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Alt/Az mounts for astrophotography?

Started by polcyn@gwtc.net, 08/23/2011 03:08PM
Posted 08/23/2011 03:08PM Opening Post
Hi all -- Are we anywhere near a time when alt/az mounts will have steppers fine enough for astrophotography? The big mountain top observatories have had them for years. IOptron sells units that GOTO and track but not well enough for astrophotography (I haven't had a chance to play with one myself).
I can guess at least part of the answer -- cost -- but are there other factors which make such a mount impractical?

Bob Polcyn
Posted 08/23/2011 03:23PM #1
Bob Polcyn said:

Hi all -- Are we anywhere near a time when alt/az mounts will have steppers fine enough for astrophotography? The big mountain top observatories have had them for years. IOptron sells units that GOTO and track but not well enough for astrophotography (I haven't had a chance to play with one myself).
I can guess at least part of the answer -- cost -- but are there other factors which make such a mount impractical?

Bob Polcyn

It's not the drive motors, it's the field de-rotator that is the stumbling block. Not only do you need a derotator that is accurate, autoguiding software must guide in alt-az mode AND guide on the third axis. So far no amateur grade equipment has met these needs...mainly because a wedge is cheaper and easier.

Uncle Rod

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Posted 08/23/2011 03:50PM #2
Are we anywhere near a time when alt/az mounts will have steppers fine enough for astrophotography?

With an Alt-Az mount, the image continually rotates during the exposure, so your stars will be little streaks if you expose for more than a few seconds. This is bad because for a lot of objects of interest, the delicious detail doesn't really come out until you expose for 3 - 4 hours, even with the most sensitive chip.

The other drawback is that the place where you get the best seeing and darkest sky is usually at or near the zenith, and that is exactly where an Alt-Az has the worst time trying to keep up with the rapidly rotating image.

An equatorial mount solves all those problems. It is essentially stationary with respect to the stars as the earth spins beneath it.

Rolando
Posted 08/26/2011 05:19AM #3
Thanks, guys. Even with the need for image rotation correction, I'd be surprised if alt/az mounts capable of astrophotography don't turn up one of these days. Being the gadgeteers that we are ......

Bob Polcyn
Posted 09/02/2011 03:02PM | Edited 09/02/2011 03:04PM #4
Steve Forbes, formerly an iOptron dealer and the guy who sold me my Minitower, made a special wedge for the MT that allowed equatorial mounting. It is now very nicely made by ADM and costs about $400. Of course, but the time you spend $800 for the minitower, plus $400 worth of upgrades Forbes did to mine to make it more stable, then add $400 for the wedge, you could buy a pretty high-quality GEM mount (Losmandy GM8, Celestron CGEM, Orion EQG etc.) that will do astro-imaging easily and better.