Accurate polar alignment is
very useful for high-power visual observations and is essential for time
astrophotography. The easiest and least accurate method of polar alignment is
sighting along the mount RA axis and making it approximately pointing towards
Polaris. The most accurate but also time consuming is the drift method, where
one assess the mount displacement from being polar aligned by observing drift
of stars observed in the high-power eyepiece. Somewhere in between, there are
polar alignment scopes. They offer reasonable accuracy (actually, you may need
to refine polar alignment using the drift method, but it is quicker if you
start being fine aligned instead of rough) and are very quick to use.
However, there are two
provisions:
I have received a polar
alignment scope for my CG-5 mount in a separate box and without any
instructions, so I had to solve both problems.
First, I had to collimate
the optical axis of my polar alignment scope with the mount RA axis. The procedure
to do this is quite straightforward:
1.
Set the mount on the
tripod without the scope or counterweights attached. Remove polar scope covers
and rotate DEC axis until you see opening in it.
2.
Plug the polar
alignment scope into the lower–end opening and secure it with thumbscrews.
3.
Peer through the polar
scope eyepiece – you will see a reticle with a circle, crosshairs and small
circlet where one of the crosshairs touches the large circle.
4.
Using altitude and
azimuth controls, set the cross to be centered
on a distant object. You can use a terrestrial landmark, but you will need to
perform a lot of adjustment in altitude, so I prefer to center the crosshairs on Polaris – it won’t move far enough
providing that the collimation is
completed within few minutes.
5.
Turn the RA axis 180
degrees and lock it. Ideally, Polaris should still stay at the crosshairs. If
it moved, the polar alignment scope need collimation.
6.
Using three
thumbscrews, move the cross-hairs half-way from their initial position to the
current position of Polaris.
7.
Repeating steps 5 and 6
will lead to an accurate collimation of the polar alignment scope and the mount
RA axis in just 3 – 4 iterations – unless you collimate in a wrong way.
8.
Once the polar
alignment scope is collimated, you can use it to speed-up setting of your mount
for photography!
Below I described a quite
convenient and quick method of polar aligning your mount without the
calculations and corrections.
1.
Print the picture below
and get it laminated. This will be your polar alignment quick reference!
2.
Set the mount on the
tripod without the scope or counterweights attached. Remove polar scope covers
and rotate DEC axis until you see opening in it.
3.
Sighting along the
mount RA axis, set it approximately polar aligned – if you were careful you
should see Polaris in the polar alignment scope field of view.
4.
Keeping the picture in
your hands, turn it to match as closely as possible the relative locations of
circumpolar constellations on the sky – the two most important are Cassiopeia
and Ursa Major.
5.
The red arrow points
the direction of displacement of Polaris from the true North Pole. However, the
polar alignment scope reverses the view so actually Polaris should get into the
red circle!
6.
Turn the RA axis of the
mount until the small circlet matches location of the red circle on the
picture.
7.
Using altitude and
azimuth controls, set Polaris to be centered
in the small circlet.
8.
Voila – providing that
reticle was turned in a right position, you got a reasonably good initial polar
alignment – could be better than 5 arc min.
9.
Now you can use drift
method to refine your polar alignment.
Just a final remark:
Rough polar alignment can be
achieved without the polar scope mounted – you can simply peer through the
opening and adjust the altitude and azimuth controls until Polaris is
reasonably centered in the field of view.
Note. The picture included
was created using TheSky software from Software Bisque.