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Equipment Discussions >> ATM, Optics and DIY Forum

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GlennLeDrew
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Re: Hypo/Dialyte testing discovery new [Re: MKV]
      #5577471 - 12/18/12 03:15 PM

My main concern with Jon's 76mm Hypo prescription above is the very nearly 10mm exit pupil. A point and shoot camera's effective aperture in daylight conditions might be near the 1mm range, meaning the scope's effective aperture is probably less than 10mm (!).

I assume all ATMs understand the effect of stopping down an aperture. And that this stopping down occurs at the exit pupil offers no escape clause in the laws of optics. What accurs at the exit pupil is projected onto the entrance pupil. Stop the exit pupil down to 1/10, and the entrance pupil is correspondingly stopped down to 1/10.

Ray trace the system with a 7.6mm stop placed at the OG (now operating by itself at f/310.) The now *very* slimmer light cone passing through both the singlet and doublet will greatly supress at least the longitudinal aberrations.


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Crayfordjon
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Re: Hypo/Dialyte testing discovery new [Re: MKV]
      #5578342 - 12/19/12 03:34 AM Attachment (23 downloads)

Thanks guys, nice of you to say that. Now here are the two images before massaging them in photoshop. All I did was size them to fit this forum requirements. Oh a point I think critics should take on board, what do imagers do but alter their images and process them to get the wonderful images that they can achieve electronically, when the actual image as seen visually through the telescope appears as a faint greenish smudge.

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Crayfordjon
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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5578346 - 12/19/12 03:36 AM Attachment (24 downloads)

And

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Crayfordjon
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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5578379 - 12/19/12 04:15 AM

Glenn I think I explained how the system works as a camera lens works in a previous post. Now, I say, does it matter! as long as you get nice images. look at the cleaned up images of the antenna and house frontage, the fine detail is very good. Do you have to use a telescope at absolutely the full aperture all the time?, usually telescopes are not, and visually the diameter of the pupil is never fully wide open to suit the design parameters of the scope in question, At best most observers never do. Now a camera lens is not used at the full aperture of the front element, it is not designed to do so, and here the Hypo is acting as a camera lens.

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GlennLeDrew
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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5578485 - 12/19/12 07:17 AM

If a 76mm aperture scope is working at a 10mm exit pupil, and the observer's pupil is 5mm, the the scope is really working at 1/2 its nominal aperture, or 38mm. Therefore the outer portion of the objective is not contributing to image formation, and so the performance is improved.

Visually, about the largest portion of the 76mm aperture one can utilize is about 7/10 of 76mm, or 53mm. How is this an improvement on the 50mm doublet if used by itself? The big singlet up front is nothing more than an aberration-inducing hunk of glass which essentially adds nothing to light grasp.

In order for the added singlet to even begin to pay its way, it must materially increase light grasp. And so the exit pupil must be not so much larger than the eye's pupil, or the camera's entrance pupil, that the effective aperture of the system be hardly larger than the doublet alone.

This is like a 7X50 bino used in bright daylight. The observer's pupil is about 2.5mm, and so the real aperture is 50 * 2.5 / 7 = 17.9mm. That is, the 7X50 bino is working instead just like a 7X18. If the bino suffers noticeable aberrations when the observer's pupils are 7mm (or nearly so), those aberrations will certainly be much diminished or eliminated in the daytime.

And so it likely is the case, too, if your camera's entrance pupil is much less than the scope's exit pupil.

How about affixing an eyepiece which delivers an exit pupil closer to a couple of millimeters (as opposed to 10mm) and take some pics for us?

Or at least supply the camera lens's focal length, from which can be derived a likely entrance pupil diameter for it. This will suggest a close-to-matching exit pupil.

I belabor these issues for three reasons:

1) In order to assess the relevance of the supporting images, it's mandatory that it be known whether the system is working at least near to full aperture.

2) To be satisfied with muddling along without understanding the test apparatus and the requirements for its effective use cannot be expected to inspire confidence in the reader.

