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Equipment Discussions >> ATM, Optics and DIY Forum

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Brian Albin
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Reged: 08/22/06

Loc: Western Oregon
An Alternative Clear Aperture Mirror Telescope
      #5602366 - 01/03/13 04:58 AM

I have been reading kfrederick’s thread about the Chiefspiegler telescope. I wonder if a different unobstructed mirror scope might draw more customer interest.

In the book “Advanced Telescope Making Techniques” by Allan Mackintosh, Volume 1; is a two tilted mirrors telescope by designer Tore Sjogren. When finished to f13 it uses spheric grind on both mirrors. The second mirror is a concave inside prime focus, so an antimagnifier. The focal ratio therefore of each mirror is greater than the final overall ratio. These slow curves should I think, minimise aberrations. The only residual aberration not corrected by the mirrors’ tilt angle, grind and spacing is astigmatism. This is brought to a value of either 1.5 diopters or 2 diopters in the two formulations provided in the book. Astigmatism correction lenses of these strengths are said to be kept in stock at spectacle stores for a small price. Such a lens is then placed under the eyepiece in the focus draw tube; making it a fully corrected system with clear aperture and few optical elements.

I think the biggest drawback (or the only one) is the second mirror is 5/8 the diameter of the primary. Once the aperture exceeds 16” the secondary would add significant cost. On big scopes the weight of the secondary would require a huskier truss set than would be the case for a Cassegrain. All in all however, the scope seems like a winner. Perhaps especially down around 10 to 12 inch aperture.
Tube length (separation between mirrors) is approximately 6/10 of final focal length.


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Mirzam
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Reged: 04/01/08

Loc: Lovettsville, VA
Re: An Alternative Clear Aperture Mirror Telescope new [Re: Brian Albin]
      #5602414 - 01/03/13 06:25 AM

It does sound like an interesting design. I'm wondering how long the f-ratios of the primary and secondary would have to be if the finished ratio is f/13? I happen to have a 12.5" f/9 sphere laying around the basement.

JimC


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Brian Albin
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Reged: 08/22/06

Loc: Western Oregon
Re: An Alternative Clear Aperture Mirror Telescope new [Re: Mirzam]
      #5602506 - 01/03/13 08:31 AM

The example he gives is: Primary f 19.3, Secondary f 41.5.

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philipdo
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Reged: 12/06/06

Loc: Flanders, Belgium, Europe
Re: An Alternative Clear Aperture Mirror Telescope new [Re: Brian Albin]
      #5602523 - 01/03/13 08:39 AM

Brian,

I experimented with this optical design a few years back. It did not work very well, mostly due to the insufficient quality and accuracy of these astigmatism correction spectacle lenses. Image quality at higher magnifications was rather poor compared to my classical catadioptric Kutter Schiefspiegler with plano-convex correction lens.


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Mirzam
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Loc: Lovettsville, VA
Re: An Alternative Clear Aperture Mirror Telescope new [Re: philipdo]
      #5602550 - 01/03/13 08:54 AM

Any reason why you could not use a Televue dioptrix?

JimC


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Dave O
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Reged: 12/21/11

Loc: Sri Lanka
Re: An Alternative Clear Aperture Mirror Telescope new [Re: Mirzam]
      #5602820 - 01/03/13 11:48 AM

From my understanding, this design is basically a Yolo with a correcting lens for astigmatism (vice the torroidal secondary). Ed Jones has applied his two lens corrector to a Yolo (creating the "Jolo") which has very good correction. As with his CHief, it can be designed to use commercially available lenses.

It does use two very long radii mirrors and for best correction, the primary is a hyperboloid (as is the Yolo).


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Brian Albin
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Reged: 08/22/06

Loc: Western Oregon
Re: An Alternative Clear Aperture Mirror Telescope new [Re: philipdo]
      #5608015 - 01/06/13 10:21 AM

Quote:

I experimented with this optical design a few years back. It did not work very well, mostly due to the insufficient quality and accuracy of these astigmatism correction spectacle lenses. Image quality at higher magnifications was rather poor compared to my classical catadioptric Kutter Schiefspiegler with plano-convex correction lens.




Hello Philip,
May I ask what focal ratio you used?


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philipdo
sage


Reged: 12/06/06

Loc: Flanders, Belgium, Europe
Re: An Alternative Clear Aperture Mirror Telescope new [Re: Brian Albin]
      #5608161 - 01/06/13 11:44 AM

Brian,
after rereading my previous post I must admit my explanation perhaps wasn't very clear. I experimented with spectacle lenses as astigmatism correctors, however I did not do this in a Yolo but in my 8" F/D 20 schiefspiegler. In this particular scope residual coma and astigmatism are corrected by a plan-convex lens. After reading the article about the Tore Sjogren scope and knowing that Anton Kutter also tried out spectacle lenses at one time, I did a comparison between OTS toroidal spectacle lenses and the usual plan-convex correction lens. Image quality at high magnification with the spectacle lens was disappointing. I tried 2 meniscus shaped toroidal spectacle lenses with different diopters but both without success. This was in a F/D 20 system. I blame the insufficient surface accuracy of the spectacle lenses and was happy to reinstall the original plan-convex corrector. I suppose that their use in a Yolo telescope will also be hampered by their surface quality. Sorry for the confusion in my previous post.

