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if you're
like most new amateur astronomers, the first thing you probably do when
you get your new telescope properly assembled is put in an eyepiece and
point it up to look at the moon. just the excitement of seeing the lunar
landscape up close is enough to keep you entertained for days. but eventually,
as you progress to finding more difficult objects, such as planets and faint
deep-sky objects, you will want to utilize all the features of your equatorial
mount, such as the setting circles or perhaps even a motor drive. a mount
is said to be "equatorial" if one of its two axes can be made parallel with
the earth's axis of rotation. aligning the telescope to the earth's axis
can be a simple or rather involved procedure depending on the level of precision
needed for what you want to do. for casual observing, only a rough polar
alignment is needed. better alignment is needed for tracking objects across
the sky (either manually or with a motor drive) at high magnifications.
still greater precision is needed in order to use setting circles to locate
those hard-to-find objects. finally, astrophotography will require the most
accurate polar alignment of all.
theory
the polar alignment procedure works on one simple principle: the polar axis
of the telescope must be made parallel to the earth's axis of rotation,
called the north celestial pole (ncp). when this is accomplished, the sky's
motion can be cancelled out simply by turning the axis (either by hand or
with a motor drive) at the same rate as the rotation of the earth, but in
the opposite direction. although residents of the northern hemisphere are
convenienced with a bright star (polaris) less than a degree from earth's
rotational axis, the ncp can still be a somewhat elusive place to locate.
rough polar alignment
for ordinary visual observing, the telescope's polar axis must be aligned
to the earth's pole. this simply means positioning the telescope so that
the polar axis is aimed up at polaris. the easiest way to accomplish this
is to rotate the telescope tube to read 90° in declination. in this position
the telescope will be parallel to the polar axis. now, move the telescope,
tripod and all, until the polar axis and telescope tube are pointed towards
polaris. finally, match the angle of your telescope's polar axis to the
latitude of your observing location. most telescopes have a latitude scale
on the side of the mount that tells you how far to angle the mount for a
given latitude (see your telescope owner's manual for instructions on how
to make this adjustment). this adjustment determines how high the polar
axis will point above the horizon. for example, if you live at 40° latitude,
the position of polaris will be 40° above the northern horizon. remember
your latitude measurement need only be approximate; in order to change your
latitude by 1° you would have to move your observing position by 70 miles!
polaris should now be in the field of view of an aligned finderscope. continue
making minor adjustments in latitude and azimuth (side to side), centering
polaris in the finder's cross hairs or low power eyepiece. this is all that
is required for a polar alignment good enough to use your telescope's slow
motion controls to easily track a star or planet across the sky. however,
in order to take full advantage of the many features of your telescope (such
as setting circle and astrophotography capability) a more precise polar
alignment will be necessary.
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accurate
polar alignment
before we can be certain that the telescope's polar axis is accurately aligned
with the rotational axis of the earth, we must first be certain that the
finderscope (which will actually be used to polar align the mount) is aligned
with the telescope's polar axis.
for polar alignment purposes, the finderscope itself can be used to accurately
align the mount's polar axis by adjusting the finder inside its bracket.
this is quite simple since the finder is easily adjusted using the screws
that hold it inside the bracket. also, the finderscope's wide field of view
will be necessary for locating the position of the north celestial pole
relative to polaris. here's how it's done:
set up your mount as you would for polar alignment. the dec setting circle
should read 90° . rotate the telescope in right ascension so that the finderscope
is positioned on the side of the telescope tube. adjust the mount in altitude
and azimuth until polaris is in the field of view of the finder and centered
in the cross hairs.
now, while looking through the finderscope, slowly rotate the telescope
180° around the polar axis (i.e. 12 hours in right ascension) until the
finder is on the opposite side of the telescope. if the optical axis of
the finder is parallel to the polar axis of the mount, then polaris will
not have moved, but remain centered in the cross hairs. if, on the other
hand, polaris has moved off of the cross hairs, then the optical axis of
the finder is skewed slightly from the polar axis of the mount. if this
is the case, you will notice that polaris will scribe a semi-circle around
the point where the polar axis is pointing. take notice how far and in what
direction polaris has moved.
using the screws on the finder bracket, make adjustments to the finderscope
and move the cross hairs halfway towards polaris' current position (indicated
by the "x" in figure b below). once this is done, adjust the mount itself
in altitude and azimuth so that polaris is once again centered in the cross
hairs. repeat the process by rotating the mount back 180° , and adjusting
the finder bracket screws until the cross hairs are halfway between their
current position and where polaris is located, and then centering polaris
in the cross hairs by adjusting the mount in altitude and azimuth. with
each successive adjustment the distance that polaris moves away from center
will decrease. continue this process' until polaris remains stationary in
the cross hairs when the mount is rotated 180º. when this is done, the optical
axis of the finderscope is perfectly aligned with the polar axis of the
mount. now the finder can be used to polar align the mount.
so far we have accomplished aligning the polar axis of the telescope with
the north star (polaris), but as any star atlas will reveal, the true north
celestial pole (ncp) lies about 3/4° away from polaris, towards the last
star in the big dipper (alkaid). to make this final adjustment, the telescope
mount (not the telescope tube) will also need to be moved away from polaris
towards the actual ncp. but the question is; since polaris makes a complete
rotation around the celestial pole once a day, how far should the mount
be moved and in what direction? let's take an example: suppose you are out
observing on august 1 st at 8:00 p.m.. a quick inspection of the northern
sky will reveal that the last star in the handle of the big dipper, alkaid,
lies above and to the left of polaris in the 10 o'clock position. now, while
looking through the finderscope (with polaris still centered in the cross
hairs) adjust the latitude and azimuth of the mount up and to the left until
polaris also moves up and to the left in your straight through finderscope.
