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freestar8n
Post Laureate
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Reged: 10/12/07

Re: Why a too-small iris results in reduced aperture new [Re: GlennLeDrew]
      #6048691 - 08/27/13 12:07 PM

Quote:

Jon,
Your diagram does not address the matter of the reduced iris. You've made the iris large enough to accommodate the full light bundle from the objective.




Oh - I just realized something much more fundamental about that diagram. All the parallel input rays on the left should be emerging parallel on output. The diagram is incorrect and combines the role of the eyepiece with the role of the eye.

In the corrected diagram, the large parallel input bundle would become a smaller, but still too big, parallel output bundle that slams into the human iris and only lets the central part through - with a corresponding loss of light.

Does anyone disagree with that?

Frank


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Ravenous
sage


Reged: 11/14/09

Loc: UK
Re: Why a too-small iris results in reduced aperture new [Re: freestar8n]
      #6048728 - 08/27/13 12:24 PM

Quote:


In the corrected diagram, the large parallel input bundle would become a smaller, but still too big, parallel output bundle that slams into the human iris and only lets the central part through - with a corresponding loss of light.

Does anyone disagree with that?

Frank



I suspect you're right - I wasn't able to understand where the extra lines at the eyepiece came from. I didn't want to ask for an explanation, as things were getting too argumentative anyway...

But I think Glenn's original post is long overdue. Newbies reading around here may get misled by some of the other posts.


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Why a too-small iris results in reduced aperture new [Re: Ravenous]
      #6048970 - 08/27/13 02:20 PM

Heh heh. I missed that aspect in Jon's diagram, where the emergent rays out the eyepiece, which formerly passed through the objective edge zone, are not parallel to the central rays. I was concentrating too much on the action ahead of the eyepiece.

This cardinal mistake (to make nothing of the assertion of full aperture use in spite of a restricting iris) by an experienced telescope maker is ample enough reason to cover this whole business.

Frank,
For the flashlight aperture test, from word one I always stressed the importance of placing the flashlight no nearer to the eyepiece than about 10 eyepiece focal lengths, to assure reasonably parallel light going into the exit pupil. This ensures that the tiny image of the light is formed near enough to the focus to make for a reliable result.

And as I mentioned in our previous discourse, if one has any doubt as to parallelism of emergent light, just measure the illuminated circle near the objective and at least a full focal length beyond this. If the two measures are the same, the result is very accurate.

That's the crux of the matter. If the circle is sharp, and is known to be parallel, and emerges sufficiently parallel to the optical axis, its diameter truly is the working aperture. Parallel light straight into the eyepiece, parallel light out the objective; that's all you need. No requirement to beetle one's brow about pupil or stop locations.

I will be cooking up a fuller treatment on this!


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Nils Olof Carlin
Carpal Tunnel
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Reged: 07/26/04

Re: Why a too-small iris results in reduced aperture new [Re: GlennLeDrew]
      #6049033 - 08/27/13 03:03 PM

You can use an oversized exit pupil, if it gives you a wider true field - it may not in practice, you should check.

If too large, the obstructed part (if any) of the exit pupil will be wide enough to be bothersome by blocking much of the eye pupil. Not least when you observe the sun or moon, and your eye pupil is much smaller than it is during deep sky observing. A low-power EP good for deep sky may be useless here.

The eye is a globe more or less, rotating in its "socket" - this means that the iris will move sideways when you shift your gaze (it is very roughly 1/2" in front of the center of rotation). Matching the exit pupil accurately to the iris will mean that even the smallest shift of gaze will give you a (noticeable) loss of light, until you have re-positioned your eye.

Choosing exit pupils that are at least 1 or 2 mm larger or smaller than your dark-adapted pupil will make observing easier.

Nils Olof


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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: Why a too-small iris results in reduced aperture new [Re: freestar8n]
      #6049127 - 08/27/13 03:49 PM

I think the exit/entrance pupil has been hammered into threads concerning singlet aperture telescopes a bit unfairly in the past referred threads, and a bit too often.
There is a balance that has to be realized between aperture, and light gathering that has been glossed over; and is backed up with what is exactly detectable by eye, vs resolution on reduced aperture.
Specifically it's all touted as theoretically possible, but what is actually visible. And by eyeball, each orb has varying degrees of acuity.
******
I repeat the question- How many of the "titans" have actually made a singlet telescope that actually has achro-description performance??????????????????????????????????????????????????????????????????
Or who has equated definitions with achro performance between what is worse/better than described conditions/capabilities in what the human eye perceives???
----
If they haven't, there should be an obvious silence.
The disgusting post of an enlarged image of the -TIP- of the antenna (full of lateral color) doesn't mention the central part of the image where the lateral color is -NILL-.
Such biased posting in the mask of a forum should be discouraged, or banned; and shows a lack of fundamental understanding of human oriented vision, and a total lack of consideration of civilized man to bolster an ego oriented bent on making their fellow man look stupid.
(been said before- if it works for those experimenting, who are -YOU to say what the definition of visual acuity should be?????)
Wake up; moderators.

Mark


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Boot
wildly diverse musical tastes
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Reged: 06/04/07

Loc: The Outernet
Re: Why a too-small iris results in reduced aperture new [Re: Mark Harry]
      #6049149 - 08/27/13 03:57 PM

Well said, Mark.



