I am exploring my options for getting into Ha observation and have a few questions.
1) I have a quality 5” ERF from 15 years ago that I originally bought to use with a Lumicon prominence filter. This worked well on my 4200mm FL long focus refractor. My question is, what amount of heat & UV passes through a red glass ERF that could potentially damage other cemented optical elements within the OTA?
2) I’ve read in other posts that people have had success with inexpensive scopes like Chinese 120mm f8 refractors. Is optical quality not as crucial with Ha viewing as with night time observation?
3) if I understand correctly, a thermally tunable filter like a Daystar Ion 0.5a filter can be tuned to also see prominences?
thanks for your time.
Just to expand on Bob's answer:
1) You can transfer UV and IR through the refractor (assuming it's just the lenses and not full of correctors or Petzval or other complex designs) all day, the heat passes through because it's high transmission and not reflection, so the glass doesn't get hot. It gets hot at the point focus as Bob pointed out. Anything at the focus point is where the heat will be experienced. This is why you can employ a front mount or rear mount or internal mount ERF as long as its somewhere in the light cone but not at the point of focus where the heat would be. The ERF reflects the thermal energy before the focus point, so the focus point has less thermal energy.
2) As Bob pointed out, inexpensive long focus achromatic doublets can be superior to a more costly ED/APO doublet or triplet. There's compromise to correction of focus of the wavelengths to get CA-free optics. When you're only dealing with a single wavelength of light, not full color, this is not an issue and you do not need color correction optics like ED/APO. This is why so many have excellent results in the inexpensive chinese achromatic doublets and no one around here is going to suggest ED/APO optics for narrowband solar, unless you want a scope that is also ideal for night use in full color. Optical quality still matters. Having a good figure and low SA is important no matter which design or substrate it is. Long focus achromatic doublets generally are well figured in red (656nm is part of visible spectrum, it's not long IR for example) and the longer focal-length will have less SA. A really expensive triplet APO at F3.9 for example would actually be a poor choice for this application compared to a cheap F8~F10 achromatic doublet.
3) You should be able to tune the filter to be simply on band, on 656.28nm and you will see both surface and prominences at the same time. You shouldn't need to tune for the surface and tune separately for prominences. It is of course possible that some features show up a bit better as you blue or red shift. For example, spicules show up a little more contrasty when blue shifted, while prominences may appear a little more bright when red shifting a bit. But ultimately if you're on band, 656.28nm, you'll see the the upper chromosphere and its features off the limb, like prominences, at the same time. It's often confused because in a single stack, the disc will be brighter than the limb because the disc has photospheric light leaking through, the parastic continuum, and so it lowers the contrast of the surface features, compared to the limb where there's no photosphere light behind the prominence so its pretty obvious and has high contrast due to no surface brightness competition with the black void of space. A double stack system will suppress that photosphere light leaking through, dimming the disc surface considerably, but then the disc and limb will be a closer relative brightness with higher contrast which makes it much easier to see the surface details and limb structures at the same time without tuning off band.
Edited by MalVeauX, 05 July 2020 - 11:12 AM.