Hi Paul,
Just by way of background, these days, for larger Dobs (e.g. 16" and up) that have
tracking, most users use motors on both axes and track simultaneously both in Az/Alt under
control of a computerized controller.
This is also exactly what most of the world's large professional 8m and 10m class
scopes do. They are all built Alt/Az but use a third motor to counter field rotation
for imaging. Modern computerized control makes this possible and practical and
large Alt/Az mounts are easier to engineer and construct than equatorial mounts.
Back in the 1970's, when 4m class scopes such as the AAT at Sidings Springs
were built, they were usually mounted equatorially. However, the AAT was the
first large telescope to rely entirely on computer control and its success, in part,
meant that computer control for all large telescopes built after it would be the go
and that therefore Alt/Az configurations would be practical as well. So the era
of large equatorial scopes pretty much came to an end.
Similarly, before computer control of amateur scopes, equatorial platforms were
popular. They can still be a good choice for smaller scopes (e.g. 12" and below)
and though less common now, there are still a few out there.
As one builder of large commercial Dobs once told me, he never intended
for his scopes to be mounted on an equatorial table. The scopes were designed to
be sitting on the ground and all the careful engineering calculations of the forces
and stresses involved go "out the window" once they are tilted on their sides.
Having said that, many users mounted large Dobs on equatorial platforms and for
example there is a 25" Obsession at the ASNSW site at Wiruna that is configured
in exactly this way.
Quote:
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Originally Posted by Eric
UPDATE - now, of course, I was wrong in some things I said And you were all too polite to point it out!
An equatorial table won't convert your Alt and Azi movements into Dec and RA - Duuuh! It is really designed for tracking purposes - keeping the object within the field of view. And it does so with one motor drive - of the table. I'm fairly sure this is good for visual, but not for photography since, if I understand correctly, you will get varying amounts of field rotation depending where you point to in the sky.
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Hi Eric! No, you were right the first time.
Mounting your Alt/Az mount atop
an equatorial table will convert it to a true equatorial platform. Field rotation is then also minimized.
Quote:
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Originally Posted by Jennifer
You're right about the field rotation. If you're polar aligned you eliminate it.
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Hi Jennifer!
Very strictly speaking, you minimize it. You never quite eliminate it.
This is true of all equatorial mounts, not just Dobs on platforms.
Because of the effects of refraction, unless an equatorial mount
is dynamically adjusted in altitude (elevation) as well there
is still some residual field rotation. For practical purposes, since imaging times
are typically short and the fields of view typically narrow, on most amateur
and professional equatorial mounts, the amount of residual field rotation is
so insignificant as to not be a serious problem for imaging. Some scopes, such as the
U.K. Schmidt though, which has a very wide FOV, can also dynamically adjust
in altitude as well whilst they track in RA.
Anyway Paul, I hope you may have found at least something in the above interesting.
Best Regards
Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place
Mount Kuring-Gai NSW 2080
Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au
And you were all too polite to point it out! 
I think you'd have to be a lot further south to make it practical.
