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Subject: Re: Altaz field rotation.
From: DTabb@aol.com
Reply To: mapug@shore.net
Date: Tue Mar 26 02:05:10 1996

Consider the following tips regarding this topic:

If your scope is moving in two axes, you have field rotation. This also
applies to polar (mis)-aligned scopes where constant corrections are required
in declination.

Field rotation is maximum at the horizons of your meridian, zero along the
east-west line through your zenith, and something in between everywhere else.
It decreases with latitude until it is zero at the poles, in which case an
alt-az scope is polar aligned.

Unless you are at the terrestrial equator and tracking an object on the
celestial equator, no object will be on the zero line for more than an
instant. Also, since the object's alt-az is constantly changing, the amount
of field rotation is too...it doesn't remain constant.

In practice, the amount of field rotation becomes an issue of what's
acceptable. The larger your imaging medium, the more apparent it will be at
the edges. So those people using large format cameras for long exposures on
film may need a field de-rotator, while those making relatively short
exposures on tiny CCD chips may not. Try the following as starting points if
acceptable (to you) means a minimum of 40 lines per millimeter of sharpness:
(lat 40d, object 45d alt, near east or west) 35mm film: 3-4 minutes; 6X7
camera: <2 minutes; CCD chip: depends on number of pixels per mm...try 2-5
minutes.

For best results, time your exposure so the object moves through your
east-west line in the middle of the exposure. Experiment to see what's
acceptable to you.

Regards,

Don Tabbutt