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Equipment Discussions >> Refractors

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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: The Demise of the Great Refractor new [Re: Sky Muse]
      #5527203 - 11/19/12 01:00 AM

Quote:

An interesting read...

http://www.aip.org/history/cosmology/tools/tools-first-telescopes.htm


First came the refractor, then only a few years later, the reflector. It was all very simple, as the refractor never vied for the reflector's position; nor the reflector for the refractor's, until the creation of the Yerkes forty-inch objective crafted by Alvan Clark...

http://www.google.com/imgres?q=alvan+clark&start=96&um=1&hl=en&am...

...the beginning of the end.

Sadly, optical lenses above forty inches in diameter tend to sag in the center, thus rendering them impractical for serious observation, with the creation of the forty-inch Yerkes objective sounding the death knell for the great refractor in unwillingly nominating its successor to the scientific community, the great reflector...

http://www.catchersofthelight.com/catchers/image.axd?picture=%2F2012%2F07%2F6...

Today, the Keck twins are the largest reflectors on the planet, with whichever one through which one chooses to observe being a toss-up.

It is most regrettable that the superior refractor, and due to said diametrical limitation, has been supplanted by mirrored contraptions, but no matter, as modern-day scientists have known no other by which to conduct their studies, and are satisfied, for the time being.

The dominance of the reflector through unfortunate chance has also resulted in a proliferation of large-apertured reflectors in the hands of the public, with said possessors substituting quantity over quality, an obstructed, indirect primary over that unobstructed and direct, and born, sadly, of far less expensive glass and metallic coatings.

Nothing is impossible, therefore there is a way to create an efficient, unobstructed objective surpassing even the greatest of the great reflectors, and beyond, the formula laying alongside cures for cancers and other seemingly uncontrollable diseases, and the eventual realisation of travel to the nearest planet outside our solar system, thus a fitting end to the dominance of inferior mirrored arrangements.

Alan




I went to the url on the Clarks. I was appalled by the number of times the author wrote Avin Clark instead of Alvan Clark. When I see that, I figure the paper isn't worth reading.


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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: The Demise of the Great Refractor new [Re: SeattleScott]
      #5527208 - 11/19/12 01:01 AM

Quote:

Jon, congratulations on hitting post # 30,000!




Jon, you need to find something else to do.


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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: The Demise of the Great Refractor new [Re: jrbarnett]
      #5527380 - 11/19/12 06:38 AM

Quote:

What if the 36-incher were a TMB triplet design, made by LZOS, instead of its Clark Doublet design? How much less "aberrated" would it have been?




The color correction would be better but the aberrations of an apochromatic refractor scale just as the aberrations of a achromat... If one considers an 6 inch F/9 apo triplet to be well corrected, scaling it to 36 inches it would have to be F/54 to maintain that same correction.


Quote:


While it is true that in a Newtonian reflector coma stays constant so long as focal ratio stays constant, focal ratio rarely stays constant, but instead (by necessity - no one wants a fire truck ladder arrangement for their 30" Dob) reduces, as reflector aperture increases.




My 25 inch is F/5, my 10 inch is F/5.

The fact that people choose to use faster focal ratios with larger scopes is true of all designs. The point here is that the chromatic aberration of the refractor do increase with increased aperture because they scale with the ratio of the focal ratio to the aperture where as the aberrations of the Newtonian only scale with the focal ratio.

A 3 inch F/17.7 achromat would be essentially color free. To scale that design, the 36 inch Lick would be F/214....

Were the 36 inch Lick a Newtonian, it could be F/17.7 and have a 122mm diameter diffraction limited circle... But there would be no reason for it to be so slow, F/4 with a coma corrector would be a realistic design but at the time it was built, coating technology was still in it's infancy.

The fact that people generally choose F/4 or even faster in a large Newtonian is a testament to the robustness of the design, a 30 inch F/4 refractor is not even on the table for discussion...

Great refractors, great refractors are small scopes, 3, 4, 5 inches...

Jon


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hottr6
Carpal Tunnel
*****

Reged: 06/28/09

Loc: 7,500', Magdalena Mtns, NM
Re: The Demise of the Great Refractor new [Re: Sky Muse]
      #5527858 - 11/19/12 12:34 PM

From a science perspective, a big refractor funded by the National Science Foundation, would be just a waste of tax-payers money.

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GeneT
Ely Kid
*****

Reged: 11/07/08

Loc: South Texas
Re: The Demise of the Great Refractor new [Re: Jon Isaacs]
      #5528575 - 11/19/12 06:57 PM

Jon, really nice post!

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rwiederrich
Goldfinger
*****

Reged: 11/17/05

Loc: Always Dark skies of Belfair W...
Re: The Demise of the Great Refractor new [Re: chboss]
      #5528665 - 11/19/12 07:44 PM Attachment (23 downloads)

Quote:

Unfortunately large refractors are a thing of the past in professional astronomy. I personally love the look of these classic instruments although they are not very practical to use. Below the image of the 65cm Zeiss refracotr of the National observatory of Japan. It is unused and serves only as backdrop of the museum housed in the dome.

There are some special application scopes as the mentioned solar telescope but they are rare these days.

best regards
Chris




Super thing that large refractors are NOT things of the past in amateur astronomy. I also love their look and to me they are very practical in their use.


