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Large refractor ?

petermartin5

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petermartin5

petermartin5

Posted
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Just curious. Not that I am thinking about buying one (!!), but I have never seen advertised a refractor larger than 6in. Are they available?Bearing in mind the advantages over a reflector, i.e. crisp images, no collimation etc thought they might be ideal for some amateur astronomers.

Guess the cost would be prohibitive?

Peter

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Macavity

Macavity

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More often seen in US magazines? Here are some random "plain" Achromats(?) - But actually available - Or at least with a(n expensive) price tag. :wink:

http://www.dgoptical.com/refractor.htm

Personally I love the following - Has a certain "Britishness" about it. The Dialyt design (single LARGE objective lens, with colour correction lenses some way along the tube?) seems like an interesting idea... ;)

30" f/12 refractor: http://myweb.tiscali.co.uk/hanwellobservatory/30_inch.htm

(end of page)

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Astroman

Astroman

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The main determining factor of fabricating a large refractor is the composition of the glass used and its refractive index. As the glass elements get larger, the grinding process is more and more difficult, maybe exponentially. Grinding a large lens is not in the least the same as grinding a large reflector. Stresses inside the glass of a large refractor quickly become disruptive to the characteristics of the light as it passes through. Most problems are quoted as "weight", but it's a bit more involved than that. Any given composition of glass can only bend light so much, no matter how curved the surface is, unlike a mirror. Add to this the second or third elements needed to reduce color aberations and it becomes something of a black art.

Large glass is costly. Exotic compositions of glass to reduce physical size of the telescope is costlier. Actually making it work is costlier still. AND it's heavy, no matter what you do. ;)

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Astroman

Astroman

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Having observed through the 24" Clark at Lowell, I will say the cost is worth it. That scope cost $20,000 in 1896. Its focal length is 386", making it just over F/16 and 36' long. Jeff Medkeff and Brent Archinal ran a little experiment one evening. They put as many Barlows in it as they could, looking for a record magnification. Jeff calculated 534,520X. What did they look at? Mars, of course! :wink: Resolution at Mars distance was 74 meters, but the seeing wasn't great. ;)

The color aberation on the Clark is pretty severe, but if you stop it down to 18". it's pretty amazing on Jupiter.

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barkis

barkis

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The main determining factor of fabricating a large refractor is the composition of the glass used and its refractive index. As the glass elements get larger, the grinding process is more and more difficult, maybe exponentially. Grinding a large lens is not in the least the same as grinding a large reflector. Stresses inside the glass of a large refractor quickly become disruptive to the characteristics of the light as it passes through. Most problems are quoted as "weight", but it's a bit more involved than that. Any given composition of glass can only bend light so much, no matter how curved the surface is, unlike a mirror. Add to this the second or third elements needed to reduce color aberations and it becomes something of a black art.

Large glass is costly. Exotic compositions of glass to reduce physical size of the telescope is costlier. Actually making it work is costlier still. AND it's heavy, no matter what you do. ;)

What you say is unquestionably true AM. but Oh dear, I think you may have shattered spoiled a few dreams there. :(

Ron. :wink:

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Kenny_10_Bellys

Kenny_10_Bellys

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One day they might get liquid lenses or some similar light-bending tech to work, then we'll all be using outrageous refractors with built-in adaptive optics, near-instantaneous variable magnification and superb light transmission. One day. In the meantime, if it's 25 grand for a 150mm that's meters long and weighs a ton I think I'll stick with my stubby 8" SCT.

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The Warthog

The Warthog

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Some of those photos have me and my 105mm suffering from performance anxiety now.

When I win the lottery, though, I'm going to have a 10" refractor on a pier, in a dome, on an island, in a lake in remote Northern Ontario. My pilot will fly me in there periodically, winter and summer, when the weather forecast is favorable.

You're all invited.

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Whippy

Whippy

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Some of those photos have me and my 105mm suffering from performance anxiety now.

When I win the lottery, though, I'm going to have a 10" refractor on a pier, in a dome, on an island, in a lake in remote Northern Ontario. My pilot will fly me in there periodically, winter and summer, when the weather forecast is favorable.

You're all invited.

Love to WH ;). Canada is my preferred place to emigrate anyways (the missus wants NZ...) and I've never been to N.Ontairio....

Tony..

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Kenny_10_Bellys

Kenny_10_Bellys

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If I win the lottery it's going to be a hollow volcano in the tropics, not freezing swampland in Canada. It'll have the 200 guards in orange boiler suits armed with sterling sub-machine guns and some sort of sliding heli-pad affair. Oh, and an underwater entrance. I always wanted to have an underwater entrance to my home. The telescope would be some humungous truss affair linked to banks of computers and displays in a seperate control room, much like Houston. And no one is invited unless they look stunning in a bikini.

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The Warthog

The Warthog

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If I win the lottery it's going to be a hollow volcano in the tropics, not freezing swampland in Canada. It'll have the 200 guards in orange boiler suits armed with sterling sub-machine guns and some sort of sliding heli-pad affair. Oh, and an underwater entrance. I always wanted to have an underwater entrance to my home. The telescope would be some humungous truss affair linked to banks of computers and displays in a seperate control room, much like Houston. And no one is invited unless they look stunning in a bikini.

Get Uzis, Stirlings are awful. I was a first-class shot with a Stirling, but only because the sights were off.

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Kenny_10_Bellys

Kenny_10_Bellys

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The Uzi, while a very effective weapon, just doesn't have the kudos of the Sterling or the Sten. Any 'guys on a mission' movie worth its salt would kit the guys out with silenced Stens or have them blazing away with Sterlings at the baddies. They have that retro appeal that the Uzi is lacking, it's too Schwarzenegger and gangster for my liking, far from the special forces or 60's superspy feel of the British weapons. Anyway, you dont see people in Star Wars running around with Uzis, it's the Sterling that is the Empires weapon of choice.

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