Hi Leon,
When discussing the topic of polar alignment, one needs to
differentiate between what is practical and what is the actual
reality.
Please allow me to explain.
If the ends justifies the means, what most amateur
astrophographers are really seeking is no discernible image
anomaly as a result of polar misalignment.
However, in reality, different parts of the sky have different
optimal polar axis points.
One practical reason for this is atmospheric refraction. The
effect of refraction is to cause objects to appear slightly
higher than they really are. At the zenith, there is no
displacement whereas at the equator, the displacement is about
half a degree. Even at 45 degrees elevation, the effect accounts
for about 1 arcminute.
To complicate the matter even further, the current barometric
pressure and temperature also impact upon refraction and the
optimal polar axis for imaging any one point in the sky and
strictly speaking have to be taken into account as well.
Refraction will also mean your circular trails are never quite
perfectly circular, not unless you image by pointing at the
zenith from the geographical north or south pole itself.
In practice, unless your exposure time is long, it is unlikely
to make a difference. However, the concept that there is some
magic point in the sky that one can align the polar axis of the
mount that results in optimal imaging in all parts of the sky is
a myth.
If one sought absolute perfection for the sake of it, the
reality would be that one would have to continually adjust the
mount in elevation whilst tracking.
And it gets worse.
Since tracking is the first differential of pointing, the
tracking rate would need to continually change as well.
And it gets worse still.
All mounts have geometric, gravitational flexure and more often
than not bearing eccentricity errors. Not only does this make
optimal polar alignment difficult, for the perfectionist, some
of these errors make it absolutely impossible. For example, if
the RA and Dec axes of the mount have not been constructed to be
at absolute right angles to each other, then there is a region
around the scope's polar axis to which it can never point. The
same is true if the mount's Dec axis and the nominal pointing
direction of the scope is not at right angles.
For this reason, the drift method is not the gold standard for
polar alignment that many mistakenly think it to be. It can't
take into account all the other errors within the mount.
A simple one, two or three star alignment is no better
as is simply perfoming a GOTO to a few stars.
Instead, to have a chance of getting closer, one needs to
perform a more sophisticated analysis, taking into account the
effects of refraction, mount errors and polar misalignment, all
at the same time. This is achieved by performing a star pointing
test on a reasonably large number of stars distributed across
the sky and then using sophisticated analytical methods to
separate the polar misalignment from the mount fabrication
errors.
All of the world's major professional observatories, including
the Kecks, Geminis', VLTs, do exactly this and almost all of
them employ a software package originally written within the
professional astronomical engineering community called TPOINT.
The Losmandy Gemini system also performs mount errors analysis
as does the Argo Navis Telescope Pointing Analysis System (TPAS)
that we happen to produce. However, that is not the point of
this response.
The real point is that you should use whatever means to polar
align your mount at a point that is optimal for the point in the
sky you plan on imaging. The best methods are to use a
sophisticated analysis technique such as TPOINT, TPAS, etc.
Any system that claims mount error compensation but only uses,
say, three stars, is lacking, to say the least. The modeling system
in your Gemini will undoubtedly give an excellent result.
However, keep in mind that a good package should always provide
statistical metrics with regards the analysis, otherwise the
results become more difficult to prove. TPOINT and TPAS both
provide such feedback.
For many amateurs, the practical reality is that aligning the
scope's polar axis to the refracted pole rather than the true
pole and combining it with guiding software that will keep
you on track will tend to provide excellent results.
Hope this is helpful and allows you to concentrate on enjoying
your imaging rather than chasing a pole that really is a
shifting target itself.
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
Chasing the elusive celestial pole
