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Thread: How much aperture for solar observation?

  1. #1
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    Default How much aperture for solar observation?



    Ok, so I'm building a solar filter for my 8" dob and I know I don't need the full 8" of aperture, but how much do I need?

    I'm thinking of taking the tube cover that I have. Punching a 4" hole through it. Gluing a piece of 4" pvc pipe over the hole, then covering the 4" pvc with Baader safety film.

    Would that do it?

    Does anyone know of a ready-made solar filter for an Apertura AD8 so I can avoid an arts & crafts project?
    Scope: Apertura AD8 w/HALO
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    Default Re: How much aperture for solar observation?

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    Default Re: How much aperture for solar observation?

    If you do go down the "arts and crafts" route, make sure the hole in the tube cover is off axis and between the secondary vanes so you have an unobstructed view. In terms of aperture needed, a 3" off-axis hole (missing both the secondary mirror and the vanes) will give you great, sharp views of the sun: equivalent to an f16 3" apo refractor. You don't really need any more aperture than that: in fact it will probably be sharper than a full aperture filter most of the time as it will cut through atmospheric turbulence a lot better.

    Oh, and make doubly sure that the tube cover and your new solar filter cannot come off!!! Plus, a trap for new players is to forget to cover the finder scope. This is a great way to get a nasty burn! So, likewise, make sure the finder cover is on and very secure. You can easily "find" the sun by simply minimising the shadow of the scope.

    Have fun,

    Dean
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    Default Re: How much aperture for solar observation?

    Bill, I agree with your thinking about using an aperture mask approach. Daytime has notoriously bad seeing and turbulence due to atmospheric heating. Thus using smaller aperture is preferable. Dean gives sound advice about off-setting it from center and between the vanes.

    In Aug of 2017 I used my ES ED80 and ES AR127 with full aperture filters. In the case of the finders, I removed them and used the shadow of the scope to know when I was in the ballpark then I could easily use the scope as its own finder to locate the Sun. Some like to use some form of solar finder as well. I also cut out a large piece of cardboard to affix around the focuser base to act as a shield while looking through the eyepiece to make certain my face was in a shadow. A couple of additional options that I've used are below. Good luck with your project.


    For the ED80 I used one of the Sun-Catcher filters from Explore Scientific. They use the same filter material as sold by Thousand Oaks Optical. To secure them to the OTA they use foam inserts inside the cardboard cell that compacts against the outside of the tube to hold it in place. I had no issues with this and the fit was quite snug. However, it appears they only sell full aperture filters, which is not really what you need in this case: https://explorescientificusa.com/pro...e-solar-filter

    For the AR127, I had bought a full aperture filter from Seymour Solar a few years back which I used. They make nice metal cells with nylon thumbscrews to secure it to the end of the OTA bending where the front trim ring sits. Once you tighten them against the tube, they can't come off without unscrewing the thumbscrews. Looks like Seymour does sell off-axis and the size you would need seems to be $122. Of course, you can build them cheaper, just depends on what you wish to do.: Shop - Seymour Solar
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    Default Re: How much aperture for solar observation?

    Quote Originally Posted by KT4HX View Post
    For the AR127, I had bought a full aperture filter from Seymour Solar a few years back which I used. They make nice metal cells with nylon thumbscrews to secure it to the end of the OTA bending where the front trim ring sits. Once you tighten them against the tube, they can't come off without unscrewing the thumbscrews. Looks like Seymour does sell off-axis and the size you would need seems to be $122. Of course, you can build them cheaper, just depends on what you wish to do.: Shop - Seymour Solar
    I see I had a mistake in the above paragraph. The work "bending" in red should read "behind". Sorry about that.
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    ---------------------------------------------------------------------------
    Astro Sky 17.5 f/4.5 Dob || Apertura AD12 f/5 Dob || Zhumell Z10 f/4.9 Dob
    ES AR127 f/6.5 and ES ED80 f/6 on Twilight-II Mount
    ES 82° 24mm thru 4.7mm
    ---------------------------------------------------------------------------
    Astronomers: We look into the past to see our future.

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    Default Re: How much aperture for solar observation?

    Thanks for the advice everyone. I think I'm going to pull the trigger on the Seymour Solar filter KT4HX linked to.
    DeanD likes this.
    Scope: Apertura AD8 w/HALO
    Eyepieces: 2" - 27mm Tele Vue Panoptic, 1.25" - Tele Vue DeLite 18.2mm, 11mm & 7mm, Vixen 9 and 6 mm SLV
    Finders: 8x50 RACI and Rigel Quickfinder
    Binoculars: 10x50 Pentax PCF WP II

 

 

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