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Thread: Do I have this right?

  1. #1
    Nimitz's Avatar
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    Default Do I have this right?



    When looking at various performance parameters of EPs versus exit pupil is the following generally correct to get the best of each parameter?

    For maximum magnification you'd want basically .5 mm
    For high magnifications something close to 1mm
    For maximum resolution and contrast 2mm
    In wide field EPs (82 deg & up) something around 3-4 mm

    If true this means that to get those wide field views you're always going to give up something in resolution & contrast as compared to smaller FOVs at the same FL?
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  2. #2
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    Default Re: Do I have this right?

    Hello Nimitz,

    you are basically right.
    The 2mm exit pupil offers a combination of high contrast and high resolution during the daylight, and we take it as a reference value also for the visual astronomy.
    Another way is to derive the maximum contrast and maximum resolution combination
    from the resolution of the human eye and from the diffraction limit of the telescope.
    Here comes as a parameter the angular resolution limit of the eyepiece, which is most often not known.
    The marketings keep quiet about the angular resolution of their eypiece, and it is often difficult to find the tests.

    Best,

    JG
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    Default Re: Do I have this right?

    Quote Originally Posted by Nimitz View Post
    When looking at various performance parameters of EPs versus exit pupil is the following generally correct to get the best of each parameter?

    For maximum magnification you'd want basically .5 mm
    For high magnifications something close to 1mm
    For maximum resolution and contrast 2mm
    In wide field EPs (82 deg & up) something around 3-4 mm

    If true this means that to get those wide field views you're always going to give up something in resolution & contrast as compared to smaller FOVs at the same FL?
    One caveat to that. The more you can back the magnification off (to a point) the better the contrast becomes. It was the primary reason I bought a focal reducer for the my SCT
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    Default Re: Do I have this right?

    Yes, with one caveat. The wide field exit pupil use is not limited to wide AFOV eyepieces. For instance in my most recent observing report I was using a KK orthoscopic 25mm giving a 3.5 mm exit pupil. Also in the mix were a 41mm and 35mm Panoptic giving 6mm and 5mm exit pupils. It was interesting to see how the usefulness of an exit pupil varied by object....
    j.gardavsky and SpyderwerX like this.

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    Default Re: Do I have this right?

    There is no direct connection between EP FOV and exit pupil. Exit pupil = telescope aperture/ magnification. So, if you have 100 deg 8mm EP and 40 deg 8mm EP, they both have the same exit pupil in the same scope.

    Largest useful exit pupil is a function of telescope design, observer eye and sky conditions. If you observing from a nice dark location with refractor and young eyes you can push exit pupil to 7mm.

    With refractors you can use even larger exit pupil, but some of light will be lost. With reflectors and CATs if you push above 6mm you may see the shadow of central obstruction.

    From light polluted location it is indeed better to keep it to 3-4mm, otherwise sky background will be too bright washing out details.
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