From: "Tom Krajci" 
To: , 
Reply-To: "Tom Krajci" 


>From: "James Lerch" 
Subject: Re: Alt-az at zenith


>I was using the drift method of polar
>aligning, and while I couldn't see any drift over 30 mins or so, the
drift >was still there, but only revealed itself over the course of several hours!

I take issue with this.  I do not think the problems in this case have been
thoroughly identified.

See: http://www.covingtoninnovations.com/astro/Polar.html And plug in
various polar axis alignment errors...and use an exposure time like 120
minutes.

A polar axis misalignment of 2-3 arcminutes gives you field rotation of
0.02 degrees.  Does that amount of rotation/trailing show on the final
image?

It is not hard to detect, through drift alignment, polar misalignment of
this magnitude in 5-10 minutes of tracking in RA. at a moderately high
power.  If you drift longer, you can detect even small alignment errors.

If your mount/scope flexure issues are under control, and you can make
precise/measured adjustments to your polar axis pointing...you can get good
polar alignment by the end of twilight.  If you are in practice, you can do
it this way:

- Put mount in position, best guess to true north and proper elevation.

- When twilight comes, use polar alignment scope to get 'decent' polar
alignment

- Use drift alignment for 5 minutes, using graduated reticle eyepiece on
a star on the meridian, and on a star 6 hours from meridian.  (Measure
amount of drift from both cases.)

- Do some trig. and apply the required correction to polar axis in form
of alt. and az movements.

- Do one last set of 5 (or 10) minute drift analysis to see if you are
within alignment tolerance.

Practice and development of solid procedures come in real handy here.

Where am I making a mistake in:

- analysis of polar axis alignment error
- tolerance for rotation in final image
- ability of operator to detect misalignemt in short amount of time
during twilight

Tom Krajci
Tashkent, Uzbekistan



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