Alt-Az mount question

[lineout]

Hi there,

Not sure where this question fitted so I put it here.

I was reading an article in either Astronomy or Aust Sky & Telescope a few weeks back where it was discussing the evolution of the large professional telescopes around the world. There was mention of the fact that all modern telescopes are now built using the Alt-Az mount and not the equitorial mount as it saves size and weight (and hence money)

These scopes are able to take astro photos with exposures of hours at a time, so my question is why amateur scopes with a Alt-Az mount can't do the same. Why do we have to mount on a wedge or an eq mount to do long exposure astro-phoyography?

Cheers

[Screwdriverone]

Hi Lineout,

It's all due to field rotation, as the ALT-AZ scope tracks an object, the rotation of that object changes in the eyepiece and therefore affects the Astrophoto.

Not so with the EQ mount as the TUBE rotates with the earth and therefore the view in the Eyepiece stays the same, hence the Astrophoto is not affected.

Computer controlled ALT-AZ mounts (like Meade and the huge observatory scopes) use field De-rotators on the eyepiece / focuser to compensate for the field rotation and therefore allowing long exposures.

Depending on how long you go with your exposures, you can get away with some field rotation by fixing it in processing and stacking software, but really only for short exposures.

I hope this answers your question.

Cheers

Chris

[lineout]

Thanks for the repsonse.

I think I understand. So are you saying that large scopes (like those in Hawaii) use field de-rotators when imaging? I assume the quality of theor de-rotators are far superior than the ones we can get (rhetorical question).

Cheers

[Screwdriverone]

Yes, that would be correct, do a search for Meade field de-rotator and you should find what I am on about, I think I saw them on Bintel's site.

Chris

[lineout]

So how do they compare to a wedge or eq mount?

[Geoff45]

If all components are manufactured perfectly and perform flawlessly, there should be no difference. Not sure what happens in practice.