Choice of alignment stars


AP GTO users,

This is a repost of a pre-egroups message. In it, I concurred with Howard
Lazarus' supposition that stars on the meridian (due South) were useful
alignment stars. The post also covered a second topic, mount leveling.
Since that part was incorrect, I'll cover the subject in a separate post.

Michael Masters

Reposted email follows:

Stars located on or very near the meridian (due South) are very valuable for
polar alignment. The reason is that when the mount is pointed to the
meridian, turning the mount's altitude (latitude) adjuster translates into a
purely vertical alignment star motion in the eyepiece. As the mount is moved
to the east or west, a horizontal component is introduced into the movement
of the alignment star when the altitude adjuster is turned. By the time the
mount is laid all the way over to the east or west, turning the mount's
altitude adjuster results in a nearly horizontal movement of the alignment
star in the eyepiece field. Since the mount's azimuth adjuster always
produces a horizontal movement no matter where the mount is pointed, this can
lead to confusion and a seeming inability to bring the mount to proper
alignment. I think your conclusion that the altitude adjustment should be
made first may be based on this characteristic.

[Note that this last point, altitude adjustment first, is in keeping with
Roland Christen's recommended method, which involves an overhead star and
resetting of the time to cause the scope to slew to the opposite side of the
mount. M.]

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