3) If it's known or even suspected that the images are taken at less than full aperture, to not disclose this smacks of misrepresentation.

While I certainly don't demand the strictest adherence to the precepts of the peer review process, at least paying some semblance of lip service to the scientific method will go a long way. Surely this is not too much to ask among a community of instrument designers and makers.


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Mark Harry
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Re: Hypo/Dialyte testing discovery new [Re: GlennLeDrew]
      #5578518 - 12/19/12 08:04 AM

I have a question:
***
What I'm looking at with just a meniscus lens sitting on the windowsill, and using a piece of sheetmetal with various holes in it...
If I use 1.25mm hole (~.050") and looking 50 yards away roughly, I can see a Frasier fir, and I can see the individual twigs as a round green stalk, only seldom seeing individual needles. If i switch to a 1mm hole (~.040") I can see practically all the individual needles on each twig, at a decidedly reduced light level.
Both cases, I am somewhat inside focus- and I can get closer to the lens, and get less magnification, somewhat sharper detail.
But to mention what's puzzling, I see that Glen is right- I am likely not using the whole aperture. But I can see the whole lens, and additional background besides- So exactly what is happening? I can use this to also check the edges of the lens- just look at any vertical line as it approaches the periphery, and view whether it has any hook or not.
M.


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Crayfordjon
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Re: Hypo/Dialyte testing discovery new [Re: Mark Harry]
      #5578714 - 12/19/12 10:44 AM

Actually the outer portion of the OG is used, field angle dictates that. What happens is that only a sub aperture zone is used by the eye for all parts of the OG diameter, this happens with a stopped down camera lens.

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Crayfordjon
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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5578758 - 12/19/12 11:23 AM Attachment (32 downloads)

This highly simplified diagram illustrates the point I just made.

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GlennLeDrew
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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5579396 - 12/19/12 06:28 PM

Mark,
In the test you describe, you're using the lens like a simple magnifier. Indeed, because you're so far back from the lens, you must employ an aperture stop (the pinhole) in order to obtain good focus via an increased depth of focus. When you examine any particular, small detail, note its apparent size with respect to the lens; this tells you the maximal fraction of the aperture contributing to the formation of any one image point.

Now this is not *quite* the same as when testing a complete system in the afocal configuration.

Jon,
Your diagram does indeed get to the issue of stopping down of the system. However, it does require a further refinement, because the OG defines the entrance aperture.

The image-forming light for an off-axis image point still passes through the central zone of the OG, along with the light for an on-axis point.
In your sketch, you have the light cone for the off-axis ray bundle passing through the same center-of-field point at the focal surface as for the on-axis bundle. An off-axis image point is displaced from the center of the field, which here would be below the optical axis.

With this in mind, a ray trace will show that all image-forming bundles from all parts of the field pass through the same reduced central zone on the OG. Again, this is due to the OG defining the entrance pupil.

Here's another way to understand this. The exit pupil is a reduced image of the OG (technically, the entrance pupil) formed by the eyepiece. And the same works in reverse; anything at the the exit pupil is projected upon the OG by the eyepiece. For example, an iris located centrally at the exit pupil, and having half the diameter of the exit pupil, will be projected onto the OG, filling the central one-half of the OG. All image-forming light, no matter where from across the field of view, which passes through the reduced iris (at the exit pupil) must necessarily pass through this central half-diameter zone on the OG; the outer zone of the OG does not contribute to image formation anywhere across the field. And again, this is because the OG is the entrance pupil.


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Crayfordjon
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Re: Hypo/Dialyte testing discovery new [Re: GlennLeDrew]
      #5580006 - 12/20/12 03:09 AM

No the stopping down of the pupil does that, all over the real aperture. The larger the pupil the larger the stopped down aperture all over the real aperture, until the limit is reached when pupil aperture equals real aperture.

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GlennLeDrew
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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5580042 - 12/20/12 04:03 AM

Jon,
Revisit your previous illustration to identify the flaw whereby you show a light bundle coming from a point off axis (at the field edge) and coming to focus at the same point as does an on-axis image point. The off-axis bundle arriving from the upper left will come to focus below the optical axis, and not at the same place as for an on-axis image point.

