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Equipment Discussions >> ATM, Optics and DIY Forum

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Mark Harry
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Reged: 09/05/05

Loc: Northeast USA
Re: Hypo/Dialyte testing discovery new [Re: GlennLeDrew]
      #5581924 - 12/21/12 06:11 AM

Glen, I understand now. I estimate the resolution is approximately (but not quite) similar to the singlet lensed Galilean telescope I made, which is around 38-40mm... roughly a 5th of the total meniscus diameter. (wider field though, where most of the color error resides.) Pretty neat fiddling with this; am learning useful tidbits along the way.
M.


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GlennLeDrew
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Re: Hypo/Dialyte testing discovery new [Re: Mark Harry]
      #5581958 - 12/21/12 07:21 AM

Jon,
I'm not trying to patronize you, or anyone. As to education, I'm just a high school grad (grade 11, age 16, 1979, in the province of Newfoundland.)

If the latest images you took through the masks were afocal also, then of course you can obtain an image simply by moving the camera laterally to pick up light from that portion of the exit pupil which is illuminated.

That is, an off axis mask offsets the reduced exit pupil from the optical axis. Try this experiment:

With the OG *not* masked, mount a piece of tracing paper in the plane of the exit pupil so that it cannot move, then trace the pupil. Install the mask centrally, and note the reduced exit pupil centered on the optical axis; trace the circle on the paper. Now locate the mask halfway from center to edge on the OG, and trace the new exit pupil. Place the mask at the edge of the OG, and trace the exit pupil again.

You should see that the exit pupil has shrunk and moved just like a miniature version, to scale, of what's going on up front.

If an iris equal in size to the reduced pupil were placed in the plane of the exit pupil, it should now be seen that light passing through portions of the OG not contributing to that smaller pupil does not contribute at all.

Reexamining the case where the full aperture contributes to image formation (where the iris is at least as large as the exit pupil), note how both the on-axis and off-axis bundles entering the OG share the identical extent of the entrance pupil. Why, in this case, does not the off-axis bundle tend to be offset from the optical axis, at least to some small extent? If not in this case, then why should it be so when the system aperture is stopped down?

Another way to look at it: Say we have a system wherein the exit pupil and iris just match in size (like the upper one in the diagram I supplied above.) The existing objective is removed and is replaced by one of identical focal length, but larger aperture. Does that new, extra annulus on the OG contribute to image formation? No, because the entrance pupil is already as large as needs be for the iris. Now, incrementally enlarge the iris, and a corresponding incremental increase in the entrance pupil will occur, until the iris diameter equals the new OG diameter divided by the magnification. The entrance and exit pupils are the apertures through which all image-forming light must pass, irrespective of field angle. And each is a 'mirror' of the other, as defined by that optical coupler, the eyepiece.

[IN EDIT: Clarified the first line of paragraph #4. Added the final paragraph.]

Edited by GlennLeDrew (12/21/12 03:41 PM)


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Mark Harry
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Reged: 09/05/05

Loc: Northeast USA
Re: Hypo/Dialyte testing discovery new [Re: GlennLeDrew]
      #5582637 - 12/21/12 02:58 PM

Yes, Glenn!
This is my whole point about this thread. Why can't the longitudinal color be solved with an achromat; and the lateral color be solved at the eyepiece???
Sure, the eyepiece to a purist will not be up to standards, but it should work well with this scope.
It works against the supposed idea to get any arbitrary eyepiece to work fairly well with any scope of various types-.
---but to get a dedicated eyepiece to work with a scope that would be essentially perfectly complimentary with each other... and to never fool around with a large piece of flint glass for matching the objective...
And the eyepiece would be cheaper, and easier to make by the ATM...
I'm persueing this on the premise that performance with short focal length eyepieces will differ very little. Just to have a low power eyepiece or two to work well at low power would be sufficient.
M.


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GlennLeDrew
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Re: Hypo/Dialyte testing discovery new [Re: Mark Harry]
      #5582740 - 12/21/12 04:08 PM

Mark,
Indeed, the color errors must be addressed, as evidenced by the smorgasbord of rainbows in evidence even with an aggressively stopped-down Hypo. Sub-aperture correction is a most attractive approach. As to lateral color, have you developed a 'feel' for the best place for the singlet with respect to the eyepiece? It would be a shame if it had to be placed behind, thus robbing some eye relief.

By the way, I'm developing the feeling that in the main, knowledgeable amateurs can discuss such things as aberrations, spot plots, interferograms and what not, with more authority than they can the relationship between entrance and exit pupil as I've been hammering away at here. Can this be so? Such fundamental concepts are important, dealing as they do with that all-important interface between objective and eye, the exit pupil.


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Mark Harry
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Reged: 09/05/05

Loc: Northeast USA
Re: Hypo/Dialyte testing discovery new [Re: GlennLeDrew]
      #5583069 - 12/21/12 07:18 PM

Well, the focus of the objective in relation to a Ramsden allows the objective to come to focus first; lateral color and all. (that's what I'm counting on) The Ramsden should compensate or remove at least a significant portion of that problem since its layout relies on being after the focus of the primary...
Your mention of "pupils" is exactly why I'm piddling around with the meniscus of my scope. Plenty of aspects to be aware of in understanding the fundamentals, etc.
May this find you well,
M.


