Quote:
Originally Posted by raymo
The DARV drift method can be as accurate as you want it; the longer you make the star trails, the more precise the result will be. If you get a perfect single star trail across the full width of your LCD screen you are as near to perfection as you can get.
raymo
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Hi Raymo,
Not quite.
It is important for practitioners to appreciate that there is no such
thing as a 'perfect' polar alignment.
It does not exist and cannot be achieved.
At best, one can find an optimal compromise depending on where you
are imaging in the sky.
One problem is that for any given elevation in the sky, the amount of
“lifting” to a star caused by refraction is different when compared to a
star at some different elevation.
What’s more, as the star advances across the sky in elevation, the
amount of “lift” is continually varying.
Plus within the FOV, points in the sky that are at lower elevations are
“lifted” more than those at higher elevations. The wider the FOV,
the more the “compression” within the image.
That also means there will still be some field rotation within
the FOV on an equatorial telescope.
So for any given point in the sky you wish to image, the optimal polar
axis will be slightly different and unfortunately continually changes with time.
So the fact that an equatorial mount moves around its polar axis in a
perfect circle can, in some ways, be considered a mechanical
compromise, as the stars don’t circle the sky in perfect circles.
What's more, since the amount of “lifting” to a star caused by refraction
is different when compared to a star at some different elevation,
that also means the tracking rate will continually vary.
So in the case of a long drift test, one is simply averaging the misalignment
error in the part of the sky the test was performed.
The result will differ at some different part of the sky.
Best Regards
Gary Kopff
Mt Kuring-Gai NSW