Once this is rectified, a correct ray trace will show that the off-axis bundle will pass through the same central zone on the OG as for an on-axis bundle.

I should post an image showing this...


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Crayfordjon
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Re: Hypo/Dialyte testing discovery new [Re: GlennLeDrew]
      #5580389 - 12/20/12 10:36 AM

You are absolutely correct, a drawing error, done in haste, but the argument still holds.

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GlennLeDrew
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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5580742 - 12/20/12 01:49 PM Attachment (22 downloads)

Jon,
In this illustration, the relevant system is at the bottom, which is the same as the one at the top except for the system being stopped down by a smaller iris at the exit pupil.

The only rays which make it through the reduced iris are those which pass through the reduced entrance pupil; the outer zone of the OG does not contribute because any image-forming light passing through here is intercepted by the reduced iris.


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GlennLeDrew
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Re: Hypo/Dialyte testing discovery new [Re: GlennLeDrew]
      #5580769 - 12/20/12 02:06 PM

An additional thing to study. In the lower illustration, follow the three rays passing through the upper edge of the reduced entrance aperture on the OG. The red, white and blue rays diverge from a point, pass through the eyepiece, and are then brought to a focus at the lower edge of the reduced iris. This shows how the eyepiece is a kind of two-way projector; it focuses the image of the entrance pupil at the exit pupil, and focuses the iris at the entrance pupil.

The eyepiece is the optical coupler between the objective and the eye (or camera) in the afocal configuration. If the iris is smaller then the exit pupil, the eyepiece projects an image of the iris onto the objective, the latter defining the new working aperture of the system.

I had assumed all ATMs were well aware of this optical principle.


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Crayfordjon
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Re: Hypo/Dialyte testing discovery new [Re: GlennLeDrew]
      #5581867 - 12/21/12 04:38 AM Attachment (17 downloads)

Please do not patronise me Glenn, If I did not know the basics of geometrical optics I would never have been able to invent the retro focally corrected dialyte. during the period I was developing the idea I performed a vaste amount of calculation and analysis involving complex application of geometric optics; as I have mentioned previously, one of the top optical designers in the UK took my basic invention and found that it could be made apochromatic, and he built one and it was!. When I invent a system I resort to experimental physics techniques and have a home optical lab to do verification experiments of any hypothesis I have come up with. Many times I have been wrong , but also, many times I have been right. I might mention that I do have a science degree, as now doubt you have. So about our argument, I quickly set up an experiment to see if I was actually wrong!, not to shoot you down. It seemsthat my argument was right, as the images I send you verify, yes made a blunder with my previous optical diagram, a greenhorn mistake, but we all make em dont we. The first image shews a mask that will be placed over the five inch OG of a non achro Hypo I currently have on test on the bench. In it is cut a circukar hole one inch diameter placed close to the edge. The second image was taken with the full; aperture of the system IE five inches. The second image is an image formed by the one inch aperture at the edge of the OG, a lousy image yes, but adequate. The final image is the mask in place over the OG of the array on the bench. Note then that the configuration I have been arguing over does work as I suggested. Now I am pulling out of this thread as I have wasted too much time as it is arguing.

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Crayfordjon
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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5581872 - 12/21/12 04:45 AM

I seem to be held up by the system not sending the rest of my images, I will try later.

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Crayfordjon
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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5581877 - 12/21/12 04:49 AM Attachment (15 downloads)

The one inch aperture at the edge image

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Crayfordjon
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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5581879 - 12/21/12 04:50 AM Attachment (15 downloads)

The central five inch image

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Crayfordjon
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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5581880 - 12/21/12 04:51 AM Attachment (15 downloads)

The aperture plate on the OG

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Crayfordjon
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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5581884 - 12/21/12 04:56 AM

Correction, image No 2 in the sequence is taken with the one inch edge stop placed over the OG.

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