Author Topic: Eyepiece question  (Read 173 times)

bamrocorna

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Re: Eyepiece question
« Reply #15 on: January 31, 2018, 10:50:27 AM »
Thanks for the responses so far, sorry I have not had a chance to read them in detail. From skimming, looks like it may be the Expanse have too long eye relief for me and I should look for shorter. Might also be interesting to try the Expanses with my eyeglasses on. BTW, I was definitely viewing at night when the blackouts were occurring. Also, since it sounds like QC may be an issue, I wonder if it matters I got the Expanse eyepieces with the ST80A? I assume they would be the same as if the EP was purchased alone, but perhaps not.

Greg Fleming

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Re: Eyepiece question
« Reply #16 on: February 08, 2018, 08:37:02 PM »
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It can be an aggravating factor,
but I almost always see it in eyepieces of lower power and very wide
eye lens.  The curvature on the Expanses is rather spectacular so
it's no surprise it's sensitive.

I tend to associate it with an EP that has been pushed really hard for
field width without a lower number of elements than the afov warrants....
Even a 6-element can have blackouts if you push it too hard...then the
eyeball is a in a tight box to make it work.

What do you mean? Exit pupil is exit pupil. A 20mm 9-element design has the exact same exit pupil size as a 20mm 3-element eyepiece.
What newly-defined characteristic of the exit pupil are you describing if it's not SAEP?

Somehow you are conflating blackout with the exit pupil.

Going high-power (thus reducing exit pupil) can make your ideal eye location even smaller,
but the aggrevation of the blackouts is pretty bad whatever your exit pupil.

The issue happens fairly independently of 3, 4, 5, 6, 7, 8, 9, 20 elements....

The blackout are mainly on the near-far axis. Thy also affect the left-right-up-down.
Move your eye and take note.

Exit pupil without blackout simply looks like moving your eye past a keyhole.

juskemenbo

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Re: Eyepiece question
« Reply #17 on: February 09, 2018, 11:07:31 AM »
OK, let me explain about eyepieces as afocal devices.

As you move your eye back from the eyepiece, the field gets smaller, but stays in focus.
That's because, once the focal plane of the eyepiece is coincident with the focal plane of the scope, the eyepiece is essentially afocal.
No matter what distance you are from the eyepiece, the field is in focus.

Nonetheless, as you back away from the eyepiece, the pupil of your eye becomes smaller than the size of the image
formed by the eyepiece (which becomes wider). You see less of the field because the edge of the eyepiece barrel is literally occulting the image.
You can move your eye back and forth laterally and see different parts of the image, and it stays in focus.
But the body of the eyepiece itself occults the edge of the field.
This sounds suspiciously similar to your description of drifting back and forth and having the edge of the field get occulted.

This only occurs when you are too far from the eyepiece, however, to see the actual field stop completely all the way around
(which occurs at the exit pupil).

Now, if you are at the exit pupil, you see the entire field to the field stop and, if the exit pupil is smaller than your eye's pupil, all the light from the eyepiece goes into the eye.

If you are too close, however, the light from the eyepiece becomes larger than the pupil of the eye and your iris starts occulting the image.
If you move back and forth, first one side, then the other, blacks out as the rays are intercepted by the iris and you get what are termed "black-outs".
This happens a lot with long eye relief eyepieces used by non-glasses wearers, or people who stand to observe or even just when you drift a bit too close to
an eyepiece. I can induce the blackouts on nearly every eyepiece that's made unless the eyepiece has such a short eye relief I automatically hold back
to preserve my eye.

So what it looks like at the eyepiece is dependent on where your eye is relative to the exit pupil.

If the eyepiece has spherical aberration of the exit pupil, however, then the exit pupil itself is not all at the same distance from the eyepiece, and moving in toward the eyepiece may make either the
center or edges of the field black out in an alternating way that resembles large kidney-bean-shaped shadows in the outer parts of the field. This is termed "kidney bean blackouts" or simply
"kidney-beaning". It is found in (primarily) widefield eyepieces without a flat exit pupil. This doesn't have to be severe (like the original 13mm Nagler)--it can be slight, but still result in an eyepiece
being sensitive to eye placement. I have run into many of these over the years.
http://www.handprint...O/ae4.html#SAEP

So, traditional "blackouts" are caused by the observer's eye position relative to the exit pupil.
"Kidney bean blackouts" are caused by SAEP in the eyepiece.
Other than cataracts or macular degeneration, those are the two sources of blackouts in eyepieces as I understand it.

The "walking past a keyhole" look simply indicates the observer is farther away from the eyepiece than the exit pupil.