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?
Answer
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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