Quote:
Originally Posted by Stonius
Gary - isn't the refractive index in the first instance mitigated by using the king rate of tracking?
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Hi Marcus,
Only partially in the time domain and by a fortuitous set of circumstances
also partially in the space domain if the elevation axis of the mount
is elevated to the refracted pole if your observatory is at mid to high
latitudes away from the Earth's equator.
Refraction distorts things in two ways.
1) It lifts objects in elevation as a function of elevation.
2) The first differential of that results in the tracking rate constantly
varying as the objects rises and falls in elevation.
One of the first astronomers to raise concern over the refraction problem
was Arthur Alcock Rambaut in the 1890's.
Rambaut was the Royal Astronomer of Ireland when astrophotography
was in its infancy.
Back in those days the clocks which produced the sidereal rates for
telescopes weren't the most accurate and that in itself caused trails
in images.
Then the refraction problem exacerbated the issue further and in
1896 Rambaut wrote a paper for the Royal Society Monthly Notices
which attempted to derive optimal tracking rates for various parts of the
sky. The idea was to introduce variations into the driving rate of the clock.
Arthur Hinks, who was at Cambridge Observatory, was one of the first
to tackle the problem of trying to determine elevation axis offsets for
various parts of the sky to try and compensate for refraction in the
spatial domain.
Quote:
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Originally Posted by Stonius
Doesn't help with the second 'atmosepheric refraction in-frame issue', but has anyone calculated how much that is an issue in exposures of up to, say, 10 minutes?
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Hinks addressed this very problem in a paper to the Royal Society in
1898.
The answer is complex and depends on what the observer's latitude is
where in the sky you are imaging.
Edward S. King was an astronomer at Harvard between 1893 to 1931.
When his eyesight began to fail he was put in charge of photographic work.
In 1902 he wrote a paper which was published in the Annals of Harvard
College Observatory entitled "Forms of Images in Stellar Photography".
It is in this paper that King comments that if the observer is at a mid or
high latitude away from the equator, then it is advisable to align the
elevation axis of the mount with the refracted pole and to adopt the
'King tracking rate'.
In Australia, observers in the far north of the country are given
a different prescription where they should be adjusting the elevation
to the true pole rather the refracted pole.
With computer control and auto-guiding, the "King tracking rate" is
essentially a thing of the past.
But most of us can readily remember when it was offered as a selection
on amateur mounts from company's such as Celestron even up to the
mid-90's.
Now and then I have given a brief presentation entitled "The Myth of the Perfect Polar Alignment"
at some star parties. It discusses the refraction problem and touches on some historical background.
It is fascinating how astronomers first attempted to tackle the refraction problem back as far as the 1890's
to image photographic plates.
Despite that, even to this day, there are amateurs out there who will "chase their tail" in pursuit of finding
some magic alignment of their mount they hope will be perfect for all areas of the sky.
Alas, even changing atmospheric pressure changes the refractive index.
Ideally the elevation axis of the mount would be motor driven to continually adjust it as well.
The UK Schmidt had such a mechanism which was required to avoid field rotation in its exposures which
had a very wide FOV.