(remember a straight through finder inverts the image, so polaris will appear
to move in the same direction as the mount is moved). how far to move polaris
will depend on the field of view of the finderscope. if using a finderscope
with a 6° field of view, polaris should be offset approximately 1/3 of the
way from center to edge in the finder's view (i.e. half of the field of
view, from center to edge, equals 3° and 1/3 of that equals 1° ). this calculation
can be approximated for any finder scope with a known field of view.
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the mount's setting circles can now be used to determine just how close
the polar axis is to the ncp. first, aim the telescope tube (be careful
not to move the mount or tripod legs) at a bright star of known right ascension
near the celestial equator. turn the right ascension setting circle to match
that of the bright star. now, rotate the telescope tube until it reads 2
hours 30 minutes (the right ascension of polaris) and +89¼° declination.
polaris should fall in the center of the finder's cross hairs. if it doesn't,
once again move the mount in latitude and azimuth to center polaris.
this procedure aligns the telescope mount to within a fraction of a degree
of the ncp; good enough to track a star or planet in a medium power eyepiece
without any noticeable drift. however, long exposure astrophotography is
far less forgiving and film will easily reveal even the smallest amount
of motion. at this point, you may be wondering why bother polar aligning
any more accurately if you can use the slow motion controls or drive corrector
to keep a guide star centered in the cross hairs of an eyepiece. unfortunately,
keeping the guide star centered in the cross hairs is only half the battle.
since, the polar axis is not perfectly in line with the earth's axis, the
stars in the field of view will slowly rotate as you guide. you will get
a sharp image of the guide star, but the other stars on the photograph will
appear to rotate around the guide star. this is also why you cannot accurately
do guided photography with an altitude-azimuth (altazimuth) style mount.
the above method of polar alignment is limited by the accuracy of your telescope's
setting circles and how well the telescope is aligned with the mount. the
following method of polar alignment is independent of these factors and
should only be undertaken if long-exposure, guided photography is your ultimate
goal. the declination drift method requires that you monitor
the drift of selected stars. the drift of each star tells you how far away
the polar axis is pointing from the true celestial pole and in what direction.
although declination drift is simple and straight-forward, it requires a
great deal of time and patience to complete when first attempted. the declination
drift method should be done after the previously mentioned polar alignment
steps have been completed.
to perform the declination drift method, you need to choose two bright stars.
one should be near the eastern horizon and one due south near the meridian.
both stars should be near the celestial equator (i.e., 0° declination).
you will monitor the drift of each star one at a time and in declination
only. while monitoring a star on the meridian, any misalignment in the east-west
direction is revealed. while monitoring a star near the east horizon, any
misalignment in the north-south direction is revealed. as for hardware,
you will need an illuminated reticle ocular to help you recognize any drift.
for very close alignment, a barlow lens is also recommended since it increases
the magnification and reveals any drift faster. when looking due south,
insert the diagonal so the eyepiece points straight up. insert the cross
hair ocular and rotate the cross hairs so that one is parallel to the declination
axis and the other is parallel to the right ascension axis. move your telescope
manually in r.a. and dec to check parallelism.
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first, choose your star near where the celestial equator (i.e. at or about
0º in declination) and the meridian meet. the star should be approximately
1/2 hour of right ascension from the meridian and within five degrees in
declination of the celestial equator. center the star in the field of your
telescope and monitor the drift in declination.
if the star drifts south, the polar axis is too far east.
if the star drifts north, the polar axis is too far west.
using the telescope's azimuth adjustment knobs, make the appropriate adjustments
to the polar axis to eliminate any drift. once you have eliminated all the
drift, move to the star near the eastern horizon. the star should be 20
degrees above the horizon and within five degrees of the celestial equator.
if the star drifts south, the polar axis is too low.
if the star drifts north, the polar axis is too high.
this time, make the appropriate adjustments to the polar axis in altitude
to eliminate any drift. unfortunately, the latter adjustments interact with
the prior adjustments ever so slightly. so, repeat the process again to
improve the accuracy, checking both axes for minimal drift. once the drift
has been eliminated, the telescope is very accurately aligned. you can now
do prime focus deep-sky astrophotography for long periods.
note: if the eastern horizon is blocked, you may choose a star near the
western horizon, but you must reverse the polar high/low error directions.
also, if using this method in the southern hemisphere, the direction of
drift is reversed for both r.a. and dec.
even with a telescope with a clock drive and a nearly perfect alignment,
most beginners are surprised to find out that manual guiding may still be
needed to achieve pinpoint star images in photographs. unfortunately, there
are uncontrollable factors such as periodic error in the drive gears, flexure
of the telescope tube and mount as the telescope changes positions in the
sky, and atmospheric refraction that will slightly alter the apparent position
of any object.
polar alignment, as performed by many amateurs, can be very time consuming
if you spend a lot of time getting it more precise than is needed for what
you intended to do with the telescope. as one becomes more experienced with
practice, the polar alignment process will become second nature and will
take only a fraction of the time as it did the first time. but remember
that when setting up your telescope's equatorial mount, you only need to
align it well enough to do the job you want.
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