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POLAR ALIGNMENT
in
DOUBLE STAR ASTROMETRY

It is perfectly possible to measure pairs with a tripod mounted telescope provided the instrument is stable and has a smooth and low periodic
error drive, and is aligned sufficiently close to the true pole to produce accurate tracking. However, if the mounting is not aligned to the true
pole, PA measures at small polar distances will be effected by systematic error, and unless the direction of the hour axis on the celestial sphere is
known, the error cannot be corrected.


A permanently mounted equatorial, even if not essential, can take your measures to a higher level of accuracy, but only if the hour axis is
accurately aligned to the true pole. To achieve the necessary accuracy the mounting and telescope must be furnished with three essential items:

i)
ii)
iii)
a filar micrometer (for measuring dec. drift)
accurate setting circles with adjustable verniers (or hi-res encoders)
differential elevation & azimuth alignment bolts, or alignment wedges.

A sidereal clock will also prove useful.


Methods adapted to deep sky astrophotography intended to minimise dec. drift at certain declinations, and between certain hour angles either
side of central meridian have no place in the polar alignment of an instrument intended primarily for double star astrometry. Mountings with
hour axes elevated above the true pole to reduce dec. drift are no more adapted to the precise measure of small polar distance PA's than
temporarily mounted instruments. In other words, alignment methods dependent upon the reduction of dec. drift are wholly unsuited to
alignment on the true pole, or for determining the direction of the hour axis.