Edited by philipdo (01/06/13 11:47 AM)


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Ed Jones
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Reged: 04/06/04

Loc: Sin-sin-atti
Re: An Alternative Clear Aperture Mirror Telescope new [Re: philipdo]
      #5608249 - 01/06/13 12:23 PM Attachment (46 downloads)

Spectical lenses are pretty much garbage if you try do do this. Off-the-shelf lenses are a much better option. Here is the Jolo.

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philipdo
sage


Reged: 12/06/06

Loc: Flanders, Belgium, Europe
Re: An Alternative Clear Aperture Mirror Telescope new [Re: Ed Jones]
      #5608283 - 01/06/13 12:46 PM

Ed,

I knew that the quality of spectacle lensens left much to be desired. I went ahead anyway because the lens had to be placed rather close to the focal plane, thus using only the central part of the lens. But even then, they didn't work. I have to agree with you, by our ATM standards these lenses are garbage, no question about it.


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ed_turco
Pooh-Bah
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Reged: 08/29/09

Loc: Lincoln, RI
Re: An Alternative Clear Aperture Mirror Telescope new [Re: philipdo]
      #5608929 - 01/06/13 05:42 PM

This is what happened when the eyeglass industry went to acrylic lenses to decrease wait times for the lenses and to radically increase profits. Glasses in an hour! -- What do you expect? Has anyone seen how these unholy pieces of plastic are turned into lenses? I have.

Acrylic lenses are *BLEEP*. Glass lenses were a whole lot better by two orders of magnitude.


Ed

Edited by ed_turco (01/06/13 05:46 PM)


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gatorengineer
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Reged: 02/28/05

Loc: Hellertown, PA
Re: An Alternative Clear Aperture Mirror Telescope new [Re: ed_turco]
      #5609154 - 01/06/13 08:05 PM

Not being a glass pusher, I understand that these designs are academically interesting. Once you get above 6" much above F15 becomes uninteresting from a real world perspective, and above 8-10inches much above F10 becomes uninteresting.

Thats why I find the CHIEF so intriguing, it can be done under F10....



As an aside, I Hope to get a permission slip from the wife and mother nature to visit Kevins 20" soon.... I have two 8 F8's in a box, and with Ed' commercially available lens F8 formula well, running outta excuses....


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Pinbout
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Reged: 02/22/10

Loc: nj
Re: An Alternative Clear Aperture Mirror Telescope new [Re: Ed Jones]
      #5609240 - 01/06/13 09:12 PM

Quote:

Spectical lenses are pretty much garbage if you try do do this. Off-the-shelf lenses are a much better option. Here is the Jolo.




just got a 10"f11.5 so before I consider it a blank, how would I go about finding lenses and all that fun math?


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Ed Jones
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Reged: 04/06/04

Loc: Sin-sin-atti
Re: An Alternative Clear Aperture Mirror Telescope new [Re: Pinbout]
      #5609480 - 01/06/13 11:44 PM

I don't think it will make a Jolo but could make a Chief or a shief. What's your preference?

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Pinbout
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Reged: 02/22/10

Loc: nj
Re: An Alternative Clear Aperture Mirror Telescope [Re: Ed Jones]
      #5609506 - 01/07/13 12:08 AM

Quote:

but could make a Chief or a shief.




what's easier to setup.


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Ed Jones
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Reged: 04/06/04

Loc: Sin-sin-atti
Re: An Alternative Clear Aperture Mirror Telescope [Re: Pinbout]
      #5609892 - 01/07/13 10:36 AM Attachment (23 downloads)

Actually a Chief would bw easier, the 4.25 inch secondary you could buy along with the lenses.

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Pinbout
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Re: An Alternative Clear Aperture Mirror Telescope [Re: Ed Jones]
      #5609924 - 01/07/13 10:48 AM

got it, thanks

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kfrederick
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Reged: 02/01/08

Re: An Alternative Clear Aperture Mirror Telescope [Re: Pinbout]
      #5609974 - 01/07/13 11:12 AM Attachment (18 downloads)

There is a step by step paper on how to build this CHief Email me I will send it. Mike Did this work .And I have a CNC file on the top part . Dave paper should be in EDs book .EDs numbers tell me little of exactly the lens go . So much more info on there exact location .Any machine shop will understand Mikes coordinate system . The top box and primary mirror are set buy a laser and a target easy accurate

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kfrederick
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Reged: 02/01/08

Re: An Alternative Clear Aperture Mirror Telescope *DELETED* [Re: kfrederick]
      #5609988 - 01/07/13 11:19 AM

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kfrederick
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Reged: 02/01/08

Re: An Alternative Clear Aperture Mirror Telescope *DELETED* [Re: kfrederick]
      #5609992 - 01/07/13 11:20 AM

Post deleted by kfrederick

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