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Thread: Counterweight caper

  1. #1
    Raymondhow's Avatar
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    Default Counterweight caper



    When doing outreach with my 8" Dob, I like to mount an 80mm finder which provides a nice alternate view. With 16mm ES eyepiece it gives 19x, so it's like half of a giant binocular. Great for the Pleiades, the moon, Double Cluster, wide view of M42, etc.

    The big finder weighs over 3 lbs, so the main problem is balancing. In the past, I used a pair of extra large welder's magnets on back of the tube.
    XT8 with 80mm finder.jpg

    This is rather unwieldy, because it doesn't provide the same balance at all angles. And the magnets have scratched my tube finish. Also, at almost every event, some kid will accidentally bump a magnet and knock it off, clattering onto the pavement. Parents would be alarmed, thinking Johnny had broken something important.

    I had acquired a 2.5 lb counterweight to use with my NMT, and got the idea to try mounting on my XT8. The weight is threaded inside, so I bought a nut and bolt to fit. This morning, I drilled a 1/2" hole on underside of the tube, opposite from the eyepiece location. Since I had the optics out, also gave the mirror a wash... it was due.
    IMG_6572.JPG IMG_6574.JPG IMG_6575.JPG IMG_6579.JPG

    The whole thing went pretty smooth, and tonight I tested it on my roof with the big finder.
    IMG_6583.JPG

    Thought I would chase some variable stars. Nope, the muck was so bad I could barely see Rigel. The moon occasionally swam into clarity, no filter needed. The weight worked pretty well though, even with my heaviest 34mm eyepiece. At 30° elevation, things did get a bit tippy. Now wish I had mounted it slightly lower, oh well. The extra weight makes the movement a bit stiff, as the little bearings are somewhat overwhelmed. But I think it will work well enough for outreach. We have an event set for later this week, if the sky will open up.

    Thanks for reading
    Raymond Howard
    Outreach Coordinator, Eastbay Astronomical Society
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    Default Re: Counterweight caper

    Hi Ray,

    Great idea.

    In reading your story and wondering how your new counterweight system could be slightly improved, I had the following thought. Can you elongate that new hole both a 1/2" back and a 1/2" forward? This gives you an inch long slot that would allow for greater balancing control. Since the counterweight screws onto the post, I assume it can be cinched down against the tube to lock its position on the tube.

    Just a thought.

    Cheers,
    JT
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    Default Re: Counterweight caper

    rather than making the hole bigger, you could use the bolt to mount a rod parallel to the tube and slide your weights around on it.

    why do i get the feeling we're going to have a vw bug mounted to your dob by the end of this thread...
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  5. #4
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    Default Re: Counterweight caper

    Very clever and a nice job, Ray. Thanks for the pics... she's a beauty.

    Clear, Dark Skies
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  6. #5
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    Default Re: Counterweight caper

    We have a club member who uses a length of medium size link chain. It attaches so that when the scope is pointed low, all of the links are suspended. As the angle of the scope increases, links start to rest on the base reducing the amount of weight. When at Zenith, only 2 or 3 links are suspended.


    Very much like this.

    https://www.google.com/search?q=chai...6Y75sub5IgeUM:


    Scott
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    Default Re: Counterweight caper

    ADM makes a bracket that would work I believe.
    My eyes
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    Default Re: Counterweight caper

    Thanks for the suggestions, guys. Variable mounting height is an interesting idea, but I'd rather keep things simple. Maybe I will drill another hole at some point, lower down.
    Raymond Howard
    Outreach Coordinator, Eastbay Astronomical Society
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    Default Re: Counterweight caper

    I found this diagram that nicely shows Dob balancing issues. Balancing when vertical can be important when the Pleiades are overhead!
    Dob balancing.jpg
    Raymond Howard
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    Default Re: Counterweight caper

    If it is possible to get a VW bug mounted, someone on this forum will come up with a way. Some of the most creative people I have ever known are here. Love the ideas.
    ken
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