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Thread: 2 Inch Versus 1.25 Inch Telescope Eyepieces?

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    Question 2 Inch Versus 1.25 Inch Telescope Eyepieces?



    Ok I am not sure what are the main differences between a 2 Inch Versus 1.25 Inch Telescope Eyepieces?

    Also since I have a 2" focuser, is it possible to put in 1.25" eyepieces, and if so what disadvantages are there?
    I know they are cheaper...

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    hi bossman , Your scope should have a 2in. - 1.25 adapter . If not these are readily available both new and used but i cant remember where i have seen them new . And some of the newer 2in. eyepieces come with an adapter , although i dont quite understand how this idea works , cutting down on the light path . Oh yea , just joking here but the main difference is 3/4 in. LOL
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    There is another thread here in which Vinnie explains. What would we do without him? I can't find it at the moment. what I remember is that if you can afford them the 2" ones are better for low mags (I think he said down to about 17mm) and are useful so you can get a wider field of view.

    I have one EP (a 13mm Hyperion) that is both 1.25" and 2" which is a good thing because when used in 2" mode I do not have enough in focus. It is not an adapter exactly - it is just a permanent extension below the 2" part that fits a 1.25" holder.

    I am sure Vinnie will be along in due course to enlighten us.

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    A heavy eyepiece is better supported in a 2" focuser. A wide field of view eyepiece will require a 2" barrel. The focal length at which the switch from a 1.25" barrel to a 2" barrel is necessary will depend upon the eyepieces apparent field of view.
    AFOV (apparent field of view): Focal length when 2" barrel needed:
    100° 17mm
    82° 20mm
    68° 27mm
    Yes, you can use a 2"/1.25" adapter in a 2" focuser.
    garskofone likes this.

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    Thanks Ted

    Just to add here, there is no advantage, or disadvantage, when using a 1.25" EP in a 2" focuser

    Also for the common Plossl type EPs the magic number is around 30mm to 32mm.

    As an aside, there are 1.25" EPs available longer than that 30mm to 32mm size and these are actually rather a waste of money. Once the EPFL exceeds the internal diameter of the barrel, you actually lose AFOV (32mm is approx 1.25")

    I should add that I recommend to anyone buying a new scope if a 2" focuser is available it is a good idea to go for it. Its a feature that is nice to have for future expansion of the EP collection, even though it may not be required initially.
    Last edited by Vinnie; 02-20-2009 at 09:44 PM.

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