling, even with a D-ERF. More on that in a minute.
Full aperture D-ERF on a Newtonian with a Quark. It works. I don't recommend it.
So naturally, why? Right? It works. But why not recommend it. A couple of us have done D-ERF's on Newtonians and the outcome is the same in terms of finding there's an issue with thermal handling at the secondary mirror. Not sure exactly how its happening, but it is. Christian has some great examples with a heat camera to show the concentration on the secondary mirror and how even with a 2nd D-ERF in front of that one is very difficult and still doesn't quite tolerate the heat. It's strange because SCT secondaries handle it. I'm not sure if it's because the mirror on the secondary of a SCT is behind plastic completely or what. But SCT designs handle this heat very well, so a SCT with a full D-ERF and a 2nd final D-ERF in the focuser eliminates virtually all heat and remains high transmision and works great, and I have this setup too (C8 Edge + D-ERF + Baader Red CCD-IR Block and throw a Quark on the back or any etalon you have and you are in business and the heat is handled). But this is not the case on a Newtonian. The primary mirror is fine and with the full aperture D-ERF it handles heat there ok. But the secondary mirror collects heat for some reason. Maybe it's due to it being struck by the energy passing by on the way to the primary mirror and then reflecting or something like that. Maybe it's the spider. Maybe its the whole apparatus. I'm not quite sure. But several people trying this setup came up with the same issue, heat issues, even with full D-ERF, and it was always at the secondary. And even with a 2nd D-ERF suspended (at great pain) in front of the secondary, it still was getting too hot.
The results are the secondary mirror has major thermal issues, it gets hot. Even though I have a D-ERF in front of it completely and a second D-ERF in the focuser after the secondary, the heat coming out is still significant. Way hotter than SCT (even though you'd think logically the designs are not too terribly different, but they really are significantly different from a thermal stand point). The result is the image under heat twitches like its nervous and bends and fluctuates, you can't get focus and keep it as it drifts constantly as heat fluctuates. It's not seeing conditions, it's heat on the mirror, not even heat in the tube, but heat on the secondary mirror itself. The only way to stop this would be a D-ERF in front of the secondary mirror, but that proves to be highly challenge to mount and still doesn't work perfectly. So again you can do this, but the heat is there, and your image will not be clear, it will twitch and spasm oddly, even with excellent seeing conditions. So it's no good for imaging and likely not healthy for your expensive imaging filters.
SCT are inexpensive too, up to 8 inches. So for a high res system of 8 inches, I would certainly pick SCT over Newtonian for this application (but specifically only for HA, not CaK). The SCT is also fine for photosphere imaging. A newtonian can be used for photosphere imaging great with solar film for cheap, any size. But for HA, the needed transmission requires a D-ERF, not even photograde film, and the D-ERF allows too much heat on that secondary. And CaK is in the same spot as HA in terms of that setup, since its ultranarrowband, a D-ERF is needed, not film. And SCT F2 mirrors are awful for CaK, where Newtonian longer focal-ratio mirrors are much better for CaK. So there's no perfect instrument for this, outside of a huge refractor. Refractor designs are ideal for solar narrowband imaging of all wavelength, simply super inconvenient in terms of size and cost. But far easier to thermally handle. SCT comes next. Newtonian last unfortunately.
Small aperture off-axis stuff works on Newtoians, but .... it's small aperture, so why bother, just use a refractor.
Anyhow, I would love to learn something new about newtonian design with respect to thermal regulation. I have the D-ERF. I have the Newt. I have HA filters to cycle it with of several sorts. But the heat is a problem even with the D-ERFs on Newts. Otherwise, they would be a super option for high res! Especially shorter wavelength options. I literally bought my F6 quartz 200mm newtonian for solar for this reason. The newt is fantastic for photosphere imaging (solar film and you're set!). But not ultranarrowband where you need a D-ERF due to transmission.
Edited by MalVeauX, 19 June 2020 - 09:39 AM.