Using the polar alignment scope

Derek Wong <dawong@...>

The following is a repost of my method of polar alignment using the AP
polar alignment scope. The instructions I received in 10/98 were a
little off, and I hope they have been fixed. IMHO using the polar
alignment scope is the most rapid way of getting a close alignment,
which may be tweaked by various other methods.


Put the polar finder into the mount and center it on a distant object,
preferably during the day. Place the object exactly within the center
circle of the reticle (NOT the Polaris circle) with the Alt-Az
adjusters, then tighten the screws. The Dec and RA mount axes should be
fixed as well.

You should have something like this:

x O x

O = object
x's = center circle of reticle of polar scope

Now loosen the RA lever on the mount, and rotate the mount 360 degrees
in RA. If you are lucky, the polar scope will be orthogonal to the
mount and the object will remain in the center circle of the reticle
throughout the entire rotation.

If the polar alignment scope is not orthogonal to the mount the object
will not stay in the center circle and you will have a situation like
the diagram below when you rotate the RA.

x O x

i i
i C i M i A i
i i

i B i

O = object
x's = center circle of reticle of polar scope
i's are points through which the center circle passes
when RA is turned a full 360 degrees
A,B,C = reference points
M = True Mount Axis

The scope starts with the object O in the center circle of the
reticle. While rotating the RA, the center circle makes its own
"circle" and passes through A,B,C and back to O. To make the polar
scope orthogonal to the mount, you must adjust the polar scope until the
center circle of the scope is on M, the mount axis.

If you follow the manual instructions (as of 10/98), you will rotate the
RA axis so the center circle is at position A, for example. Then you
realign the scope so the object O is again in the center circle.
However, you are still the same distance from the pole, so in effect you
have not gotten any closer to your goal!

You must watch the entire movement of the center circle when you rotate
the scope 360 degrees. The center circle makes another "circle" (quotes
differentiate this) around the mount axis M. All you need to do is
estimate the position of the mount axis by aligning the scope halfway
between points O and B (or A and C). Even though the actual pole may or
may not have an object, it is easy enough to estimate and if repeated
this method nails alignment right on.

Derek Wong

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