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Triplet/Doublet for HA Solar

Equipment Refractors Buying Advice Sun
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Nick Grundy avatar
Nick Grundy avatar
I'm looking to buy a refractor to dedicate to solar imaging (becoming hooked). 

I'm using the Quark Chromosphere with the ASI432mm mostly. I'm looking for something in the 90-130mm aperture range and according to my estimates, the ideal is going to be around F8. 

I've seen a lot of doublets in use with Solar imaging and wondering if that is because they are more affordable/lighter or is it because the blue wavelength where they typically struggle isn't as important with Solar?

Is there any reason a carbon fiber tube will do better in a solar imager than an aluminum one? or vice versa? Focusing for solar is a struggle to get really accurate so I'd hate to get a new scope that was going to make me do that more often over a session. 

appreciate all the insight!

CS
Nick
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andrea tasselli avatar
andrea tasselli avatar
In solar imaging you'll do well even with a good, long achromat, don't need fancy stuff. Carbon tube is fine if you want high stiffness without pay back the price in weight and I can't see any advantage in daylight part from being light. Being black, not so much.
Nick Grundy avatar
Nick Grundy avatar
I'm assuming that doublets are popular for use with the daystar because they do well enough with light in the red wavelength and blue doesn't really matter?
andrea tasselli avatar
andrea tasselli avatar
That's right, secondary spectrum does not count as the imaging is monochromatic to a very high degree. In fact you could achieve even better results with a singlet optimized at the Ha wavelength. Obviously other wavelengths in the Solar photosphere (such as Ca) would not do as well. You'' probably want an APO for that.
Luka Poropat avatar
Luka Poropat avatar
A doublet is more than fine for Ha solar. 
You will be more limited by the Ha filter you will be using and its bandpass.
The tube material will not be really the problem, your main issue will be the seeing and the temperature of the ground. Ideally avoid concrete or asphalt areas, something near grass will work good. Best seeing conditions usually happen in the morning before temperature inversion.
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Minh Lết avatar
Minh Lết avatar
A doublet is lighter, cheaper and works the same for Solar in Ha as a triplet. I am using a doublet with a doublet stack quark/pst etaalon and here's what I got https://www.astrobin.com/cmqsij/
Nick Grundy avatar
Nick Grundy avatar
So it’s apparent that the same factors in DSO scopes aren’t exactly the same for solar ha. 

is aperture still as important?  (In comparison to the quality of the optics)  

if there’s a choice between 120mm higher quality or 150mm lower quality, which do you all think will be more impactful? (If it’s close at all, the 120 just takes up less space which is nice)
andrea tasselli avatar
andrea tasselli avatar
Small is fine with Solar Ha and I'd go with a fine 4" refractor rather than an equally fine 6" because, other than the obvious questions of practicality and cost, the 4" is much less seeing limited than the 6" and much less susceptible of temperature transients. In fact I'd think a 3" would give the 4" a run for its money. Well, unless you live under skies with exceptional daytime seeing.
andrea tasselli avatar
andrea tasselli avatar
Put your money in the etalon, not in the scope, that's my rule.
Nick Grundy avatar
Nick Grundy avatar
andrea tasselli:
Put your money in the etalon, not in the scope, that's my rule.


since that part is done with the Daystar, I'm now scouting some good doublets. Since i've usually got crap seeing here near San Francisco, I'm thinking about a decent 4-5" doublet now; ideally something the looks good in the red wavelength. (i've always been drawn to the FC-100 series with their lightweight for visual use.)

I last tried the quark on my FOA-60 and I'm near certain that I saw undersampling firsthand. My SER files came up gray and muddled and totally lacked detail. (it's native f8.8 and my calculations pointed to F7.5ish as more ideal, accurate?)

does anyone have a fancy technical way to focus in ha for solar? I find myself constantly second-guessing if i'm in focus.
Eddie Bagwell avatar
Eddie Bagwell avatar
Before my Lunt, I was using the DayStar Quark with the ES127 Doublet with great results. For focusing, slew to a sunspot and stretch the histogram to show more contrast to achieve a good focus point. In SharpCap, just press the stretch button. For the best image exposure, adjust the gain or exposure until the histogram bottoms out around 70%.
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