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Thread: Are lower number eyepiece sizes always a smaller viewing glass?

  1. #11
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    Default Re: Are lower number eyepiece sizes always a smaller viewing glass?



    Eye lens size isn't a good guide to quality.

    All that a small eye lens usually means is that you have a scaled design eyepiece (and the smaller it gets, the shorter the focal length with less eye relief (for scaled designs, the eye relief will be some fraction of the focal length; exactly how much depends on the eyepiece design type).

    Note that a scaled design doesn't mean that it's necessarily a bad design at all; plossls are decent eyepieces, and quite a few folk really like orthoscopics for planetary and similar use. Some older scaled designs don't do so well by comparison with more modern ones.

    The non-scaled "premium" designs often have bigger eye lenses, wider AFOVS and longer eye relief; that doesn't always mean the quality is necessarily better. A lot still comes down to quality, and similar form factor doesn't guarentee similar results.
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    Default Re: Are lower number eyepiece sizes always a smaller viewing glass?

    No that is not the case at all, I have some very expensive eyepieces with very small eye lenses.
    The quality of the eyepiece has nothing to do with the size of the lens.
    These run from $350-450USD used each for example.
    SMC.jpg
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  3. #13
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    Default Re: Are lower number eyepiece sizes always a smaller viewing glass?

    Sorry, wrong wording. As I know i've seen expensive, small eye lens eyepiece. But I had never seen a cheap higher magnification eyepiece with a larger eye glass area.

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    Default Re: Are lower number eyepiece sizes always a smaller viewing glass?

    Which eyepieces in particular are you looking at?
    Refractors: Antares 105 f/15, Celestron 150 f/8, Stellarvue NHNG 80 f/6.9, TAL 100RS f/10, TS 102 f/11, UR 70 f/10, Vixen SD115s f/7.7
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    The weakest link in the optical chain is the large nut located directly behind the eyepiece/ camera. - Gabrielle
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    Default Re: Are lower number eyepiece sizes always a smaller viewing glass?

    I guess I'm looking into some higher magnification around 10mm or lower, but noticing they all have ridiculously small viewing glass areas. Obviously, more glass=more cost, but with a few exceptions, they all seemed to be the super small eye glass

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    Default Re: Are lower number eyepiece sizes always a smaller viewing glass?

    The design of the eyepiece and AFOV will determine the lens size, a 5mm Plossl only has 4mm eye relief so a large eye lens would be pointless. The picture of the TMB Supermonocentrics (Hasting Triplet) I posted above have a 30°AFOV so again a large eye lens is not required.
    My 5mm Speers-Waler has 12mm eye relief and a 82° AFOV with a 1.25" eye lens.

    Typically anything with long eye relief or wide FOV will have a larger eye lens but will be more expensive due to the extra elements required.

    Depending on the AFOV your looking for and budget, the Siebert Optics StarSplitters are a good eyepiece.
    60° AFOV and typically 10mm eye relief.
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    Refractors: Antares 105 f/15, Celestron 150 f/8, Stellarvue NHNG 80 f/6.9, TAL 100RS f/10, TS 102 f/11, UR 70 f/10, Vixen SD115s f/7.7
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    Ya gotta keep this Apo/
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  7. #17
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    Default Re: Are lower number eyepiece sizes always a smaller viewing glass?

    Hello Starlord,

    there are 2 points regarding your questions.

    1. The series of the Abbe othos, Plössls, amd most (if not all) symmetric eyepieces, decrease their total size with decreasing focus lengths, so they are geometrically downscaled, and also their eye relief is downscaled.

    2. In the wide field eyepieces, the downscaling of the focus lengths is achieved with the Smyth lens (Tele Vue, Pentax), or with Barlow lens (some wide field planetary eyepieces of Takahashi, Long Perng, ...).
    Contrary to ad 1., the resulting size, and the resulting weight of the eyepiece may increase with decreasing focus length, see:
    http://www.astronomyforum.net/member...D160013es.html

    From the technical point of view, the diameter (d) of the eye lens is given by the eye relief (e) and apparent field of view (AFOV),
    d ≥ e*tan(AFOV)
    From another technical point of view, the diameter of the eye lens is often adjusted to the inner diameter of the eyepiece lens cell, and what is too much, it will be reduced by a baffle in front of the eye lens to prevent eventually emerging stray light rays.

    Best,

    JG
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    Default Re: Are lower number eyepiece sizes always a smaller viewing glass?

    Quote Originally Posted by Starlord101 View Post
    I guess I'm looking into some higher magnification around 10mm or lower, but noticing they all have ridiculously small viewing glass areas. Obviously, more glass=more cost, but with a few exceptions, they all seemed to be the super small eye glass
    What is your budget? Explorer Scientific 82 degree series are well performing EPs with fixed top lens diameter.

 

 
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