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Old 23-12-2008, 05:29 PM
gary
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Join Date: Apr 2005
Location: Mt. Kuring-Gai
Posts: 5,636
Quote:
Originally Posted by AlexN View Post
Even WITH guiding, your results are always better if your polar alignment is perfect ...
Hi Alex,

Thanks for the post.

I thought it timely for just a quick "heads-up".

As I remind enthusiasts, there is no such thing as a "perfect
polar alignment". In other words, there is no Holy Grail
angle by which you can set your mount in elevation in order
to achieve zero field rotation for all zenith distances.

Instead, there is only an optimal polar axis elevation angle
for any one point in the sky.

The problem is due to atmospheric refraction. Particularly for
large zenith distances, it impacts upon both the optimal
point to which one should align the polar axis and also affects the
instantaneous tracking rate.

As a 'whole-sky' compromise, it is usually best to align the polar
axis with the refracted pole.

However, the reality is that for most enthusiasts performing digital
imagery these days, autoguiding has become the norm and
imaging times are usually kept modest so that any residual
field rotation is rarely an issue.

Quote:
Originally Posted by iceman
For long exposures, drift alignment is the only way to ensure you get it as accurate as you can.
Another common myth among enthusiasts is that the drift test
is the Gold Standard with regards polar alignment.

The problem with a drift test is that it does not take into
account refraction nor the geometric, gravitational flexure
and eccentric bearing errors within the mount/OTA.

Therefore best practice is to perform a mount error
analysis from a star pointing test and from that derive the
two polar axis misalignment terms as part of that process
and then finally correct the elevation term for the refracted pole,
which is dependent upon your latitude, temperature and barometric
pressure.

The above prescription would be recommended, say, for anyone
with a mount in a fixed observatory or anyone with a portable mount that
plans on leaving it setup for an extended session.

The techniques I broadly described above above are the same
as employed in most professional observatories and many amateurs
do the same. For example, Monte is a regular contributor on the
forum and commonly uses this approach when imaging.

As many of the fine images that appear on forums and in competitions
are testimony to, if imaging times are kept short, then a drift test alone
can deliver the goods.

Best Regards

Gary Kopff
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