Quote:
Originally Posted by PRejto
I don't know what equipment you are using (mount!) but polar alignment is always a compromise. There really isn't such a thing as "perfect alignment" unless you are speaking of a small area of the sky for a relatively small period of time. Refraction changes with altitude and with temperature and humidity so it is impossible (as best I understand!) to align perfectly and have no issues with tracking.
|
This is absolutely correct.
Quote:
|
Originally Posted by codemonkey
Since starting out in astrophotography I've been limited to exposures of <= 30 seconds, so this has never been an issue for me, but I've been wondering... how do you align your mount so precisely for exposures that are 20-30mins long?
|
There 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.
I have talked extensively on this subject at venues such as the
IceInSpace Astrocamp in a presentation entitled "The Myth of the
Perfect Polar Alignment".
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.
Arthur R. Hinks in the Monthly Notices to the Royal Society in 1898
derived a nice table that showed for various declinations and
hour angles in which you propose to image suitable offsets by which
to raise the scope's polar axis.
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.
So for example at 25 degrees south latitude there in Qld at 100m above
sea level, the refracted pole is about 117 arcseconds above the true pole.
So you should be aligning the polar axis 117 arcseconds above the true
pole.
If you were to go up a mountain to 1500m, the refracted pole at the
same latitude is around 102 arcseconds.
So you can appreciate from these numbers alone it tends to make a
nonsense of claims by many amateurs that their mount is aligned
with the pole to within some small number of arcseconds.
The question then is, which pole? True or refracted?
And which part of the sky are they imaging in and how long for?
It also says that one can waste an inordinate amount of time chasing
something that does not exist. There is no such thing as a perfect
polar alignment.
By the way, the drift test is by no means the gold standard for aligning
a mount. It simply gives a rough compromise between the two areas
of the sky which you performed the drift test on.
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