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.
Caption: The North Celestial Pole (NCP) is the point in the sky
around which all the stars appear to rotate. The star Polaris
lies less than a degree from the NCP and it can be used to
roughly polar align a telescope. However, for accurate polar
alignment, the polar axis of the telescope's mount needs to be
aligned to the true NCP.
Rough Polar
Alignment:
Caption:
By matching the latitude angle of the telescope mount
with the latitude of your observing site, you can
easily approximate the position of the North Celestial
Pole (NCP).
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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.
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.
Caption:
Even with the telescope positioned 180º around the
mount, the telescope (and finderscope) should still be
pointing at the same object in the sky.
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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.
Caption:
When rotating the finderscope 180º around the
polar axis , the cross hairs will rotate around the
point which the polar axis is pointing (indicated by
the "X" in Figure B). Adjusting the
finderscope and the equatorial mount until an object
remains centered in the cross hairs indicates that the
finderscope is aligned with the telescope's polar
axis.
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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 1st 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 finderscope with a known field of
view.
Caption:
The true North Celestial Pole (NCP) lies less than a
degree away from Polaris in the direction of the last
star in the handle of the Big Dipper (Ursa Major).
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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.
Precise Polar
Alignment
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.
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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