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GlennLeDrew
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Re: Hypo/Dialyte testing discovery new [Re: Mark Harry]
      #5583098 - 12/21/12 07:35 PM

Jon,
It suddenly occurred to me that the behavior you ascribe to off-axis imaging-forming light passing through the OG is exactly as occurs in Galilean telescopes. In such, the negative ocular locates the exit pupil lies inside the instrument, which of course is inaccessible (as if that would matter in any event ). In the Galilean, the field of view depends directly on the OG aperture; reduce the aperture and the FOV is reduced in step.

But in positive systems having the exit pupil behind the ocular, reducing the aperture can only reduce illumination uniformly across the FOV, provided the exit pupil is not larger than the iris. In the latter case, illumination does not change, doing so only when the exit pupil becomes the smaller.


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Mark Harry
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Reged: 09/05/05

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Re: Hypo/Dialyte testing discovery new [Re: GlennLeDrew]
      #5583683 - 12/22/12 08:17 AM

Glen,
The correction at the eyepiece has to be made past the objectives primary focal point 'somewhere'. I could either make an actual RAMSDEN (think I have some spare lenses that I could do so) or use an achro, with a singlet for a 3-element; 2-group eyepiece of sorts. Haven't really done much yet, for I still have to make the 4.5" achro.
I did use spotplots in OSLO of the dialyte, and a Ramsden eyepiece. Sized them to be identical scale, and printed both out on the same sheet, and it APPEARS that it might work pretty well.
M.


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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

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Re: Hypo/Dialyte testing discovery new [Re: Mark Harry]
      #5645480 - 01/27/13 04:56 AM Attachment (18 downloads)

I built a Hypo to my own specs using an F:40 127mm OG and a75mm Bino OG reduction lens. I took some images afocally. Visually at X30 where the exit pupil is 2mm, the same as my eyes, I was looking through the full aperture of the scope. I could only see a very faint colour splitting at the edge of the field, the images are very sharp and bright. The scope is as good as an achromatic.

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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5645481 - 01/27/13 04:57 AM Attachment (18 downloads)

Terrestrial snow scene.

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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

Loc: UK
Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5645487 - 01/27/13 05:04 AM Attachment (17 downloads)

This is an image taken in th optical bench array, the star cluster is produced by pinholes in a foil overlaying an LED source.

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wiseone
Vendor (Zerochromat)


Reged: 01/07/08

Loc: North Wales, UK
Re: Hypo/Dialyte testing discovery new [Re: Mark Harry]
      #5645491 - 01/27/13 05:12 AM

Hi Mark, I am enjoying this thread, and to add my two penn'orth, you can see what a simple achromat plus dialyte corrector elements does. Have a look on my web site - www.firefly85.com. I haven't pushed this yet, as there were some mechanical issues to sort out. This has now been done, and I plan to get this going this year.
Best wishes,
Peter


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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

Loc: UK
Re: Hypo/Dialyte testing discovery new [Re: wiseone]
      #5645508 - 01/27/13 05:32 AM Attachment (22 downloads)

Faux Jupiter.

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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

Loc: UK
Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5645510 - 01/27/13 05:34 AM Attachment (17 downloads)

Another snow scene, note lack of colour in these images.

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Crayfordjon
Vendor - Zerochromat
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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5645514 - 01/27/13 05:39 AM Attachment (20 downloads)

Finally for now, antennae. In this you can see the small amount of colour that is left in the images.

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Ed Jones
Carpal Tunnel
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Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5645786 - 01/27/13 10:45 AM Attachment (18 downloads)

The closest doublet I could find to your design is a Ross L-AOC306 which gives the spot diagram below for C,D and F. The little black dot in the center is the airy disk with a 72 micron blob of CA around it. Why don't I see all this color in your photos?

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GlennLeDrew
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Re: Hypo/Dialyte testing discovery new [Re: Ed Jones]
      #5645985 - 01/27/13 12:36 PM

Ed,
Imagery at sufficiently low magnification/image scale will not reveal all aberrations evident visually at small exit pupil diameters. And as I've harped upon here previously, if an afocal camera has an entrance pupil smaller than the exit pupil, the resulting image will evince an artificial improvement due to the smaller-than-full-aperture system pupil.


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ed_turco
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Reged: 08/29/09

Loc: Lincoln, RI
Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5646742 - 01/27/13 07:19 PM

Quote:

I built a Hypo to my own specs using an F:40 127mm OG and a75mm Bino OG reduction lens. I took some images afocally. Visually at X30 where the exit pupil is 2mm, the same as my eyes, I was looking through the full aperture of the scope. I could only see a very faint colour splitting at the edge of the field, the images are very sharp and bright. The scope is as good as an achromatic.




If the exit pupil is 2mm and the magnification is 30x, you are looking thu a 60mm telescope, not a 127mm one.


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GlennLeDrew
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Re: Hypo/Dialyte testing discovery new [Re: ed_turco]
      #5647193 - 01/27/13 11:44 PM

Indeed, a 127mm objective and a 2mm exit pupil must provide 63.5X.

If both the magnification of 30X and the 2mm exit pupil are correct, the system is being stopped down to about half aperture. If this is the case, the culprit could be a too-long focuser drawtube for the faster-than-f/4 light cone behind the 70mm doublet.


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Crayfordjon
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Reged: 06/17/09

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Re: Hypo/Dialyte testing discovery new [Re: GlennLeDrew]
      #5648295 - 01/28/13 02:10 PM Attachment (12 downloads)

Star field test for colour with the F:40 Hypo. the field is 3 degrees wide.

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Crayfordjon
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Re: Hypo/Dialyte testing discovery [Re: Crayfordjon]
      #5648299 - 01/28/13 02:11 PM Attachment (13 downloads)

The last image is the raw image, this image has been sharpened

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