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An Alternative Clear Aperture Mirror Telescope

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#1 Brian Albin

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Posted 03 January 2013 - 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.

#2 Mirzam

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Posted 03 January 2013 - 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

#3 Brian Albin

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Posted 03 January 2013 - 08:31 AM

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

#4 philipdo

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Posted 03 January 2013 - 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.

#5 Mirzam

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Posted 03 January 2013 - 08:54 AM

Any reason why you could not use a Televue dioptrix?

JimC

#6 Dave O

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Posted 03 January 2013 - 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).

#7 Brian Albin

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Posted 06 January 2013 - 10:21 AM

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?

#8 philipdo

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Posted 06 January 2013 - 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.

#9 Ed Jones

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Posted 06 January 2013 - 12:23 PM

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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#10 philipdo

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Posted 06 January 2013 - 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.

#11 ed_turco

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Posted 06 January 2013 - 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. :ohgeeze:

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


Ed

#12 gatorengineer

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Posted 06 January 2013 - 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....

#13 Pinbout

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Posted 06 January 2013 - 09:12 PM

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?

#14 Ed Jones

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Posted 06 January 2013 - 11:44 PM

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

#15 Pinbout

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Posted 07 January 2013 - 12:08 AM

but could make a Chief or a shief.



what's easier to setup. :grin:

#16 Ed Jones

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Posted 07 January 2013 - 10:36 AM

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

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#17 Pinbout

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Posted 07 January 2013 - 10:48 AM

got it, thanks

#18 kfrederick

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Posted 07 January 2013 - 11:12 AM

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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#19 kfrederick

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Posted 07 January 2013 - 11:19 AM

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#20 kfrederick

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Posted 07 January 2013 - 11:20 AM

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#21 kfrederick

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Posted 07 January 2013 - 11:24 AM

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#22 Arjan

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Posted 07 January 2013 - 02:41 PM

What lenses did you use?

#23 Dave O

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Posted 07 January 2013 - 08:52 PM

What lenses did you use?


Jack's CHief used Newport Lenses (KPX211 and KPC064). It was designed using the mirror and lenses that he had already purchased. I think that Ed's design above (using lenses with slightly different radii) provides slightly better correction (based on the provided spot diagrams).

#24 Dave O

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Posted 07 January 2013 - 09:32 PM

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


I feel I should add a caveat here ...

Ed's CHief is a fairly simple design and the lenses used for the corrector can be purchased 'off the shelf" placing it within the range of many ATMs; but, it is not a project I'd recommend for a beginner. The tolerances for the placement of the lenses are very tight ... they must be accurately positioned and they must remain accurately positioned as the telescope points to all areas of the sky if it is to perform anywhere near what is predicted by theory.

Kevin 'solved' the placement problem on his Wide Band CHief by incorporating all the optical elements in a 'top box' which was machined (using a CNC waterjet) to very tight tolerances. This apparently has worked very well for him; but, all of his optics were custom figured, their radii and conics were precisely known, and the design was reoptimized as each of the elements was completed.

CotS Lenses will have more deviation from the 'spec' than Kevin's lenses did and this may make the use of a machined 'top box' less effective (unless the individual lenses can be accurately measured and the design reoptimized for the actual values prior to constructing the top box). In fact, Jack S. had attempted to construct his CHief using a CNC'ed 'top box'; but could not get the scope to come to focus. Some of Jack's problems resulted from a lack of 'stability' in the OTA ... the OTA flexed too much and the optical elements could not be kept in alignment with each other. However, I suspect some of the problem may lie in the fact that the 'top box' does not (easily) allow for even slight adjustments to the tilt(s) and spacing of the corrector lenses to compensate for any deviations between the actual specs of the lenses and the nominal/design/published specs of those lenses.

I believe that for the greatest chance of success with a CHief, one needs to design the corrector to allow for some adjustments of the lenses .... Ed has posted some designs and I believe he has determined that a holder could be machined to properly position the two corrector lenses with respect to each other, and then the entire corrector could be adjusted as a single unit in the telescope making collimation much easier ... perhaps he will elaborate more on this ...

#25 kfrederick

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Posted 08 January 2013 - 07:55 AM

Jack can speak for himself but I think he gave up when Mike told him to.He said the top needed to be .003 and the truss carbon fiber .I knew this was wrong and pleaded with him to try as we planed . But he quit and made a newt .If this is wrong Jack clear it up if reading this.






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