Quote:
Originally Posted by Barrykgerdes
A thing to remember also is that orthogonality is less of a problem in ALT/AZM telescopes than in a polar mounted telescope. Getting a true polar alignment is virtually impossible if the orthogonallity is more that a few arc seconds out when using the standard procedures. This is why a drift alignment always seems to give different results than the manual described method.
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It is also worth pointing out that when one performs detailed analysis of fork
mounts of this type, that RA-to-Dec non-orthogonality, what is referred to as NP
and the Hour Angle component of Dec-to-optical axis non-orthogonality, what is referred
to as CH, is sometimes only part of the story as far as systematic mechanical
error influencing the pointing error residual is concerned.
Specifically it is not uncommon to discover various eccentric bearing error
and associated run-out effects in both the RA and Dec axes that often have
magnitudes as great as the NP and CH errors. For example, on commercial
SCT's eccentric bearing errors with magnitudes of 300 to 900 arc seconds
are not uncommon. The readers luck may vary.
It should also be kept in mind that the effects of refraction mean that
there is no such thing as a true polar alignment. At 45 degrees from the
zenith, refraction can account for around 1 arc minute difference in elevation.
In turn, refraction is a function of local barometric pressure and temperature.
The pressure of course can be influenced by the observer's altitude above
sea level. The effects of refraction then mean that rather than being some
specific point in the sky to which one can align the mount's RA axis which
is then good for the whole sky, there is really different specific optimal points
at a specific moment in time when the scope is pointed at a given zenith distance.
In other words, the optimal polar axis keeps moving as the scope tracks.
For this reason, the drift test is by no means the 'gold standard' when it
comes to determining the optimal alignment point as it does not take into
account a whole range of phenomena, including non-orthogonalities,
eccentricities, flexures, refraction, etc.