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Thread: 2" 15mm and 10mm eyepiece

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    Default 2" 15mm and 10mm eyepiece



    Hey all,

    Recently I have taken the plunge and got myself a skywatcher ED120 OTA, it came with a 28mm eyepiece, It was suggested to me that I get these eye pieces, I have been looking around and I honestly cannot find a 2" 15mm and a 10mm eye piece in Australia, it's beginning to frustrate me a little bit, can somebody where I can find such objects?

    I am also after a televue 2" 2x barlow type lens thing.

    Do you think this would be a good investment with both of these?

    Thanks,
    Fraser.
    Astronomy compels the soul to look upwards and leads us from this world to another.

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    Default Re: 2" 15mm and 10mm eyepiece

    There's no direct benefit to having a 2" barrel versus a 1.25" barrel. If you're looking for eyepieces and using the barrel diameter as your primary selection criteria, then you're going to be disappointed with the result.

    Choose your eyepieces based on focal length, AFOV, image correction, and build quality/coatings. The barrel diameter, frankly, is irrelevant. Eyepiece manufacturers ship their eyepieces in 2" barrels when the field lens must be larger than the 1.25" barrel will accommodate.

    You might find this post useful for selecting eyepieces: Choosing the Right Eyepieces
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    Default Re: 2" 15mm and 10mm eyepiece

    Would this also apply to the barrel diameter of Barlow lenses?

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    Default Re: 2" 15mm and 10mm eyepiece

    The required diameter of a barlow lens is defined by the diameter of the field stop where the lens is used. Barlow lenses do not sit at the focal plane; they sit in front of the focal plane. On a fast scope, the light cone may diverge enough where the barlow sits that a 2" format is necessary to prevent vignetting. In general, however, let your eyepiece selections drive your barlow choice. If you have 2" eyepieces that you wish to barlow, you're going to need a 2" barlow, end of story. But if you are only intending to barlow your 1.25" eyepieces, you can very likely get away with a 1.25" barlow without any ill effects, unless you're using it in a very fast scope.

    I should note that Harry Siebert recommends larger barlows for quality reasons:


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    Default Re: 2" 15mm and 10mm eyepiece

    So really, the 2" and the 1.25" Have no real difference apart from the FOV, so maybe I shouldn't get the 15mm and the 10mm 2" eyepiece, maybe I should consider getting a zoom eyepiece, just for the fact that you can't use eyepieces for astrophotography, so I might invest in one of those, or maybe get a high quality barlow.

    With the powermates, would it be better to get a 2" One or a 1.25? I want to use it for Astrophotography in the future, but since I have a new scope, it's opened up to a 2" world to me.
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    Default Re: 2" 15mm and 10mm eyepiece

    Barlows and Powermates are good to have in 2" format, since you never know what eyepiece you want to use with them, and you might want them for photography. You can always use an adapter for 1.25" eyepieces.

    For short focal length eyepieces like 10mm and 15mm, there is absolutely no point in having it in 2" format. Here is a spreadsheet that shows you what eyepieces will work with any telescope.

    Eyepiece Spreadsheet

    Enter the scope's aperture and focal length, and the design AFOV of the style of eyepiece you want to use. The spreadsheet will show a sample of eyepiece focal lengths over the entire usable range for that scope, including the barrel size required. If it shows 1.25", it means that a 2" barrel will be of no benefit. If it shows 2", it means that you need a 2" barrel to get designed field of view. If it says "limited by barrel" after the AFOV column, it means that even a 2" barrel is not big enough, and the eyepiece will not show its designed AFOV.

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    Default Re: 2" 15mm and 10mm eyepiece

    The telescope creates a linear FoV, the size of which is directly proportional to the focal length.
    An EP "admits" a portion of this linear FoV and turns it bak into an angular FoV so we can admit the light into our eye.

    If the EP only "admits" a linear FoV smaller than 1.18" the EP can be in 1.25 format
    If the optical path through the lenses in the EP are never wider than 1.18" there is no reason the EP cannot be in 1.25" format.
    There is no optical advantage to an EP using a larger barrel than the optics require.

    There is ONE EP currently that requires a 2" barrel in the 10mm FL class (9ES120). As far as I can tell, this EP actually does need the 2" barrel for the light path inside the optics.

    There are a small number of EPs in the 10mm-15mm class that HAVE 2" barrels, but other than the 9ES120 the optics don't actually need the 2" barrel. The 13E is available in 1.25+2" barrels, the corresponding 14ES100 comes only in 2" format. Optically, the light path in both "fit" inside a 1.25" barrel. The 17E light path does not.

    Some optical designes use a Smythe lens to diverge the incoming light cones to get a wider apparent FoV.

    A Smythe lens is like a barlow in that it diverges the incommnig light cones, but unlike a barlow in that it creates aberation of the opposite sign of the main lens. Taken together the aberations cancel, and wider fileds of view are possible.

 

 

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