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Thread: Trying to Decide If and What Solar Telescope to Buy

  1. #1
    Join Date
    Mar 2004

    Trying to Decide If and What Solar Telescope to Buy

    I've been batting this idea around for a year or two after posting here and discovering that companies do not make H-alpha filters that can be mounted on a normal camera lens (e.g., a 77mm threaded narrow-band Ha filter). While I'm quite pleased with the outcome of of my solar eclipse photos and subsequent movie that I did with an 800mm lens equivalent (400mm + 2x) with a solar filter equivalent (two stacked ND filters), for general astrophotography of the sun and especially for the upcoming Transit of Venus event, I'm still thinking about doing this (a small black dot moving across a solid yellow surface with maybe a sunspot or two is not nearly as interesting as one that shows supergranularity, flares, filaments, etc.).

    I'll start out by saying that I still have reservations about doing this because:

    • I already have a good system going for a 400mm or 800mm lens that hooks directly to my camera because, well, it's a camera lens.
    • Solar telescopes are friggin' expensive.
    • The solar 'scopes I see have a 1.25" eyepiece barrel that I'd need to use a T-ring and -mount for my camera, and ...
    • ... I've read some bad things about the stability of the focusing mechanism and am not sure how it would behave with a 5-lb camera body attached.
    • I don't have a good mount system other than my tripod and AstroTrack that could be both stable and handle the weight of both the telescope and an attached camera.

    With those stated, can anyone allay my fears for the third, fourth, and/or fifth items?

    I'm currently looking into the Coronado SolarMax II telescopes as those seem to be all that's out there for consumer-grade systems. My current 400mm lens is an f/5.6, while with the extender I generally shoot at f/11 for best sharpness. With a 1.25" eyepiece barrel, my understanding is that I'm going to get an effective focal length that's longer than that quoted by the product, but I'm not sure what that "crop factor" will be. That also means (I think) that I'll get a slower system with the camera attached.

    Thus, my starting point is looking at the SolarMax II 60 (the SMT60-10 model versus the -15 model). Which also happens to be more than I would like to spend at $1.5k. I mean, that's more than I paid for my 400mm camera lens.

    But then re-reading the older thread about an H-alpha filter for a normal camera lens, I'm thinking that I may almost be better off getting a dedicated astronomical CCD for once. SBIGs seem like the way to go for amateur imaging, and the ST-8300M appears to be popular and has a good QE ... and while I could use that for other imaging I do, it tacks another $1800 onto the price. And its minimum shutter speed of 0.12 seconds makes me wonder whether it could handle the sun just through an Ha filter.

    So I guess this is more of a stream-of-consciousness "Please give me advice" thread. So ... can anyone please give me advice?

  2. #2
    Join Date
    Dec 2006
    It's too late to do this for the transit. Getting a good solar setup tuned and going takes time which isn't available.

    For really detailed H alpha images of the sun use a good video camera such as a DMK or Flea 3 mono camera that can take 60 frames per second. Software such as Registax can then be used to get tremendous detail from these images. A video DSLR can be used as well if your seeing is unusually good from your location. The lower frame rate reduces your chances of catching enough high resolution frames before the sun changes too much blurring the image if working under average seeing.

    But for the transit I'd use the system you know and are familiar with rather than risk something new and untried.

    If going H alpha in the future you can go either the complete scope route or buy the needed filters for a present scope. I use that system. No matter which brand you go with you will have two filters, one on the front of the scope and one usually near the eyepiece. One of the filters is the narrowband sub angstrom filter, the other plugs gaps these filters leave and if on the front of the scope reduces the solar energy to safe levels for the optics.

    Some put the H alpha filter in front of the objective. This has the advantage of being both an energy reduction filter as well as H alpha and works with parallel light rays from the sun making the filter easier to make. Its disadvantage is it greatly limits aperture and resolution since potential resolution is directly proportional to the diameter of the optics. Large filters of this time for high resolution are extremely expensive limiting the aperture to 40 to 90mm in most cases. For larger apertures a relatively inexpensive pre filter is put over the front of the scope that blocks heat from entering the scope and plugs gaps in the H alpha filters frequency coverage. This way a relatively small filter can be placed near the eyepiece allowing for large aperture at much lower cost. But the light cone of such filters needs to be very shallow, usually about f/30. This can be achieved with a Power Mate but does raise the focal length and reduces the amount of sun you can capture in an image.

    There is no perfect solution here. You really need to evaluate your local seeing conditions with an inexpensive white light filter such as Baader film on a 6" scope to see if going the larger aperture, high resolution way is even possible with your local seeing conditions before you can decide this method is even possible for you. Then stop it down to 90mm and 60mm to see again what loss of resolution you see. In some locations even 90mm is beyond local seeing and a less expensive 60mm filter might be all you can use effectively.


  3. #3
    Join Date
    Mar 2004
    If I can get a system on or before Friday (I do realize that I'm cutting this close), then I can devote a lot of time between then and Tuesday to getting things set up and tested.

    I also think I should clarify a bit: I do not actually own any sort of telescope, unless my 400mm f/5.6 camera lens counts. I can use - when not otherwise occupied - a 16" and/or 18" telescope on campus when they're not being used, but that's about it. Buying a telescope for my own use is FAR down the line for me at this point.

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