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Precise polar alignment
Polar
alignment of your equatorial mount can be done in several ways. The
most precise way is to use the drift of a star in declination (north or
south on the sky) in your field of view.
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 using latitude and north celestial pole alignment techniques.
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.
Make
the appropriate adjustments to the azimuth of 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.
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. If this is done in the southern hemisphere, swap south and
north in the above instructions.
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As
the earth rotates around its axis, the stars appear to move across the
sky. If you are observing them using an altitude-azimuth (alt-az)
mount, they will quickly drift out of view. Readjustments to get them
back in view are awkward and frequent or require computerized tracking.
In
order to avoid these problems for either visual astronomy or
astrophotography, you need a different type of mount that’s oriented or
aligned to make following the apparent motions of the stars much easier
than with an alt-az mount.
A telescope on an equatorial mount
can be aimed at a celestial object and easily track the daily motion,
keeping it in your eyepiece. It works by first polar aligning or
inclining it at an angle equal to your latitude and pointing one axis
(called either the polar axis or right ascension (RA) axis) in the same
direction as the earth’s rotational axis (towards the celestial pole).
Once the polar axis is parallel to the earth’s axis and turned at the
same rate of speed as the earth, but in the opposite direction, objects
will appear to stand still when viewed through your scope. There is no
rotation of the field of view and tracking can be extremely accurate,
making the equatorial mount perfect for astrophotography. It has two
motions: in RA (east-west) and in declination (dec, north-south). With
the use of setting circles, a polar-aligned equatorial mount can
quickly find celestial objects.
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.
Aligning
the telescope to the earth's rotational 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.
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Rough polar alignment of your equatorial mount can be done in several ways. The easiest way is to use your latitude.
The
polar axis of an equatorially mounted scope must point at or be
polar-aligned to the north celestial pole, the point in the sky around
which all the other stars appear to rotate. The pole is directly above
the north point on the horizon. So your axis must point both north and
also be tilted up at an angle. Since the altitude of the north
celestial pole is always equal to your latitude on the earth, you can
just use the scope’s latitude adjustment to raise the polar axis to the
right angle.
Your latitude will equal the altitude of the north
celestial pole.
Either
look on a map, use Google Earth or an almanac to find your observing
site’s latitude. Unlock any latitude adjustment screws on the sides of
the mount and turn the latitude adjustment screws until the index on
the polar axis reads your latitude. Tighten the adjustment screws if
needed to secure the latitude setting. (You may also need to loosen the
center pivot bolt by turning the hex nut to allow the equatorial mount
head to be tilted.)
Now complete the polar alignment by turning
the entire mount (not either axis – both should be clamped tightly) to
align the upwards end of the polar axis with north on the horizon. If
doing this at night, north is located directly below Polaris, the Pole
Star.
Another
frequently used method is to point to Polaris. This star is located
only one degree from the north celestial pole, the point in the sky
around which all the other stars appear to rotate, and where the polar
axis of a properly aligned equatorial mount should point.
First, set up the mount so that the polar axis is pointing north.
Second,
unlock the declination clamp and move the scope in declination so that
the tube is parallel to the polar axis. Your declination setting
circles should read 90 degrees in this orientation. Clamp the
declination lock.
The last steps involve moving the entire mount. Don’t use either the RA or Dec motions to change the position of the tube.
Third, move the mount in altitude and azimuth until Polaris is in your finder’s field of view or centered in your finderscope.
Fourth,
tweak the position of the mount by again moving the mount, this time
centering Polaris in the eyepiece field of view. Altitude can be
adjusted using the latitude adjustment screw or shortening-lengthening
tripod legs.
The alignment is now good enough for visual purposes.
To
refine this alignment, get a chart showing the offset of Polaris from
the pole and move the mount so that this point in the sky is centered
in the eyepiece field of view. You now have an excellent polar
alignment well within one degree of the true north celestial pole.
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Accurate polar alignment
You
can use your finderscope (finder) to accurately polar-align your
equatorial mount. This is more accurate than a rough alignment using
the mechanical scales on your mount or roughly pointing the whole mount
towards Polaris.
To
use your finder as a polar finder is a three-step process. First, the
optical axis of the finder must be aligned to the mechanical polar axis
of your equatorial mount. Second, Polaris will be centered in the
finder (polar axis pointing towards Polaris). Since Polaris is nearly
one degree away from the true north celestial pole (NCP), the last step
will offset the finder’s view and the polar axis to get true polar
alignment.
