Refractors are the perfect optic arrangement for solar with respect to thermal handling because the lenses do not absorb or keep any thermal load, they transmit all of it. Obstructions gather heat and transfer heat and mirrors absorb some heat and do not reflect everything. And when we're talking 1~2%, that's a lot of heat at the end of the day from a large aperture when you start to examine the load the focal plane. The unfocused beam is where you do all of the heat regulation because its not at focus and the intensity is spread out over a larger surface area basically. Within a refractor, you can do this at the opening (full aperture) or internally (sub-aperture) because the light cone changes size. You can put a sub-aperture energy rejection filter near the focuser and it will still handle all this energy by reflecting it out (dielectrically coated only) before it gets close to the focus plane and point of focus where all the intensity would be a problem. There is common worry that this allows air in the tube to heat up and be turbulent and have "tube currents." This may be true for some, but so far, I have not had this issue with a sealed tube in 100F+ weather (Florida) directly pointed at the sun with only a 50mm sub-aperture D-ERF and am able to do it with the full 150mm clear aperture of my refractor and if seeing is good (sub-arc-second) I can image high res short wavelength this way (and long wavelength of course, much easier).
You are not going to damage the refractor's lenses at all doing any of this. The thermal energy is transmitted, not absorbed. It's the filters down line or camera that will be damaged if thermal energy is not handled.
To keep your filters and/or camera from cracking under intensity of heat, you simply reject most of the heat before it ever comes to focus. So this is done with, again, dielectrically coated filters before the point of focus. It can be the full aperture size before your scope, or internally as sub-aperture within the scope somewhere. Many of us opt to use it near the focuser because you can get away with a mere 50mm filter to do this, saving a lot of cost to have a working system that is thermally stable.
I use the Baader Red & Blue CCD-IR Block imaging filters for this. They're affordable. They're good for imaging. They're good as sub-aperture internal dielectrically coated energy rejection filters because they reflect all the UV and shorter IR and most visible spectrum energy right back out of the OTA when seated in an unfocused beam that fits through 50mm (ie, near the focuser) of the light cone. This removes enough intensity for me to not crack common filters right after, at focus. These do such a good job at handling heat that I commonly will do just the Baader Red CCD-IR block filter as the internal ERF at the full 150mm aperture of the refractor, then just a Baader ND3 (10 stop) ND filter to reduce transmission (because transmission is so high that my sensor saturates even at 0.032ms and zero gain without it; obviously this is imaging-only, not for visual) and then my camera sensor. The filters never crack. The camera never cracks or melts or overheats. I do the same way for Calcium K and Gband with a Blue version of this Baader imaging filter as an internal D-ERF.
Also, refractors are the cheapest way to image solar! Big aperture refractors are rather inexpensive. 102mm to 150mm is a lot of aperture in solar. And achromatic doublets are all you need! In fact, an achromatic doublet is preferred to an ED/APO; it has to do with longitudinal focus. A fairly long achromatic doublet is inexpensive and very much ideal for narrowband solar imaging. You can get 120mm aperture achromatic doublets for peanuts. Just add a good focuser. Even 150mm achromatic doublets are affordable. I'm using an old yard cannon 150mm F8 celestron achromatic doublet that I picked up used for like $250. You can buy them new for more, and others, but they're all fairly inexpensive if you consider what you're getting for solar. I mask the aperture to 120mm F10 often when seeing isn't ideal or when doing short wavelength in less than ideal seeing. I can freely change aperture without having to have a new D-ERF because its internal. So simple! That's the beauty of single wavelength imaging. It's far more expensive to attempt to use any aperture mirror based optic because it requires a full aperture or sub-aperture but front mounted energy rejection system. Solar film is cheap and does this great but that only works for photosphere imaging and does not work for ultra narrowband imaging like HA and CaK. So the cost to get a full aperture D-ERF for a cheap mirror is hugely more expensive than a basic refractor which can be thermally handled for cheap with internal sub-aperture D-ERFs.
Edited by MalVeauX, 03 December 2020 - 11:35 AM.