Re: Re: [APML]OT-Drift Alignment

["westergren" <westergren@netzero.net>]

Steve,

Thanks for sharing your equations for polar alignment error and the source
for Richard Hook's paper on the subject.  I tried your equations and
compared them to my spreadsheet process.  I think the units for your drift
rate R below are actually arc-minutes per minute rather than arc-sec per
minute.

I also use my STV as you do for guiding and for drift aligning.  It really
does work great and gives me the polar alignment accuracy for long FR-free
astrophotos.

Don


> Here's an equation for the accuracy required for polar alignment:
>
> E = 750 * S * cos (D) / ( T * F * A)
>
> Where:
>
> S = worst case length of trailing allowed in microns
> D = declination of the object being imaged in degrees
> T = time duration of the exposure time in minutes
> F = focal length of the system in mm
> A = sky angle between the guide point and furthest part of image in
degrees
> E = required accuracy of polar alignment in degrees
>
> A second useful equation is:
>
> R = 0.262 * E
>
> where:
>
> R = Drift rate of a star in arcseconds per minute (without guiding)
> E = polar alignment error in degrees
>
> The first equation lets you determine how close you must be when drift
> aligning and the second lets you know if you've achieved that level of
> accuracy. Using my STV, I simply set the dec axis agressiveness to zero
and
> use a timer to figure the approximate drift rate. The STV lets you detect
> drift very rapidly and also measure it since it reads out in arcseconds.
> This can be done with a reticle eyepiece but it takes longer and is less
> accurate.
>
> There is an excellent paper by R. N. Hook in the British Astronomical
> Association Journal 2/89 from which I derived these equations. Hook
> presented results for 30 micron trails but this was easily generalized.
>
> Steve...
>


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