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  1. #1
    gcbryan's Avatar
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    Default Eyepiece as finder



    What is considered the "normal" eyepiece regarding magnification?

    The lowest mag I have at the moment is about 40x (23mm Panoptic).

    Today I decided to order a cheap ($30) 40mm plossel just to use as a finder. Therefore I don't really care how good it is in general. It should (correct me if I'm wrong) be good enough to use as a finder on my 80mm refractor.

    I have a finder of course but it's hard to use in some straight up orientations.

    When I try to find Uranus for example I'm looking for the "circle" in Pisces. I can find this with my 15x70 binoculars but then when I try to do the same with my 23mm ep it's much harder.

    I figured using a 40mm eyepiece would bring me down to x23 and should make this much easier. Then I can just center (for example) Uranus and then use my better eyepieces.

    Anyone here with a really good 30-40 eyepiece where you use this very much for actual viewing?

  2. #2
    Zeiss's Avatar
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    Default

    what about just getting a EP with a wide FOV to use as a "finder"
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  3. #3
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    Default

    Most of my viewing is with 70x to 150x.

    I've tried your method with my ETX70 but found I need an rdf or laser pointer to get me into the FOV.

    Most finder scopes I've played with have about a 5 degree FOV. A standard plossl has an apparent FOV of maybe 50 degrees. With a 25x it will give you only a 2 degree FOV. A superview ep will give 3 degrees or better.

    I think you will be happy if you have an rdf and a superview ep.

  4. #4
    gcbryan's Avatar
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    Default

    I also use a green laser pointer just held manually in line with the OTA to get into the general area.

    We'll see how the 40mm works out. At the price I thought it was worth a shot.

  5. #5
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    A cheap 40mm plossl (in a 1.25 inch barrel) typically has a 43 degree apparent field of view. You may not get a much wider field of view than through your panoptic.
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  6. #6
    gcbryan's Avatar
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    Yeah, I think I was a little to quick to order that. Oh well, maybe I'll find some use for it.

  7. #7
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    Default

    "Finder eyepiece" is insufficiently specific. Let me explain:

    A finder EP is chosen such that it delivers a 7mm exit pupil (or an exit pupil matched to your eye's entrance pupil. At this exit pupil the view is bright, big and easy to accomodate into your eye.

    Such an EP specification can be found in your main telescope optics as well as in your finder scopes optics. So with a 50mm objective (typical of a finder scope) 7X magnification gives a 7mm exit pupil. With a 500mm objective 70X magnification gives a 7mm exit pupil. Either can/could be utilized as a finder. Big bright wide FOVs.

    Now, if we knew the focal length of the finder scope (or main scope) THEN we could tell you what EP would deliver the 7mm exit pupil you are looking for.

  8. #8
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    Default

    Quote Originally Posted by gcbryan View Post
    I also use a green laser pointer just held manually in line with the OTA to get into the general area.

    We'll see how the 40mm works out. At the price I thought it was worth a shot.
    I made a cradle for my lp from 1/2 inch pvc. It fits nicely if you cut a 6 inch piece in half with a hack saw. I then mounted the pvc on some aluminum angle and flat to be able to align it with my scope. It works much better for me than hand held.

 

 

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