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Stargazing Is there any material I can use for an aperture solar filter?

  1. Sep 29, 2018 #1
    I live in a small town and would like to make some observations today. However i dont have a solar aperture filter, i only have an eyepiece filter of which i do not trust. So is there any household items, materials i could use safely to save me from from ordering on the net and waiting however for the film to arrive. Someone said DVDs split in in half (1/2 depth/thickness) works but is that good advise or just bad advise?

    I should say that the telescope is a D= 76mm( but can be reduced). SO any ideas would be great or am i just hoping for something that does not exist.

    Thank you.

    P.S
    I have no idea what this prefix i need to select is about, i am not at school im just an enthusiastic beginner of astronomy.
     
  2. jcsd
  3. Sep 29, 2018 #2

    DaveC426913

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    Gold Member

    I bought a piece of welder's glass from my local "Princess Auto" (a hardware warehouse).

    I covered the scope objective with cardboard, cutting a small hole in it, and then taped the welder's glass over that.
     
  4. Sep 30, 2018 #3
    NICE ONE!!!!! Such a simple idea. It seems quite obvious now you say it. Thank you.
     
  5. Sep 30, 2018 #4

    davenn

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    Science Advisor
    Gold Member

    NEVER EVER use an eyepiece filter for solar filtering

    not good advice

    76mm is a good aperture size :smile:
    Easiest thing to use is 3 to 4 layers of a mylar survival blanket over the front of the scope
    Start with 4 layers, if it is too faint, reduce to 3 layers.
    I used film filters for 40+ years for viewing sunspots and solar eclipses

    when you get a bit more money (~US$50 - $170) you can get a commercial solar filter
    here's the same thing over the front of one of my telephoto lenses …..

    2015_03_14_3302sm.jpg

    one of several suppliers
    http://thousandoaksoptical.com/shop/solar-filters/full-aperture-solarlite/


    when you get lots of money, you can get a proper solar telescope like this one of mine ……

    IMG_1093sm.jpg

    Now the mylar and the commercial filter glass ways are great for showing sunspots on the surface of the sun
    And a warning, because we are at solar minimum, there are very few spots to be seen. The sun is pretty lank most of the time.

    That solar scope of mine shows prominences on the limb and other features on the face of the sun.
    It is a very narrow Hydrogen Alpha (Ha) 0.7 Angstrom and less.

    with that you get images like this …….

    180909 Ha Cap_14.jpg

    180930 Cap 003.jpg

    180930 Cap 053.jpg



    See my solar imaging thread on PF, here ……

    https://www.physicsforums.com/threads/solar-imaging-and-techniques.925656/


    cheers
    Dave
     
    Last edited: Sep 30, 2018
  6. Sep 30, 2018 #5

    Tom.G

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    Science Advisor

    Note that the quote below is for DIRECT viewing, not thru a telescope. If you use full aperture of your 76mm telescope, add 4 to the shade numbers in the article.
    Quoted from: https://eclipse2017.nasa.gov/safety

    Viewing with Protection -- Experts suggests that one widely available filter for safe solar viewing is welders glass of sufficiently high number. The only ones that are safe for direct viewing of the Sun with your eyes are those of Shade 12 or higher. These are much darker than the filters used for most kinds of welding. If you have an old welder's helmet around the house and are thinking of using it to view the Sun, make sure you know the filter's shade number. If it's less than 12 (and it probably is), don't even think about using it to look at the Sun. Many people find the Sun too bright even in a Shade 12 filter, and some find the Sun too dim in a Shade 14 filter — but Shade 13 filters are uncommon and can be hard to find.

    For those interested, the 'add 4 to the shade numbers' was calculated from:
    (I'm using 1mm pupil diameter here for a safety factor >2. Online search says minimum diameter is 1.5mm.)
    Assume eye pupil diameter of 1mm. Area of 0.785sq.mm.
    Telescope aperture 76mm. Area of 4 534sq.mm.
    Area ratio: 5776 (That's the additional factor of intensity reduction required.)

    Welding filter are rated as Shade Number = log10(attenuation ratio)
    log(5776) = 3.76
    So add 4 to the shade number and you get 1/10,000 of the light thru.
    If you can't find any dark enough you can stack them. The shade numbers add.

    Cheers,
    Tom
     
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