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: The Demise of the Great Refractor new [Re: Ziggy943]
      #5528901 - 11/19/12 09:53 PM

I suppose it's forgivable, as "Alvin" is the common spelling.

Similarly, I got that all my life, and still do..."Allen" here and "Allen" there, on most occasions when someone took my name for this, that or the other...

Lady: "Sir, what's your name?"
Me: "Alan."

I'd even spell it for them, but to no avail, and yes, I doubted their credibility, too.

Cheers,

Alan


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: The Demise of the Great Refractor new [Re: rwiederrich]
      #5528922 - 11/19/12 10:00 PM

Quote:

Super thing that large refractors are NOT things of the past in amateur astronomy. I also love their look and to me they are very practical in their use.




I agree, however for objects near the horizon I see some "practical" dificulties with your setup.

best regards
Chris


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: The Demise of the Great Refractor new [Re: rwiederrich]
      #5528953 - 11/19/12 10:13 PM

That's because refractors look like a telescope, the original design, instead of a "cannon", or a howitzer, or as someone else recently suggested...

...a "water heater"...



...and here I have two "water heaters"...

By the way, yours is quintessential.

Regards,

Alan


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wiseone
Vendor (Zerochromat)


Reged: 01/07/08

Loc: North Wales, UK
Re: The Demise of the Great Refractor [Re: Astrojensen]
      #5529349 - 11/20/12 03:52 AM

It is possible to make an excellent refractor larger than 40 inches! John Wall has made a 30 inch with a singlet objective and corrector lenses downstream of the cross-over point. After having seen John's wonderful design, I was moved to improve on it by using OSLO. John is old school, and used traditional optical design techniques to produce an outstanding telescope. OSLO gives you a design can that can be used up to 40 inches or more. The resultant configuration is hugely better than a traditional doublet, with a tiny weight of optics, and at a much lower cost.
I re-designed the optics for the 28 inch Greenwich refractor to fit inside the existing tube, as the current optics are not in good shape. The conservators put a brake on this, even though externally, there would be no change.


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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
Re: The Demise of the Great Refractor new [Re: wiseone]
      #5529805 - 11/20/12 10:51 AM

Peter is right I am old school in that I make all my own optics. I made a thirty inch retrofocally corrected dialyte of my own design, the OG is green float glass which nowdays is very transparent and suitable for such work.I achieved good colour correction with this scope after researching the design for a number of years. It is not a Schupmann, that scope is a catadioptric refractor, the retro uses a totally different design concept. Peter took my ideas and thoroughly analyzed it in OSLO coming up with a Apochromatic version and has since branched out into many ramifications of the idea. It is now the largest refractor in the UK.

Edited by KWB (11/21/12 01:24 PM)


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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
Re: The Demise of the Great Refractor *DELETED* new [Re: Crayfordjon]
      #5529808 - 11/20/12 10:52 AM

Post deleted by KWB

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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
Re: The Demise of the Great Refractor *DELETED* new [Re: Crayfordjon]
      #5529810 - 11/20/12 10:53 AM

Post deleted by KWB

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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
Re: The Demise of the Great Refractor *DELETED* new [Re: Crayfordjon]
      #5529814 - 11/20/12 10:54 AM

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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
Re: The Demise of the Great Refractor *DELETED* new [Re: Crayfordjon]
      #5529825 - 11/20/12 10:57 AM

Post deleted by KWB

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rwiederrich
Goldfinger
*****

Reged: 11/17/05

Loc: Always Dark skies of Belfair W...
Re: The Demise of the Great Refractor new [Re: chboss]
      #5529855 - 11/20/12 11:13 AM

Quote:

Quote:

Super thing that large refractors are NOT things of the past in amateur astronomy. I also love their look and to me they are very practical in their use.




I agree, however for objects near the horizon I see some "practical" dificulties with your setup.

best regards
Chris




Really..how is that exactly?


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wiseone
Vendor (Zerochromat)


Reged: 01/07/08

Loc: North Wales, UK
Re: The Demise of the Great Refractor *DELETED* new [Re: Crayfordjon]
      #5529869 - 11/20/12 11:18 AM

Post deleted by KWB

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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: The Demise of the Great Refractor new [Re: Sky Muse]
      #5529977 - 11/20/12 12:04 PM

Quote:

I suppose it's forgivable, as "Alvin" is the common spelling.

Similarly, I got that all my life, and still do..."Allen" here and "Allen" there, on most occasions when someone took my name for this, that or the other...

Lady: "Sir, what's your name?"
Me: "Alan."

I'd even spell it for them, but to no avail, and yes, I doubted their credibility, too.

Cheers,

Alan




The difference here is that it is an extensive research paper, not a casual comment. Even so, we wouldn't misspell George Washington.


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: The Demise of the Great Refractor new [Re: Ziggy943]
      #5530082 - 11/20/12 12:49 PM

Still, I've read of many "George"s, but only of one "Alvan".

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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: The Demise of the Great Refractor new [Re: Sky Muse]
      #5530250 - 11/20/12 02:00 PM

Me too. Alvan was Shobal's son.

Regards,

Jim


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