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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
up vote 8 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

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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Actually you can 'zoom' a telescope (in fact change focal length) just not on the fly as with a zoom lens. You can increase the focal length with a barlow lens - simila to a teleconverter, or reduce it with a focal reducer (and increase the relative aperture). They both come before the ocular or camera in this case. What you cannot modify on a telescope is aperture which is always wide open.

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I used my father's Schmidt-Cassegrain len (i.e. a mirror lens) to make a gigantic telephoto lens. It was gave utterly phenomenal magnification, but wasn't that great quality wise.

I took this picture with it:

As you can see it's just a massive mirror lens and it was a real pain to focus.

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In a previous career, I worked a lot with wildlife, including shorebirds. I've never worked as a wildlife photographer, but I did work with some people who photographed shorebirds professionally. There were spotting scopes that users had purchased adapters for to use the scopes with a DSLR. I'm not sure what brand the adapters were or for which spotting scopes, but the images were very useable. I would imagine a similar system could be set up for use with an astronomy scope.

If you Google "Digiscoping" you have a good chance of finding a system that will work for you.

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I have a Celestron C5 spotting scope (5" diameter and a focal length of 1.250 meters) photographing anything from 26 feet to infinity with excellent results. Also with the right filters, I photograph sunspots and granulations. Then at night I can shoot Saturn and Jupiter. I would recommend the Celestron C5 to any photogapher.

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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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