The alignment of the optical axis of the finderscope
with the mechanical polar axis of the mount can be done either at night
with Polaris or (perhaps more easily) during the daytime on a distant
building or landmark. If you do align during the day, use the latitude
adjustment screws and the tripod to level the polar axis to make it
easier to do.
NOTE: Any star can be used to get the finder
optical axis and polar axis aligned. Polaris is chosen for convenience
and also because it will be used in the second step.
You’ll flip
the mount several times and recenter Polaris (or the landmark) in the
finderscope each time, successively getting closer to alignment of the
finderscope and polar axis.
Working
with Polaris, start by setting up your mount as you would for polar
alignment. The Dec setting circle should read 90 degrees. Unclamp the
RA and rotate the mount until the telescope and finder are all the way
to the left or right, Dec axis horizontal. Get Polaris in the field of
view of the finder and centered in the crosshairs by moving the mount
(using fine adjustment screws). Now move (flip) the mount all the way
to the opposite side (180 degrees or 12 hours RA away from the original
position). Note the shift of Polaris off the crosshairs. Actually the
finderscope and the crosshairs themselves have rotated in a small
semicircle around where the polar axis points. You can see where that
is by looking through the finderscope as you rotate the mount, watching
for the center of motion. Clamp the mount and turn the three setscrews
around the finderscope to move the crosshairs over this pivot point.
Recenter Polaris by only moving the mount.
Even with the telescope positioned 180 degrees away around
the mount, the telescope (and finderscope) should still be
pointing at the same object in the sky.
Repeat
the flipping-setscrew-recenter Polaris procedure. Each time you go from
one side to the other, the off-center distance of the crosshairs from
the pivot point will be smaller. After three or four repeats, the
crosshairs won’t move when you flip the mount. You will be pointing at
Polaris. Your polar finderscope optical axis is now pointing in the
same direction as the polar axis.
(If
you did this during the day with a landmark, now wait until dark. Set
up your mount as you would for polar alignment. The Dec setting circle
should read 90 degrees. Use the latitude scale and adjustment screws to
center Polaris in the finder.)
When rotating the scope and finder 180 degrees around the polar axis,
the crosshairs will rotate around where the polar axis is pointing
(this pivot point is the "X" in the right-hand figure). Adjusting the
finder and the equatorial mount until an object remains centered in the
crosshairs during the rotation aligns the finderscope with the
telescope's polar axis.
Steps
one and two are done and the polar axis of the telescope is aligned
with Polaris, but as any star atlas will reveal, the true pole lies
about ¾-degree away towards the last star in the Big Dipper’s handle
(Alkaid). To make this final adjustment, the telescope mount will need
to be offset from Polaris towards the actual NCP.
Since Polaris
makes a complete rotation around the NCP once a day, how far should the
mount be moved and in what direction? One easy way is approximation.
Guesstimate the direction by using Alkaid. Guesstimate the amount by
knowing the field of your finderscope and dividing it by the ¾-degree
distance of Polaris from the pole.
Example: On August 1, at 8PM, Alkaid is above and to the left of Polaris in the 10 o’clock
position. You have a 6-degree field-of-view finderscope. Starting with
Polaris on the crosshairs, use the fine adjustment screws to shift the
mount in altitude (latitude) and azimuth up and left by one-eighth
finder field (6 divided by 0.75).
The
true North Celestial Pole (NCP) lies less than a degree away from
Polaris in the direction of Alkaid, the last star in the handle of the
Big Dipper (Ursa Major).
Now
use the setting circles to check how close the polar axis alignment is
to the NCP. Unclamp the axes and swing the scope’s tube to a bright
star of known RA and Dec near the celestial equator. Turn (set) the RA
setting circle to this star’s RA. Now move the tube until the RA
setting circle reads 2 hours 31 minutes and the Dec circle reads +89
degrees 15 minutes. These are the coordinates of Polaris and the Pole
Star should now be in the finder’s crosshairs. If it’s off, once again
move the mount in latitude (altitude) and azimuth to center Polaris.
Now
you have a polar alignment for your scope within a fraction of a degree
of the NCP. This is excellent for visual purposes and short-exposure
photographs piggybacking on the main tube. However, guiding corrections
and field rotation will still be problems for long-exposure
astrophotography, which demands the most precise polar alignment.
NOTE:
At the completion of this process, Polaris may very well not appear in
the center of your main scope's eyepeice field. This is because the
optical axis of the finder and the polar axis are now parallel. But the
finder's optical axis and the main tube's optical axis may not be
parallel. So centering a star in your finder won't necessarily center
it in your eyepiece. To overcome this, once you've achieved polar
alignment, you can realign the finder with the main tube optics to
return the finder to normal operation with the main scope.
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