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Equipment Discussions >> ATM, Optics and DIY Forum

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Wes James
Postmaster
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Reged: 04/12/06

Re: Hypo/Dialyte testing discovery new [Re: kfrederick]
      #5567249 - 12/12/12 09:56 AM

I don't understand what the issue is of using an inexpensive, small sensor'd digital camera... digiscopers do it all the time with extremely effective and sharp results. The sensor in the typical small digital point-n-shoot is about the same size as the human eye- so putting a small digital point-n-shoot behind an eyepiece is as if your eye was placed behind the eyepiece. The main requirement for digiscoping is that high-range zoom camera's don't work... typical of a digiscoping candidate is a camera with only a 4:1 zoom range.

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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Hypo/Dialyte testing discovery new [Re: Wes James]
      #5567896 - 12/12/12 04:19 PM

In the case of a stopped-down system, as occurs when the aperture of the afocally employed system (camera or eye) had a pupil smaller than the telescope's exit pupil...

Assuming the camera's iris is correctly located in the plane of the exit pupil (as it should be for evenness of illumination, and especially when the iris and exit pupil are close to the same size), an image of the iris is formed at the object glass. And so all image forming light contributing to the image passes through this reduced central aperture at the OG.

And even if it were such that off-axis image-forming light passed through the OG offset from the latter's center (as though an off-axis mask were installed), the same reduced fraction of its aperture would be employed.

The point is that no one image point anywhere in the field is being formed by the full aperture of the OG, and so aberrations are reduced. Make up a 'telescope' using a single-element objective, which with a suitable eyepiece produces an exit pupil of about 2mm for daytime testing, or better yet 5mm for nighttime. The chromatic and spherical aberrations will be awful. Now place a mask up front which reduces the exit pupil to 1mm, or a bit less (starting from the larger, low-light exit pupil will make for a more dramatic change). The image, while dimmer, will be of rather better quality.

This is why I'm stressing the importance of knowing whether and by how much the afocal imager is reducing the effective aperture by virtue of its small iris. Any aperture reduction artificially improves performance, especially when the reduction is significant. And for the Hypo (in its simpler configuration), which is limited to larger exit pupils, images are only properly representative when the camera's iris at least comes close to equaling the exit pupil diameter.

One way to obtain an idea of the actual working aperture is to find the diameter of the circular obstruction which just blacks out the image. With the camera's iris located at the exit pupil plane, the eyepiece will form an image of the iris on the OG, the diameter of which is the iris's diameter times the telescope magnification. An obstructor having the same size as this projected iris will, when positioned on top of it, will just fully block all image forming light. Experimentally try successively smaller obstructors until one is found which just blacks out the image when positioned just so; this is, to reasonable accuracy, the effective aperture of the system.


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Hypo/Dialyte testing discovery new [Re: GlennLeDrew]
      #5568064 - 12/12/12 06:35 PM

It occurs to me that using a front end occulting disk to ascertain the working aperture with camera can only work with full-auto cameras only when the camera's iris is already fully open. If the light level is high enough during picture taking to cause the iris to stop down, placing an occulter up front will cause the iris to open up wider than when actually taking the pics.

This leads to one approach to follow when assessing via afocal imaging. Do this at a low enough light level to ensure that the camera's iris is fully open. By zooming in at least somewhat, the increase in focal length should further increase the camera's own linear aperture. And as long as the coupling of exit pupil and iris allows (what with reduced eye relief distance), a shorter focal length eyepiece will aid in getting the exit pupil down to a size more in line with the camera's entrance pupil.

A closer match in the pupil diameters will provide in the pictures a more realistic visual impression of performance.

Does your camera, or the manual, provide specifications on the lens re. the focal length and aperture ratio ranges?


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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
Re: Hypo/Dialyte testing discovery new [Re: GlennLeDrew]
      #5568627 - 12/13/12 03:21 AM

Thanks Kfred, you see my point, My thinking is holistic, not reductionist, reductionists nick pick over small details and see faults in everything, nothing ever positive comes of it, the old saw that a camel is a horse designed by a committee is germain here. I am now pulling out and handed it back to Mark. Thanks Mark.

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wiseone
Vendor (Zerochromat)


Reged: 01/07/08

Loc: North Wales, UK
Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5568668 - 12/13/12 04:35 AM

I sent some design results to Mark a week or so back for a 6 inch retro. The front lens and field lens are both BK7, and are easy to make. The corrective lens set is just a couple of small identical doublets. The tolerance to decentration is very loose, so it is not a problem to actually make. It is diffraction-limited over most of a 1 degree field of view, and would make a great all-round long focus refractor (f12), and under 4 feet long when folded. If anyone is interested, I can post the spot diagrams and Ray Intercept Curves.

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kfrederick
Carpal Tunnel


Reged: 02/01/08

Re: Hypo/Dialyte testing discovery new [Re: wiseone]
      #5568760 - 12/13/12 07:27 AM

Yes post them sounds great . Thanks

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wiseone
Vendor (Zerochromat)


Reged: 01/07/08

Loc: North Wales, UK
Re: Hypo/Dialyte testing discovery new [Re: kfrederick]
      #5568854 - 12/13/12 09:12 AM Attachment (26 downloads)

Here are the spot diagrams. First on-axis

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wiseone
Vendor (Zerochromat)


Reged: 01/07/08

Loc: North Wales, UK
Re: Hypo/Dialyte testing discovery new [Re: kfrederick]
      #5568855 - 12/13/12 09:14 AM Attachment (22 downloads)

Off-axis

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wiseone
Vendor (Zerochromat)


Reged: 01/07/08

Loc: North Wales, UK
Re: Hypo/Dialyte testing discovery new [Re: wiseone]
      #5568859 - 12/13/12 09:16 AM Attachment (26 downloads)

And Ray Intercept Curves. By the way, the whole refractor can be made from BK7 and F2 glass types, with only a small loss in performance. Easy for experienced ATMs to make!

