Thread: Celestial Pole
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Old 14-10-2019, 08:30 PM
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gary is offline
Join Date: Apr 2005
Location: Mt. Kuring-Gai
Posts: 5,299
Hi Peter,

It will precess and nutate in about the order of 50 arc seconds a year.


Also keep in mind that here is no such thing as a perfect polar alignment
and equatorial mounts are really just engineering compromises.

Even if one were to align an equatorial mount's polar axis perfectly
with the celestial pole, field rotation will still occur.

How much and to what extent is trigonometrically relatively complex
and is a function of what area of the sky you are imaging and the
time you are imaging for.

At best, one can find an optimal compromise depending on where you
are imaging in the sky.

One problem is that for any given elevation in the sky, the amount of
“lifting” to a star caused by refraction is different when compared to a
star at some different elevation.

What’s more, as the star advances across the sky in elevation, the
amount of “lift” is continually varying.

Plus within the FOV, points in the sky that are at lower elevations are
“lifted” more than those at higher elevations. The wider the FOV,
the more the “compression” within the image.

That also means there will still be some field rotation within
the FOV on an equatorial telescope.

So for any given point in the sky you wish to image, the optimal polar
axis will be slightly different and unfortunately continually changes with time.

What's more, since the amount of “lifting” to a star caused by refraction
is different when compared to a star at some different elevation,
that also means the tracking rate will continually vary.

What comes as a surprise to some amateurs is that the problem
of field rotation due to refraction for long exposure times was first
extensively studied by professional astronomers, such as Arthur A.
Rambaut, as far back as 1893.

The photographic plates in those days were not very sensitive and long
exposure times were the norm.

Astronomers realized that not only field rotation was upsetting their
long exposures on equatorial mounts but that the first differential, that
is the tracking rates, would have to be dynamic as well.

In a theoretical sense, guiding cannot compensate for the field
rotation due to refraction either. One would still need to either
have the camera revolving on a third axis -a derotator - or would one
would have to dynamically change the mount's alt axis as well.

Conveniently the field rotation caused by refraction is in the
opposite sense to that caused by elevation.

A good compromise is to attempt to align the mount's polar
axis not with the true pole but the refracted pole.

The effect of refraction is to "lift" the apparent position of an object
so the refracted pole is always above the true pole.

For your latitude in Adelaide, an appropriate amount to raise the mount's
polar axis would be about 85 arc seconds above the true pole.

Last edited by gary; 14-10-2019 at 08:55 PM.
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