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Thread: Exit pupil in fast scope

  1. #1
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    Default Exit pupil in fast scope



    I've been toying with the idea of an ultra-short focus Dobsonian scope, say a 14" f/3.5. That's 356mm aperture and a focal length of 1245mm, barely longer than my 8" f/5.9. The attraction being, like the 8" it would be a sit-down scope for me.

    So I'm wondering, what's the minimum magnification I could expect to use? With the 8", I'm happy to use my 34mm ES 68 degree, which gives 35x and an exit pupil of 5.8mm. That is comfortable, and gets all the Pleiades nicely in view.

    With the 14" f/3.5, the same eyepiece would give 36x. But with an exit pupil of 9.9mm, yikes!

    So my question is, would I still get a decent view of the Pleiades, despite getting less of the pupil? Or, would the secondary become too prominent in the view? I suspect the latter, because when I've tried a 50mm Plossl eyepiece with my 8", it gets me 24x but with exit pupil of 8.5mm which puts the secondary into the view.

    Comments?
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    Default Re: Exit pupil in fast scope

    In my mind you would want to keep around a 6mm exit pupil max to mitigate the secondary shadow. Something around a 20mm with a 68º AFOV would likely encompass M45 with an f/3.5 scope.

    If you pushed your AFOV some, you could lengthen the eyepiece a hair more. That fast though, the coma might be a booger.
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    Default Re: Exit pupil in fast scope

    If F=1245 then M=F/f and your exit pupil is d=D/M=Df/F where f is the eyepiece focal length, d is your pupil say 5 mm and D is the aperture of 14x25.4=355.6. So f=Fd/D=1245*5/355.6=17.5 mm. Anything more will lead to a loss of brightness because it gets blocked by your pupil.
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    Default Re: Exit pupil in fast scope

    Thanks guys. I wouldn't worry about loss of brightness with the Pleiades (or Beehive) at an outreach event, but would worry about the secondary shadow ruining the view. The 20mm eyepiece suggestion (giving 62x) sounds about right for a minimum mag, as that would give an exit pupil of 5.7, about same as the 34mm in the 8" f/5.9. But that wouldn't get me the whole Pleiades, unless maybe I went with ultra-wide monster eyepiece. Not that I should be looking at the Pleiades with a 14" scope anyway...

    Yes, coma becomes a booger... I'd probably use a corrector. Which raises the question, how does a coma corrector affect the exit pupil?
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    Default Re: Exit pupil in fast scope

    Most coma correctors slightly increase the focal length and focal ratio. My 200mm f/4 (800mm FL) becomes, with the Paracorr, f/4.6 and 926mm. You would recalculate the exit pupil accordingly.
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    Default Re: Exit pupil in fast scope

    You have a 80mm refractor that will give a 6.15mm exit pupil and 3.85° TFOV using the supplied 23mm eyepiece
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    Default Re: Exit pupil in fast scope

    Quote Originally Posted by KathyNS View Post
    Most coma correctors slightly increase the focal length and focal ratio. My 200mm f/4 (800mm FL) becomes, with the Paracorr, f/4.6 and 926mm. You would recalculate the exit pupil accordingly.
    Thanks Kathy, that makes sense. But I'm curious, does that mean the Paracorr only improves coma to that extent? In other words, does your view appear like an uncorrected f/4.6 would?
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    Default Re: Exit pupil in fast scope

    Quote Originally Posted by Gabby76 View Post
    You have a 80mm refractor that will give a 6.15mm exit pupil and 3.85° TFOV using the supplied 23mm eyepiece
    Yes, and I do like to show off the Pleiades with the big finder. I also like to use the main scope as well, if I can get the whole main portion into view as I can with 8" Dob and 34mm eyepiece.
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