Alley in Pisa, Italy

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This might earn me stupid question of the week honors, but reading this question made me wonder whether it would ever make sense to use a astronomy telescope + camera mount as an earthbound zoom lens?

I assume they are "slow" but can they focus? It seems they might be useful in certain circumstances, such as shooting shorebirds such as terns and gulls which typically stand around without much movement.

EDIT - ok I got my terms confused here - I meant telephoto not zoom. I was thinking of the utility of a telescope used as a telephoto prime lens versus a made-for-cameras telephoto prime lens. They surely provide differing characteristics with respect to price, aperture, and distance?

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Never heard of someone do that but they do use spotting scopes for cameras though. –  Itai Jan 8 '13 at 2:24
    
+1 I've been wondering this myself for a while. I suspect that the answer is that because telescopes are optimised for a different use, telescope won't be able to compete with a similarly priced telephoto lens. –  Chinmay Kanchi Jan 8 '13 at 2:53
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3 Answers 3

up vote 5 down vote accepted

would ever make sense to use a astronomy telescope + camera mount as an earthbound zoom lens

As far as I know, telescopes generally (always?) have a fixed focal length. Instead of changing magnification by moving internal lens elements as a zoom lens does, the magnification of a telescope is changed by switching eyepieces. So technically, no, you can't use a telescope as a zoom lens.

That said, yes, you can use a telescope as a long focal length lens. Telescopes I've tried in the past had no problem focusing on objects much closer than celestial objects, perhaps a few hundred meters away, so focusing shouldn't be an issue.

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I have updated my question, please reread it Caleb. –  Andrew Heath Jan 8 '13 at 22:59
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I have a celestron to micro-4/3'rds adapter that I have used for exactly this purpose.

I had difficulty focusing, more due to the nature of the camera, than the lens.

Here's a sample.

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It makes no sense to "zoom out" on an astronomy telescope, you want to zoom in as far as possible, so a fixed focal length makes sense and is cheaper. Plus, zoom lenses are always lower image quality than primes (aka fixed focal length).

Are you confusing the term "zoom" with "telephoto"?

Lots of folks have used small telescopes on earth. Probably used to be a prime usage back when college dorms were single sex, the guys would aim at the girls' dorm.....

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I have updated my question, please reread it Pat. –  Andrew Heath Jan 8 '13 at 23:00
    
Actually, you often do find yourself wanting to zoom out when using a telescope on astronomical objects. For example, one of my telescopes has a 2000mm focal length. I cannot get both clusters of the Double Cluster in a shot (or in the eyepiece), same for M31, the Andromeda Galaxy. There is a whole market of "wide field" telescopes which have a larger field of view just for these objects. –  Paul Cezanne Jan 9 '13 at 11:43
    
@paul, are they really zooms? or just a different type of 'scope for different objects to view. I bet that they use two "prime" scopes for the two use cases. –  Pat Farrell Jan 9 '13 at 17:48
    
oh, primes, absolutely. Sorry for the confusion. It just that with certain scopes you WANT to zoom, you just can't... –  Paul Cezanne Jan 9 '13 at 18:04
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