Refraction Polar Alignment
Milton Hill Observatory
Est 2003
- Choose a bright, easily located star close to the equator
(to maximize
the drift rate) and near your meridian. Center it on the
crosshair of
your eyepiece.
- Track the star until its drift in declination becomes
noticeable.
Ignore any drift in right ascension. If the star drifts northward, move
the polar axis east. If the star drifts southward, move the
polar axis
west. Repeat, making finer adjustments, until the drift becomes
negligible.
- Choose another bright, easily located star near the eastern
or western horizon and center it in your crosshair eyepiece.
- Track the star until its drift in declination becomes
noticeable. Ignore any drift in right ascension. If you are looking east and the
star drifts northward, move the polar axis down. If you are
looking
east and the star drifts southward, move the polar axis up.
(Reverse
the corrective action if you are looking west.) Repeat,
making finer
adjustments, until the drift becomes negligible.
- Normally you have to offset the Alt axis from the true pole
to the
refracted pole in order to zero out the RA drift. I would
suggest
pointing your scope to a star near the celestial equator and meridian
line and then adjusting the height of the altitude axis until the drift
is zeroed out. There will always be drift elsewhere in the
sky because
of atmospheric refraction. You cannot eliminate drift
entirely over
the whole sky with the sidereal rate because of the variability of
refraction.
Last update 30-Oct-2008