Re: [APML] Declination drift error quantified?

[Jerry Lodriguss <jml@astropix.com>]

 From Tom Polakis:

Thanks for everybody's replies regarding my questions about declination drift.

Covington's Web applet is great, and it tells me the following:  if I
observe an object near the meridian for my usual ten minutes and I'm off
the Pole by 1 degree, the object drifts in declination as much as 2 1/2
arcminutes from the center.  With a 50-degree eyepiece observing at
typically 250x with the 20-inch. that's nearly halfway from the center to
the edge of the field, which is quite annoying!  In fact, I think my rough
alignment error has been even worse than 1 degree.

With a red-dot finder and a simple offset in the correct direction, I think
I can get several tenths of a degree accuracy, which means 1' of error over
10 minutes of time.  That's acceptable, presuming the little polar scope is
indeed parallel with the polar axis.  The good news is that for low hour
angles -- and that's all you can get with a tracking platform -- drift is
largely insensitive to Polar _altitude_ pointing error.

Back to my comment about 250x, I find that most experienced visual
observers prefer a magnification near this value for the final, detailed
look after stepping up from a lower, finding power.  For my purposes, rough
polar alignment is rarely good enough for visual observing.  Sure, I'm
using a Dob which is easily nudged in altitude, but why do that when a few
minutes of up-front effort eliminates that small hassle?

Thanks again for the help.

Tom Polakis

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Tom Polakis
Tempe, AZ
Arizona Sky Pages
http://www.psiaz.com/polakis/

Astrophotography, Tips and Techniques
for Digital Enhancement in Photoshop:
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