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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: Why a too-small iris results in reduced aperture new [Re: Boot]
      #6049268 - 08/27/13 04:48 PM

Thanks, Joseph.
M.


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Why a too-small iris results in reduced aperture new [Re: Mark Harry]
      #6050022 - 08/28/13 02:53 AM

Mark,
This thread was motivated by an evinced lack of understanding that a restricting iris reduces the working aperture commensurately. This was confirmed by a post made here, in which a supporting diagram was badly in error.

Furthermore, my issue was with the unknown, or at least uncertain, variables involved with the images used to support claims of performance on the Hypo. A reasonable deduction based on what scant evidence was provided had the actual working aperture reduced to but a fraction, which is known to reduce aberrations. I wanted to see images which assuredly used at least near to the full available aperture.

Someone could easily post images taken with a stopped down achromat which could be taken for apochromatic performance. Would this reflect actual performance? No.

And so the aim to provide at least the basis for understanding the relationship between pupils in a telescope. For all parties.

Finally, this thread has not been concerned with flagellating any particular optical system, being content to confine itself to optical concepts.


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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: Why a too-small iris results in reduced aperture new [Re: GlennLeDrew]
      #6050234 - 08/28/13 08:40 AM

I want it to be understood;
Glen did -NOT- post the pic I mentioned in my previous post!
Mark


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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

Loc: UK
Re: Why a too-small iris results in reduced aperture new [Re: Mark Harry]
      #6050527 - 08/28/13 11:44 AM

Freestart8, the fault is mine, I omitted to draw in the full aperture of the exit pupil dia, the iris dia shewn is for the human iris which is typically around 2mm, whereas the exit pupil is 10mm. The diagram in not in error, but was drawn hastily, I feel that some of the more veherment critics are all to ready to knock people off their pedestals, an occupation all too familiar with those who are intellectually challenged.

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Ravenous
sage


Reged: 11/14/09

Loc: UK
Re: Why a too-small iris results in reduced aperture new [Re: Crayfordjon]
      #6050551 - 08/28/13 11:55 AM

Are you saying those 45 (ish) degree lines coming from out of the eyepiece represent the user's view of the edge of the field?

Not vehemently critical I hope, I am just trying to understand just how useful certain optical configurations might be.


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Why a too-small iris results in reduced aperture new [Re: Ravenous]
      #6050604 - 08/28/13 12:24 PM

Jon,
in your diagram, two groups of rays, one from on-axis and the other from the OG's edge, both pass through *the same point on the optical axis*. This means these two ray bundles are from the same point in object space, namely on the optical axis. And so the ray bundle from the edge zone on the OG must emerge out the eyepiece *parallel to the optical axis*. Not at the angle shown.

In order for your illustration to be correct, the edge zone ray bundle must intercept the focal plane *below* the on-axis focal point. Then it would correspond to an edge-of-field image point, whose rays would emerge out the eyepiece at an angle equaling the apparent semi-field angle.

I belabour this point not because I'm intellectually challenged , but because the diagram can all too easily give the impression that all image points over the full field come to the one focus point on the optical axis. We know this is impossible, and I'm sure you'll agree that we must avoid any pitfalls which militate against a full and correct understanding.

I know, because in spite of great care in my illustrations (and text), I'm still not getting through to all. If there were even little mistakes, confusion would only be all the more profound.


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freestar8n
Post Laureate
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Reged: 10/12/07

Re: Why a too-small iris results in reduced aperture new [Re: Ravenous]
      #6050796 - 08/28/13 02:15 PM

Quote:

But I think Glenn's original post is long overdue. Newbies reading around here may get misled by some of the other posts.




I'm realizing there is a whole context here that I don't know about - and the thread is in fact at a much more basic level than I thought. In that case I'm not even sure the word "pupil" is needed - just show a picture of light hitting the eye - and not going in. I don't have any idea where the misunderstanding lies, or how to address it - assuming there is in fact a disagreement.

This very scenario is described succinctly in the military standardization handbook on optical design, which can be downloaded free. It is summed up in one paragraph in section 4.4.2.4 - and it includes the subtlety described by Nils above, where it is in fact beneficial to have a pupil that is a bit oversized . So there is benefit in having it slightly oversized, to make sure the eye pupil is always fully illuminated - but the throughput is always limited by the eye pupil.

Frank


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Why a too-small iris results in reduced aperture new [Re: freestar8n]
      #6050970 - 08/28/13 04:12 PM

Frank,
The primary thrust here is to provide some understanding of the way in which the eyepiece behaves as an optical coupler, whereby it projects upon the objective an image of the iris, and vice versa. And along the way to touch upon other aspects of performance that result. In such a diverse community of people, one can't overlook the simple, basic fundamentals.

I suspect that if you polled a hundred experienced observers and ATMs who've not looked here (and maybe a few who have!), the number who could describe (or even be peripherally aware of) how the eyepiece can project an image of the iris onto the objective might well be counted on two hands, perhaps even one. Well, that's my inner cynic peeling out.


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p1taylor
professor emeritus


Reged: 02/19/08

Loc: West Midlands UK
Re: Why a too-small iris results in reduced aperture new [Re: GlennLeDrew]
      #6052867 - 08/29/13 04:00 PM

this is a silly carrion to me what deferens dos it make, as long as you see what you are looking at.



peter


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