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Thread: Exit pupil help

  1. #1
    michael131313's Avatar
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    Default Exit pupil help



    I was thinking of getting an ES 68 24mm EP for my Starblast 4.5, 114mm, f/4, 450mm focal length. However, this has an exit pupil of about 6.3mm and I am 70 years old and can only expect my eyes to have about a 5.3 pupil width. I think it would be better to get an ES 68 20mm with an exit pupil of 4.96mm. I would like opinions about this being correct. Thank You.
    ES AR 102 102mm, f/6.5, Orion Starblast 4.5 114mm,f/4 , Celestron 10x50, GSO SV 30mm, ES 68° 20mm, ES 82° 14mm, 11mm, 8.8 mm, 6.8mm, 4.7mm. Twilight 1 mount.

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    Default Re: Exit pupil help

    It depends on what you want to do. Certainly it assures that you'll not waste photons and lose contrast for viewing. But sometimes you may want a "finder" eyepiece and won't mind effectively stopping down your scope to a smaller aperture.

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    Default Re: Exit pupil help

    Either would work well for you. I wouldn't be worried about a 6.3mm exit pupil even at 70. I might have some reservations of an exit pupil closer to 7mm and higher, but I think 6.3mm would be fine. The image might not be quite as bright as it would be to someone much younger due to light loss, but should still be bright and crisp.
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    Default

    With a refractor, an oversize exit pupil is no problem - I often use a 40mm eyepiece with my f5 refractors, for a wider TFOV with an 8mm exit pupil.
    With scopes with a central obstruction, like reflectors and SCTs, though, at some point you start seeing the shadow of the secondary.

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    Default Re: Exit pupil help

    Quote Originally Posted by jerryTheC View Post
    With a refractor, an oversize exit pupil is no problem - I often use a 40mm eyepiece with my f5 refractors, for a wider TFOV with an 8mm exit pupil.
    With scopes with a central obstruction, like reflectors and SCTs, though, at some point you start seeing the shadow of the secondary.
    Very true. I had missed the bit about it being for a Starblast which has a 30% obstruction....

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    Default Re: Exit pupil help

    I have all three of the ES 68 1.25 inch eyepieces. In my opinion, the 20mm is actually the best one. It does not have as wide of a chunk of the sky, but it still has the same size field for your eye to look at. However, I have used the 24mm in my Infinity 80 f/5 refractor and in my Polaris 130 f/5 reflector with no problems. If you have a problem, it would be with your f/4 StarBlast. What I expect is that the field will be a bit darker with the outside portion showing lighter. If you are concentrating mostly on the StarBlast, I would consider the 20mm to be the best option, unless you really want that extra sky in the view.
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    Default Re: Exit pupil help

    Are you certain of your pupil diameter? There is quite a bit of person to person variation. It might be that your pupils are large for your age or the other way around. If they are large, the shorter focal length EP could be an OK choice. If your pupils are smaller than average even the longer FL EP exit pupil might be too big.

    I have several suggestions.
    1) Contact your optometrist prior to your next eye exam and see if he will measure your pupil diameter for you in low light conditions
    2) google "pupil size chart" and under "images" you will find a bunch of gauges for measuring pupil diameter. With someone's help in low light or with a mirror you should be able to get a good idea of the size of your pupils


    resources:
    Web page on measuring:
    The proper procedure for testing pupils | OptometryTimes.
    Chart of variation person to person:
    Electroretinograms and Normative Data | InTechOpen.
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    Default Re: Exit pupil help

    Thank you all for the help. I appreciate it.
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    ES AR 102 102mm, f/6.5, Orion Starblast 4.5 114mm,f/4 , Celestron 10x50, GSO SV 30mm, ES 68° 20mm, ES 82° 14mm, 11mm, 8.8 mm, 6.8mm, 4.7mm. Twilight 1 mount.

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