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Shortest Focal Length Useable Eyepiece

Eyepieces Observing Optics
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#26 MartinPond

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Posted 10 May 2021 - 11:02 PM

I find that you can go  beyond 50x(aperture, inches) or 2x(ap, mm),

    but it gets dimmer and very sensitive to dust and floaters

    (including stuff floating outside, on your cornea).

  You also need a very, very solid tripod for the shakes.

 

I usually keep to 25x(aperture, inches) or 1x(ap, mm),

  ..it usually looks cleaner and more colorful, and my eyes can

   see the little details.  Dim makes for a sloppy image

    for your cortex to process.

 

Like it says above,  highest power might only be used for splitting stars,

   so...rarely.  

 

Something like a simple eyepiece (Plossl, 4mm as you posit)..

   can be dust-sensitive and like wearing a contact, relief-wise.

   A more complex eyepiece, or a Barlowed longer-fl simple eyepiece

    is less annoying.


Edited by MartinPond, 10 May 2021 - 11:05 PM.

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#27 Sarkikos

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Posted 11 May 2021 - 06:45 AM

Yes, you are correct.  I'm starting to get a handle on the terminology and so maybe you or another can correct me if I'm wrong. My inkling now is that if I look down on the eyepiece and I see the size of the glass, the defined diameter is the exit pupil.  I have some vintage eps, Galoc Saturn ones, and the glass on the 0.24 inch also labeled 6.0mm is about 3/16 in diameter. 

 

It would make sense to call that diameter the exit pupil and so I think I was on the right track with the field stop, and now I think I get it that the eye relief is the distance one's eye is away from the eyepiece glass when focus is achieved. 

The eyepiece magnifies the image coming from the primary mirror (or objective lens).  The exit pupil is the actual width of the beam of light exiting the eyepiece at your eye.  That is why some observers will say that you are "wasting light" if the exit pupil is wider than the pupil of your eye.

 

The exit pupil is not defined by the size of the eye lens of the eyepiece.  Thinking like this will only confuse matters.  Don't think about the physical appearance of the eyepiece. 

 

The field stop of the eyepiece directly determines the apparent field of view, not the exit pupil.  The field stop and primary mirror or objective lens focal length together determine the true field of view.   TFOV = (Field stop / Objective focal length) * 57.3.  A slightly high value for TFOV can be estimated by AFOV / Magnification.  

 

The only attribute of the eyepiece that matters when calculating the exit pupil, is the focal length of the eyepiece.  The exit pupil is calculated by the focal length of the eyepiece divided by the f-number of the primary mirror (or objective lens).  A 4mm eyepiece used in an f/5 telescope (e.g., a Newt with a 250mm primary that has a 1250mm focal length) will yield a 0.8mm exit pupil.  Any f/5 telescope of any aperture will produce a 0.8mm exit pupil when used with a 4mm eyepiece.

 

Eye relief is the distance from your eye to the eye lens of the eyepiece when your eye is at the exit pupil.

 

Mike


Edited by Sarkikos, 11 May 2021 - 06:57 AM.

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#28 MartinPond

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Posted 11 May 2021 - 08:27 AM

You can see the exit pupil by moving your eye back

   from the eyepiece in daylight (not the sun!) and noticing

   the bright circle in the face of the eyepiece.

   Standing back from the eyepiece, this is actually an image of the 

    telescope objective's opening.


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#29 Jon Isaacs

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Posted 12 May 2021 - 08:45 AM

Yes, you are correct.  I'm starting to get a handle on the terminology and so maybe you or another can correct me if I'm wrong. My inkling now is that if I look down on the eyepiece and I see the size of the glass, the defined diameter is the exit pupil.  I have some vintage eps, Galoc Saturn ones, and the glass on the 0.24 inch also labeled 6.0mm is about 3/16 in diameter.

 

It would make sense to call that diameter the exit pupil and so I think I was on the right track with the field stop, and now I think I get it that the eye relief is the distance one's eye is away from the eyepiece glass when focus is achieved.

 

 

There is a terminology lot to understand.

 

The exit pupil is actually the "beam" of light that enters your eye.  As Martin said, if you back away from the eyepiece, you can see a bright disk in the eyepiece. That's the exit pupil

 

The diameter of the exit pupil is equal to the aperture divided by the magnification. Try experimenting with different eyepiece focal lengths and see the diameter of the disk change.

 

Jon


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#30 woodsman

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Posted 13 May 2021 - 12:11 PM

There is a terminology lot to understand.

 

The exit pupil is actually the "beam" of light that enters your eye.  As Martin said, if you back away from the eyepiece, you can see a bright disk in the eyepiece. That's the exit pupil

 

The diameter of the exit pupil is equal to the aperture divided by the magnification. Try experimenting with different eyepiece focal lengths and see the diameter of the disk change.

 

Jon

I'll do that. Thanks! 




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