Quote:
Originally Posted by ZeroID
The Holy Grail would be to have a perfect alignment and perfect sidereal rate and not need any corrections from a guide scope but .... I think I just saw pig fly past ...
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Hi Brent,
At last October's IceInSpace Astro Camp which is held north of Sydney, I gave a
presentation on 'Busting the Myth of the Perfect Polar Alignment'.
In a nutshell there is no magic point in the sky to which you can align the RA
axis of the scope and achieve no field rotation.
Likewise, there is no magic single constant rate at which you can drive the scope
and achieve perfect tracking.
Strangely enough this was well understood by the very first professional
astrophotographers as far back as 1893 but is still commonly misunderstood
by many amateurs today, even many accomplished ones.
The reason the Holy Grail is a myth is, of course, due to the distorting effects of atmospheric refraction.
The details are surprisingly complex but the practical realities are that for many
parts of the sky in which people image nearer the zenith, if the mount is aligned to
the refracted pole rather than the true pole, a constant sidereal rate is used and
the exposure time kept short enough, then the field rotation can often be kept small
enough to be acceptable.
But these very issues plagued practitioners in the late 1800's and early 1900's
using emulsion plates and one of the technical challenges they had to contend
with was that whereas we take variable time keeping under computer control
for granted, they had to do this by purely mechanical means.
The other philosophical upshot of all of this are that equatorial mounts are
really just engineering compromises. Without additional facility to change
their elevation dynamically, from a theoretical point of view, they just provide
an approximation of the ideal.
What I like to call "God's Equatorial Mount" would have exactly the same limitations and also suffer from field rotation.
That does not mean in practice equatorial mounts can't work well enough to
exceed user's requirements. But from an engineering standpoint, there is nothing
intrinsically "magical" about their design that gives them zero field rotation,
irrespective of how they are aligned.
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