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ed_turco
Pooh-Bah
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Reged: 08/29/09

Loc: Lincoln, RI
Re: Hypo/Dialyte testing discovery new [Re: wiseone]
      #5568970 - 12/13/12 10:37 AM

I hate to jump in here and be a jerk. This is not a disparagement of anyone or the designs mentioned.

Why not use a corrected singlet in a refractor, say f/12 or so and use a filter? Any thoughts?


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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

Loc: UK
Re: Hypo/Dialyte testing discovery new [Re: ed_turco]
      #5568995 - 12/13/12 10:48 AM

Ed I did this using R G and B interference filters, refer back to page 6. Yes, monochromatic scopes work fine, even if images are red green or blue, great on the moon but a bit dim for fuzzies to be seen. The spherochromatic ab disappears using discreet filters and the images using a singlet OG are extremely good.

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Scout92
member


Reged: 12/20/11

Loc: Hoquiam, WA.
Re: Hypo/Dialyte testing discovery new [Re: Mark Harry]
      #5570710 - 12/14/12 11:40 AM

Would a six inch +meniscus from suplusshed work?

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Scout92
member


Reged: 12/20/11

Loc: Hoquiam, WA.
Re: Hypo/Dialyte testing discovery new [Re: Mark Harry]
      #5570796 - 12/14/12 12:40 PM

Mark I am fascinated by your experiment and am tring to understand John's hypo as well, however I have very little understanding of optics so it is coming slowly. Keep up both the good work and the posting I would love to see what you come up with and maybe try to create one of my own. Take your time and explain at your pleasure...I am all ears!

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ed_turco
Pooh-Bah
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Reged: 08/29/09

Loc: Lincoln, RI
Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5570906 - 12/14/12 02:06 PM

Who needs fuzzies? I'm a Lunatic!

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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
Re: Hypo/Dialyte testing discovery new [Re: ed_turco]
      #5571967 - 12/15/12 03:22 AM

Scout 92, you should try to build a Hypo keeping STRICTLY to my specifications, and use a low power eyepiece to get anything from X16 to X25. You will then see what a Hypo can do. If you dont like then OK, no sweat, but at least you tried to see one for yourself, instead of just shooting it down in flames because it does not conform.

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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]
      #5571971 - 12/15/12 03:26 AM

Scout 92 again, I know the meniscus you are talking about, it comes from surplus shed, be careful, they are suspect, and they are a meniscus lens, which aint a very good lens for inaging.

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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: Hypo/Dialyte testing discovery new [Re: Crayfordjon]
      #5572058 - 12/15/12 06:29 AM

The Hypo can be built, as John says- and stick to a low power setting.
My dialyte is different. It requires tighter tolerances, but not nearly as much as an achro, or apo. Also, I don't know of any place that sells what I am discussing here- I have to fabricate most, if not all the parts (though a low-medium power eyepiece I believe, can be made expeditiously from S/S parts)
I think the way it's going to work out (dialyte) is that by correcting lateral and longitudinal color in 2 different places, good imaging and correction can be had without the tight tolerances. So far, longitudinal color can be made by changing spacings by 1/16-1/8" when tinkering with OSLO. If all goes as planned, I will wind up with something that performs about as well as an achromat.
****
At present, I'm roughing in the curves on the "zero" power achromat. Think of finish- generating a curve of ~17" with 220 grit on the flint! (SF1, really soft stuff!)
****
I would be a bit cautious about the 6" PCX lenses from the Shed- heard (though haven't confirmed personally) that they could be a bit sloppy. But they could be cleaned up with a pitch polisher???
M.


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Hypo/Dialyte testing discovery new [Re: Mark Harry]
      #5572062 - 12/15/12 06:36 AM

And how does a meniscus objective singlet compare to a PCX? Or DCX? The altered shape is expected to have some impact, methinks, at least for the 'faster' lenses like this f/17 from the 'Shed.

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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: Hypo/Dialyte testing discovery new [Re: GlennLeDrew]
      #5572068 - 12/15/12 06:55 AM

I made this meniscus (concave back) so that I can use typical aspherizing technique to make this concave surface a hyperboloid. It affords tilting the RGB color correction on the longitudinal graph so that they're straight up and down; shrinks the spotplots quite a bit. Left spherical, it tips these lines off vertical, and blows the spotplots.
****
The shallower the concave back, the more correction it needs. There is a point where you can deepen the curve by bending the meniscus, and just have paraboloidal correction; and even spherical. But that makes for pretty steep curves which necessitate a thick blank to start off with. (a lot of grinding there)
So I picked curves that would fit a typical BK7 moulded blank that used tooling I already have on hand. (call me lazy)
M.


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Aljr
member


Reged: 12/08/10

Loc: BRAZIL
Re: Hypo/Dialyte testing discovery new [Re: Mark Harry]
      #5572171 - 12/15/12 08:45 AM Attachment (16 downloads)

Quote:

Words of Mark Harry: I would be a bit cautious about the 6" PCX lenses from the Shed- heard (though haven't confirmed personally) that they could be a bit sloppy. But they could be cleaned up with a pitch polisher???
M.



Mark, you're right. This lens PCX is hardly polished, but it is made of white glass. It is necessary to make a new polishing. I can say that the lens "PMN(positive meniscus)" is/has superior quality.
I used the lenses PCX "Surplus Shed" in Hypo as if these were "good quality lenses". That's how I took a "giant / big man" with feet of clay.
I would like to warn that "Stipe Vladova" of Croatia, was the person who first tested the PCX lens.


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