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Pupil size too small... or too large?


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I have always been fascinated by wide angle low power views and tend to push the limits in this. Conventional wisdom suggests that an exit pupil size of (say) 6 mm about matches the pupil dilation of the young (I'm not THAT young!). So I normally work to around 5mm. Clearly "Not all light enters the eye", but what would an overlarge exit pupil actually LOOK like? Cheap and cheerful achromats are often provided with objective lens covers, which double as a "stop" to reduce the various(!) aberrations. Could the effect be the same? :shock:

Or is experimenting with overlarge exit pupils a definite :nono: :)

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I've still got no idea what all this exit pupil stuff is all about. Is it just the hole that you look through? I got a 32mm Meade plossl in the post yesterday (beautiful little thing), and the "exit pupil" seems to be 29mm!!

Am i missing something entirely here?

Help me out here!

Andrew

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The exit pupil is how large the image an eyepiece produces, if the exit pupil is larger than the observers pupil then the light is wasted.

exit pupil= Aperture / Magnification

or

exit pupil= (Eyepiece focal length) / (focal ratio of primary or objective)

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Andrew

Divide the focal length of the eyepiece by the F number of your scope.

eg

30mm eyepiece in an F5 scope gives 30/5= exit pupil size of 6

If the size of the pupil in your eye is say 4mm due to background

light or age then 2mm of the exit light is wasted...

Think that's how it works :lol:

Edit: blimey, Gaz was quick on that one :)

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Sorry to omit the details of the calculation! It's one of those things I assume "everyone (else) knows" - So perhaps it's pertinent? FWIW, I used to look through "uncle's" 7x50 binoculars (exit pupil: 50/7 = 7mm) , during daytime. Now I only have a pair of cr*p 10x50s! Interesting though, Gordon. I have been tempted to buy at least ONE (cheap) "more than 32mm eyepiece" 2" eyepiece - just to try in my reffractor. Such things are sometimes worth investigating, even if they defy (strict) "theoretical" ideas? Doubtless I'll never get/stay "rich" ! :)

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An exit-pupil larger than the eye-pupil is remarkably comfortable to use as eye-placement is less critical. It is why I prefer to use a 7x50 binocular when observing in company over a 10x50 - people find the 7x50's larger 7mm exit-pupil easier to use.

Consider also that reducing magnification will increase the exit-pupil size but, when using a scope with a central obstruction such as an SCT or Newtonian, if you reduce the magnification too much the shadow from the central obstruction can become visible...

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An exit-pupil larger than the eye-pupil is remarkably comfortable to use as eye-placement is less critical. It is why I prefer to use a 7x50 binocular when observing in company over a 10x50 - people find the 7x50's larger 7mm exit-pupil easier to use.

Consider also that reducing magnification will increase the exit-pupil size but, when using a scope with a central obstruction such as an SCT or Newtonian, if you reduce the magnification too much the shadow from the central obstruction can become visible...

Something I notice very much when using my 32mm ep with my Newtonian. My 5mm X-cel gives me a 1mm exit pupil with the Newtonian, but I can still move my eye around to see different parts of the FOV. It all gets to sound like optical technobabble after a while. As long as I'm comfortable with the view, I don't care about such details.

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