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  1. #1
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    Default Confusion about maximum focal length eyepiece



    I have read that a 31.75mm eyepiece is the maximum focal length for a scope with a 1.25" focuser. So the argument goes that a 40mm EP would vignette in any 1.25" scope. I find this hard to believe, but my knowledge is limited. Can someone help?

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    You have got it wrong. 31.75mm is the metric equivalent of 1.25", the diameter of the barrel, not a limit on the focal length. There is no reason why you should get vignetting regardless of the focal length, as long as it fits in the focuser. On the other hand, an eyepiece with a larger diameter than the focuser can be expected to give vignetting.

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    Here is Celestron's explanation of vignetting: What is vignetting?

    What happens with a telescope when vignetting occurs is that the field stop (the narrowest part of the optical train -- the part that forms the smallest ring of light) is smaller than the intended stop for the telescope.

    There are lots of components in a telescope that can cause vignetting. For example, a dew shield can cause vignetting when using a low-power, wide-field eyepiece, as the observed patch of sky might be large enough that the incoming light cone intercepts the edges of the dew shield, thus reducing the amount of light entering the scope. The focuser drawtube can also introduce vignetting, if the light cone arriving from the primary mirror or objective lens is wider than the opening of the focuser drawtube. Similarly, if the effective light cone (the parts of the light cone that the eyepiece needs to project a complete image of the desired field of view) is wider than the 1.25" opening at the bottom of the eyepiece, then you'll get vignetting.

    Determining whether or not a particular eyepiece will suffer from vignetting (and thus a narrower AFOV than is advertised on the barrel) is not something I know how to do offhand, but in general I believe that it's most common with faster focal ratio scopes combined with longer focal length eyepieces -- in these situations, the light cone is already wide (due to the fast focal ratio of the scope), and when combined with a long focal length eyepiece (which results in a very low magnification and a huge swath of sky that has to be directed into the eyepiece), this can cause vignetting for eyepieces with 1.25" barrels.

    In particular, I'm pretty sure that a 32mm Plossl eyepiece (AFOV=52deg) is about the longest you can use in a fast (f/5 or so) scope without vignetting. A 40mm 1.25" Plossl will vignette in this case -- and the AFOV will shrink to ~40deg.

    Eyepiece manufacturers obviously do not like vignetting and will package their eyepieces in barrel sizes that make sense to avoid it. That's why long focal length large AFOV eyepieces tend to come in 2" formats -- so that the focuser and eyepiece barrel don't vignette the image.

    I'm sure there are some more subtle things about this topic that I'm not revealing, and I may have gotten some of this flat-out wrong...it's an area where I'm not 100% up to speed. So if somebody more knowledgeable would like to correct me, please do...

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    31.75 is indeed the diameter of the barrel, and therefore the width of the largest possible focal plane. In other words, the image that the eyepiece looks at can be no larger than 31.75mm across. What focal length eyepiece will work with that depends on the AFOV of the eyepiece design.

    A 50 deg AFOV design will max out at 34 mm. A 70 deg design will max out at 22 mm. An 82 deg design will max out at 18 mm. Any eyepiece with longer focal length than these limits will want a focal plane that is bigger than the barrel of the focuser will allow, and will therefore vignette.

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    Thanks all!

 

 

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