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  1. #1
    Michel Hardy-Vallée's Avatar
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    Difference between a camera lens and a telescope?

    Art's thread about the Canon 5200mm lens suddenly made me realize that this focal length was actually longer than that of most refracting telescopes.

    But refracting telescopes of just one meter are huge ugly brutes mounted on a giant stand inside a building of their own. How is it that an even longer lens can be so small in comparison?

    And what's the difference between a telescope per se and a camera lens? The only difference I can see is that a camera lens projects a real image on a surface, whereas a refracting telescope projects a virtual image, but other than that I don't understand.

  2. #2
    Lee L's Avatar
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    Michel,

    Astronomers are kinda funny about what dimension they are talking about in referring to telescopes. A "one meter scope" might refer to focal length or to physical aperture. A one meter focal length telescope might be an f:15, in which case it's physically pretty small, and likely a small simple refractor or a small portable Maksutov. A mirror telescope (as opposed to a refractor) is often referred to by physical aperture, and a one meter mirror can be relatively large, and mounted in a cage with "folded" optics that make the physical length of the telescope 1/3rd or less than the focal length.

    My guess is that the Canon 5200 in the ad is some sort of folded mirror telescope, of which there are many different designs. Here are three common types of telescopes, two of them mirrors. http://www.stargazing.net/naa/scope2.htm

    You're right about both camera lenses and telescopes projecting an image. If you can put film at the focal plane, either a camera lens or telescope can be uses to take photos. If you put a short focal length optic (and eyepiece in telescope parlance) in a position to focus on the aerial image projected by the primary optics (the lens or mirror) so that you can view it with your eye, both a telescope and a camera lens can be used in the same way. Put a camera telephoto on a support, then put a short lens (50mm or less) with its back end toward the rear of the telephoto lens and you can focus with magnification on the aerial image from the telephoto. Use a 400mm telephoto and a 50mm normal back to back with the 50 focused on the aerial image formed by the 400 and you have a 400/50 power telescope, or 8x. Doesn't matter if the telephoto is a mirror or refractor. Some optics, mirror lenses in the 500-1200mm range were sold with both camera and eyepiece mounts.

    The only obstacle to photographing with any telescope is the ability to place a camera body in a position where the film is at the focal plane of the telescope. There are other design parameters for given telescopes, certain sensor coverage, angular coverage, pure light gathering power (pure aperture is more important than f-ratios on infinitely small points of light like stars), etc. So a telescope may not cover a given camera format, but the principles and optical arrangements aren't that different.

    Lee
    Last edited by Lee L; 10-02-2008 at 02:37 PM. Click to view previous post history.

  3. #3
    Bosaiya's Avatar
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    Telescopes and camera lenses tend to have a huge difference in quality, at least for the photographer. Chromatic abberation is the norm, edge-to-edge sharpness can suffer, etc. I've used an adapter in the past to mount cameras on scopes and been less than thrilled by the results. But if you're just in it for the Wow! factor you might be pleased.

  4. #4
    Lee L's Avatar
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    Quote Originally Posted by Bosaiya View Post
    Telescopes and camera lenses tend to have a huge difference in quality, at least for the photographer.
    Just as with camera lenses, there is a huge range of quality variation in telescope lenses/mirrors. You can't make a valid blanket statement about telescopes any more than you can make a blanket statement about cameras that covers both a Holga and a Leica.

    A mirror only system cannot produce chromatic aberration.

    Lee

  5. #5
    Bosaiya's Avatar
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    Good point! How about "Telescopes that are affordable by mere mortals"?

  6. #6
    Lee L's Avatar
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    Quote Originally Posted by Bosaiya View Post
    Good point! How about "Telescopes that are affordable by mere mortals"?


    Maybe you can get some Martian financing.

    You can get good telescope optics starting at the prices where good camera telephotos start, and there's an active market in used telescopes. There are good astrophotography forae online that also have trading and selling sections, but I don't visit them, so can't give specific recommendations. A place to start might be Astromart. There are also good books on astrophotography and magazines that review them in photographic terms from time to time. Astrophotography is very demanding of optics, and photographing pinpoint stars will tell you a lot about a lens.

    Lee

  7. #7
    Michel Hardy-Vallée's Avatar
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    Lee, thanks a lot for the explanations, it's becoming clearer to me!

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    To add to the above comments, it is usual to denote telescope dimension by the diameter of the lens or mirror. In astronomical terms the 5200mm mirror lens at f14 would be about 370mm. If the telecope had a simlar f atop it would have a focal length of about 15000 mm. Having said that this lens is pretty much a telescope (http://www.canonfd.com/mirrorlenses/pages/page10.html) as it weighs in at 120kg!

  9. #9
    Struan Gray's Avatar
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    Perhaps the biggest difference is that telescope objectives are optimised for on-axis performance, whereas photographic lenses sacrifice some of the on-axis resolution in order to perform well across a wider image plane.

    There are exceptions, but even the objectives from so-called 'wide-field' refractors are poor as taking lenses (or desirably swirly - it depends on your point of view). You can make a very good telescope objective from an air-spaced doublet, a type of lens which has been regarded as barely adequate for photographic use for over a hundred years.

    Things are changing somewhat now that many people are using digital sensors for observation and recording. This has led to a greater desire for good flat field performance, but it doesn't come cheap.

  10. #10
    Joe VanCleave's Avatar
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    For visual observing, a moderate-aperture Newtonian reflector on a dobsonian (alt-azimuth) mount is great performance for the money. These scopes are getting better and better all the time. They usually represent a tradeoff between optical quality and the build quality of the mount. Some folks are also upgrading their mirrors in these scopes with premium quality, small optical shop products, for a price. But, being alt-az mounts with no motor drives, these are for visual observing only.

    There are more and more good quality, small aperture refractors on the market, no doubt driven by the desire for quality digital imaging. For this purpose, a high-quality mount is important; such mounts are often much more expensive than the telescope itself. See the refractor forum at Cloudy Nights for lots more info.

    ~Joe

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