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Stargazing How is the dark hole formed?

  1. Apr 26, 2017 #1

    sophiecentaur

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    I bought myself a nice 32mm (2" fitting) eyepiece with a 'wide view'. I was disappointed to see a dark patching the middle when I looked through the scope in daylight. I have never been aware of this when looking at (brilliant) wide objects at night. With its wide angle view, it gives the impression of looking out of the spacecraft window. I bumped into a Q and A about various things and it did include this question. The given answer was because of the wide exit pupil of the lens and the narrow aperture of the eye in daylight.
    I wonder if anyone has a source of a diagram that would make that explanation any clearer. I can't sketch anything out that convinces me.
    At least reading about it implies my lens is not, as I first thought, a duffer.
     
  2. jcsd
  3. Apr 26, 2017 #2

    jedishrfu

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    Don't know if this answers it but there's a diagram showing the exit pupil:

    https://en.wikipedia.org/wiki/Exit_pupil

     
    Last edited: Apr 26, 2017
  4. Apr 26, 2017 #3
    Can I as it the instrument has a central obstruction ? Normally if the exit pupil if wider than your eyes pupil you just loose light.
    Regards Andrew

    PS At low magnification you can see the central obstruction if the exit pupil is larger than eyes pupil. I came across this a long time ago when playing with a short focus reflector with a large central obstruction. In fact I could look round it as I shifted my head across the exit pupil
     
  5. Apr 26, 2017 #4

    Drakkith

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    Here's some (poorly done) raytraces I made real quick by hand. Hopefully the forum doesn't bug out and eat them. And hopefully you can read them. None of this is drawn to scale.

    A. Three on-axis rays coming into the telescope. The dashed line represents the path the blocked ray would have taken.
    20170426_142052_HDR.jpg
    B. Rays entering and exiting eyepiece. Note that the dashed ray from earlier would exit the eyepiece closer to the axis than the other rays.
    20170426_142041_HDR.jpg
    C. Rays after leaving the eyepiece and entering the eye. The dark-adapted eye accepts all the rays while the day-adapted eye rejects all the rays because of the size of the pupil.
    20170426_142001_HDR.jpg
     
  6. Apr 26, 2017 #5

    sophiecentaur

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    Thanks a lot chaps. I knew PF would sort me out on this one. That dark region really does look like an 'obstruction' that you can avoid by moving the head a bit.


    Pupil / teacher jokes: AAArrrrgh!!!! o_O
     
  7. Apr 28, 2017 #6

    davenn

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    I will assume the telescope is a Newtonian style one
    and if so, the darkened area in the middle of the view is likely to be the obscuration caused by the secondary mirror

    It will more obvious at some focus points than others ( depending on if you are focussing on a nearby object (tree at the end of your back yard)
    or an object at infinity focussing ( something at least a few km's away and out to stars, moon etc)

    Dave
     
  8. Apr 30, 2017 #7

    Chronos

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    What you are describing sounds like kidney beaning, an issue common with wide field eyepieces. You can compensate by keeping your eye centered on the optical axis of the EP or backing away slightly from the eyepiece. It is especially noticeable when your pupil is not fully dilated - e.g., in daylight.
     
  9. Apr 30, 2017 #8

    sophiecentaur

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    I looked at a number of links which discuss kidney beaning and I am not sure it that's what I have been seeing. Strangely, I could only find one image from a google search. Afair, what I have seen is not the same as the picture below.
    image017.jpg
     
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