[APML] Declination drift error quantified?

[Jerry Lodriguss <jml@astropix.com>]

Hi,

I received this question from Tom Polakis, and I could not answer it, if 
anyone on the list could I would really appreciate it and will pass the 
answer on to Tom.

Thanks,

Jerry

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I am using a equatorial tracking platform for visual use.  It was set up
by the manufacturer for 33 degrees latitude, which is within about one
degree of any of the sites I regularly use in Arizona.  My current polar
alignment procedure is simply to level the platform, and visually sight
along the center of the platform until it looks about to be pointing
north.  This is often claimed to be good enough for visual use, but in
reality, it is not.  At a magnification of 500x, I often see a declination
drift of several arcminutes over a ten-minute observation.  Since I'm
pretty anal about the aesthetic quality of a centered object, this has
become unacceptable.

Tom Osypowski has introduced a polar alignment tool for his platforms.
One version uses a magnifying scope with an 0.8-degree-offset Polar
reticle, while the cheaper unit uses a "red-dot" 1x sight.  Tom claims the
latter is close enough for visual use (there's that belittling phrase
again!), but I wonder.  Would anybody be able to give me a formula or
reference that quantifies the amount of declination drift near the
meridian as a function of azimuth error of the mount?  Assume that the
tracking rate and Polar elevation are dead on.

In a Web search, I was able to find one reference to this subject in a
message by Matt BenDaniel from October 2000.  He quotes Covington, who
says that the maximum declination drift for a 30-minute exposure with an
alignment error of 2 arcminutes would be 15.7".  I don't imagine that the
equation is so clean as to be linear (i.e. for an error of 1 degree, the
drift is 30 times as high).  And by maximum drift, does that refer to
pointing at the celestial equator?

Getting a feel for the amount of declination drift will influence my
judgement as to whether the red-dot finder is really close enough.
Thanks.

Tom

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