Is an F/1 Refractor possible?

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Is an F/1 Refractor possible?

Postby david48 » Sat May 21, 2011 6:10 pm

Old-fashioned refracting telescopes used OG's with a high F ratio, usually F/15. So "traditional" 3" refractors had OGs with a focal length of 45". Thus, after allowing for the additional focal length of the eyepiece, the telescopes were about 4 feet long.

Which made them rather unwieldy and inconvenient. But this inconvenience was supposed to be the inevitable downside of having a long focus OG to ensure good image quality.

Modern refractors seem to have abandoned the old F/15 ratio, and operate at about F/10 or F/8. Yet despite this reduction in focal ratio, they produce images which are apparently considered satisfactory. And the telescopes are undeniably shorter and more convenient to handle.

The handling qualities are even further improved in the Meade ETX-60, -70, and -80 scopes, which operate at (I think) F/5 or F/4.

How much further could this be taken - could a useable telescope be made with a F ratio of F/3, F/2 or even F/1?

A 3" refractor with an F/1 ratio would only be 4" long, and constitute a powerful pocket observatory!

What stops such a telescope being marketed?
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RE: Is an F/1 Refractor possible?

Postby brianb » Sat May 21, 2011 9:13 pm

[quote]What stops such a telescope being marketed? [/quote]
You need about 12 elements with aspheric curves made from exotic glass to get a camera lens to work well at f/1.4 (even then it won't be diffraction limited until stopped down to about f/4) ... so cost would be a very significant barrier. In any case there is no benefit to going below about f/6 for visual work, you just get issues with eyepieces ... the cheap fast achromats you cite are pretty horrible optically compared with a slow achromat or a fastish triplet / quadruplet.

If you want a short tube, you can fold the optical path - as is done in binoculars, and SCT / Mak reflectors.

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RE: Is an F/1 Refractor possible?

Postby les » Sun May 22, 2011 11:40 am

HI There,
Interesting theory,a 3" f1 refractor fitted with a high power 4mm eyepiece would give a magnification of 18X,but why stop at f1,what about f0.5 or f0.25 or a focal point within the lens itself,giving a magnification of 0X.Cant see that catching on though somehow.[:D]
Regards Les.
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RE: Is an F/1 Refractor possible?

Postby brianb » Sun May 22, 2011 11:56 am

[quote]a focal point within the lens itself[/quote]
Schmidt cameras?

Stanley Kubrick comissioned a camera lens (35mm format) working at f/0.7 for filming the candlelit scenes in "Barry Lyndon" by natural light. It certainly worked but the images made with that lens have an ethereal soft focus quality (possibly intended from the artistic point of view) rather than being obviously of high optical quality.

With efficient CCD sensors, the need for ultra fast lenses to capture low light images has diminished.

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RE: Is an F/1 Refractor possible?

Postby david48 » Mon May 23, 2011 12:06 pm


[quote]
Stanley Kubrick comissioned a camera lens (35mm format) working at f/0.7 for filming the candlelit scenes in "Barry Lyndon" by natural light.
[/quote]

Thanks Brian - your mention of the camera used in "Barry Lyndon" is interesting, because as a Kubrick fan, I recently read the biography by John Baxter, and in it Baxter says:

"[i]Kubrick scoured the world for lenses that admitted more light....the best was a Zeiss 50mm developed by NASA for use on the Moon. Its large aperture reduced the depth of field severely, but Kubrick asked California engineer Ed DiGuilio to fit it to a camera. After inspired engineering, DiGiulio succeeded, but had to rebuild the camera."
[/i]
I found the "50mm" bit slightly puzzling, as that doesn't sound all that big for a lens: perhaps it's a misprint for "150mm",
because Baxter also mentions the "giant 165mm Graflex lens of Kubrick's boyhood"?

I take your other point about the expense of making short f/ratio lenses. But some appallingly expensive telescopes get advertised in Astro magazines. If an f/1 refractor could be made, wouldn't it find a market among rich dilettante "astronomers"
who just want to have the latest "cool" scope.

Cheers David


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RE: Is an F/1 Refractor possible?

Postby david48 » Mon May 23, 2011 12:40 pm

[quote]ORIGINAL: Les
Interesting theory,a 3" f1 refractor fitted with a high power 4mm eyepiece would give a magnification of 18X,but why stop at f1,what about f0.5 or f0.25 or a focal point within the lens itself,giving a magnification of 0X.Cant see that catching on though somehow.
[/quote]

Yes, but the OG could have a very strong Barlow lens stuck on the back of it, so the eyepiece provided high magnification.

In fact, why should the OG and eyepiece be separate at all? Couldn't both be cast in a single unit of glass.
Then you'd have the OG, a Barlow lens, and an eyepiece, all stuck together, to make a one-piece glass "window".

Like the one, that C S Lewis's [i]Sorn[/i] showed to Dr Ransom on Mars, in "[i]Out of the Silent Planet[/i]":

[i]"[The Sorn] directed his attention to something like a small window, he leaned forward and looked....he saw perfect blackness and floating in the centre of it, a bright disc - his eyes took in the markings: Northern Europe and North America - it was the Earth he was seeing."[/i]

Couldn't this "window" be achieved by modern optical ingenuity?

Cheers David
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RE: Is an F/1 Refractor possible?

Postby brianb » Mon May 23, 2011 4:50 pm

[quote]I found the "50mm" bit slightly puzzling, as that doesn't sound all that big for a lens[/quote]
50mm focal length is quite reasonable for 35mm format movie film, in fact it's a moderate telephoto - 35mm movie format is "half frame" (18x24mm film gate) so 50mm focal length is 1.6x the frame diagonal.

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RE: Is an F/1 Refractor possible?

Postby brianb » Mon May 23, 2011 4:50 pm

[quote]Couldn't this "window" be achieved by modern optical ingenuity? [/quote]
Demonstrate how & make yourself a bundle.

But the laws of physics are against you.

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RE: Is an F/1 Refractor possible?

Postby sftonkin » Tue May 24, 2011 1:56 pm

[quote]ORIGINAL: david48

Couldn't this "window" be achieved by modern optical ingenuity?
[/quote]

David, why don't you get yourself an optical raytrace program (eg [url=http://www.optics-lab.com/]Optics Lab[/url] or [url=http://www.lambdares.com/software_products/oslo/oslo/optical_system_setup/]OSLO[/url]) and try out some of your ideas. Briefly, an f1 light cone will give you a lovely demonstration of common optical aberrations/Seidel errors.
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Re: Is an F/1 Refractor possible?

Postby Luxfre » Fri Mar 10, 2017 10:47 pm

Not only possible, I have one I made. 3 element 6 in. Lens X 6 inch focal length.
Will send photos soon.
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