Quote:
Originally Posted by Robh
This is something we quite often don't think of. Refraction at the horizon is about 34' of arc. The apparent diameter of the sun is about 32' of arc. As the sun begins to set and its disk appears to be sitting on top of the horizon, it is actually below the horizon. Refraction also accounts for the flattening of the sun's disk at the horizon.
Atmospheric refraction is about 1' at 45 degrees and 5' at 10 degrees above horizon. At the height of the South Celestial Pole it is about 2'.
Rob
|
Rob,
Not only that, refraction is why there is no such thing as a "perfect polar alignment",
a commonly misunderstood concept by many enthusiasts. Instead, for any given
point in the sky you want to image, the optimal point at which to align the polar
axis will differ. As a compromise, it is best to align with the refracted pole.
In reality, most imaging times are kept sufficiently short so that the effects of
not altering the scope's elevation dynamically doesn't become appreciable in the image.
As far as its impact on pointing analysis, refraction needs to be accounted for
and again it is this complex entanglement of various phenomena that makes
whole-sky star pointing tests combined with software analysis the best
prescription for improving a scope's pointing performance.
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