POLAR ALIGNMENT TECHNIQUES
- For photographing or imaging any astronomical objects
using exposure greater than a few seconds it is necessary to
carefully align the telescope mount so that the primary axis of
rotation ( the polar axis ) is
oriented so as to be "exactly" parallel to the Earth's
axis of rotation. This can be a long process for those who are
not experienced at it, but we hope that the following guidelines
will make the task a bit shorter and a lot easier.
- A POLAR SCOPE can be useful for approximate polar
alignment IF it is well designed and well aligned
with the polar axis of the telescope.
- Due to the mechanical imperfections that are inherent in
telescope mountings AND the effects
of refraction a POLAR SCOPE can only provide an approximate
alignment setting for the telescope.
- The main function of using the polar scope is to provide
for a frst order approximation to polar
alignment which then translates directly into a savings of
time when performing the accurate polar
alignment using the DRIFT PROCESS.
- The DRIFT PROCESS is the most accurate method available
for portable mountings. If you use
the drift method properly you will be able to do long exposures
with virtually any lens or telescope and
NEVER see the image ruining effects of FIELD ROTATION ( Photos
taken near the horizon are still
subject to the effects of differential refraction which is
altogether a separate and different effect!! )
- Once you have approximate polar alignment, point the
telescope to a star
that is at about 6 hours HOUR ANGLE and
is about 15 degrees above the EASTERN HORIZON...
- If the star DRIFTS TO THE NORTH ...
The POLAR AXIS is TOO HIGH
- If the star DRIFTS TO THE SOUTH ...
The POLAR AXIS is TOO LOW
(reverse the above instructions if using a star near
the western horizon)
CONTINUE MAKING ALTITUDE CORRECTIONS UNTIL YOU SEE
NO DRIFT
IN DECLINATION OVER A SPAN OF 5-15 minutes.
- NEXT, point the telescope to a star
that is ON THE MERIDIAN and
is near the celestial equator.
- If the star DRIFTS TO THE NORTH ...
The POLAR AXIS is TOO FAR TO THE WEST
- If the star DRIFTS TO THE SOUTH ...
The POLAR AXIS is TOO FAR TO THE EAST
CONTINUE MAKING AZIMUTH CORRECTIONS UNTIL YOU SEE NO
DRIFT
IN DECLINATION OVER A SPAN OF 5-15 minutes.
- If you do not use a polar scope or other means of
obtaining an approximate initial alignment you
will probably find it to be more productive it you start your
drift alignment using a star on ther meridian and
move to a star near the horizon after you have "roughed in
" the azimuthal alignment of the
mount.
- If your equatorial head can not be kept level during
these adjustments, it will be necessary to
repeat these steps a second time ( or more ) in order to account
for errors that are introduced as each axis
is adjusted.
See "A MANUAL OF ADVANCED CELESTIAL PHOTOGRAPHY"
for more comprehensive information on this and other topics of
interest for astrophotography.
-- THE MATERIAL HERE AND ELSEWHERE ON WEB PAGES BY BRAD WALLIS
and/or ROBERT PROVIN IS INTENDED FOR PERSONAL USE ONLY AND MAY
NOT BE USED IN ANY PUBLICATIONS WITHOUT THE EXPRESS CONSENT OF
THE AUTHORS
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