Polar Alignment Made Easy!
(Here's one for German Equatorial Mounts or "GEMs".)
In order to use your scope to its fullest potential, it must be polar aligned.
Polar alignment means the telescope drive motor turns the telescope in the exact
opposite direction as the rotation of the Earth. This has the effect of making the
stars appear stationary, they don't drift out of the eyepiece, making it possible to
track any deep sky object for extended periods of time without any adjustments by the
The first step in polar alignment is alignment of the finderscope optics
with the main optics. I do this while it is still light enough to see objects on the
ground or on Earth. After the Sun goes down, but before twilight ends is good. The first
thing I do is make sure the cross hairs in my finder scope are aligned with the two axis'
of the telescope's optics. I do this by aiming the finder at an object on the ground and
turning the telescope in declination, (up and down) and making sure an object in the finder
follows along the cross hair in one direction. If the object appears to slide to either
side of the cross hair, I loosen one of the screws and turn the finder a small amount and
try again until the object follows the crosshair.
Next is lining up the main optics
with the finder scope. Using low power on the main scope, point it at something on the ground
in the distance some distance away. Choose an object
that is distinctive-a lone tree atop a mountain or an insulator on a telephone pole. When
this object is centered in the eyepiece, check the finder scope view. Use the three adjusting
to move the crosshairs so they come together at the exact point on the object that appears
in the eyepiece. Make sure the screws are tightened once the finder is aligned or it may
slip when the scope is moved.
The last step in collimating all your optics is to line
up the Telrad sights, (if you have one, and who doesn't?). With the scope still aimed at
a distant object, and your Telrad on,
turn the three little dials at the back of the Telrad until the object appears in the center
of the center circle. You may have to wait a while for the sky to darken enough to see the
Telrad bullseye, but once it is set, it should stay pretty well aligned, even after you take
it off after a night of observing.
Now, on to polar alignment. Your tripod should
be equipped with a "wedge", a device that allows you to set the angle of the scope to parallel
your latitude. What this really means is your forks will be pointed to the same height in
the sky as Polaris. Close counts! :-) When I set up in the field, I bring along several
bricks of different thicknesses. I put them under the legs so I can adjust the angle and
give me a more stable, nearly level base. If your wedge is adjusted close to your latitude,
you can use the bricks to aim it exactly.
Next, turn your scope in declination so that the main
objective points away from the base and your declination setting circle reads 90 degrees.
Turn the scope in right ascension until the forks are level with each other. The mark you
use to read RA should be directly in the middle of the drive base. The RA reading does not
matter at this point. Now, point the main objective as close to North as possible. It
be within a few degrees of Polaris. What you want to do is move the mount around back and
forth and up and down enough to get Polaris in your finder scope. Once this is done, you
want to move the scope until Polaris is 1/2 way between the edge of view and the middle where
the cross hairs meet. The direction here is important, too. The North celestial pole is
not exactly on Polaris, but very close to 1/2 a 6 x 30 finder scope field away in the direction
of the last star in the handle of the Big Dipper, also called Alkaid or Eta Ursa Majoris.
All you have to do, once you get Polaris in your finder, is turn the scope tube in RA until the
RA crosshair runs along a line from Polaris toward Alkaid. Then, place Polaris on the
crosshair, 1/2 way towards Alkaid and you are close enough to polar alignment to keep an
object in your main field of view for at least an hour.
While this level of accuracy
is good enough for viewing, it is still insufficient for astrophotography. The most
accurate, and also the most tedious method I've tried so far is Called "Drift Alignment".
It takes some time and patience, but it's well worth the effort. You need to pick and
locate a bright star that's close to the celestial equator and just East of due South.
A higher power eyepiece with a reticle crosshair helps here. Align the star as close
to the center lines as possible. With the drive on, watch the star as it drifts away from
center. The farther you are from aligned, the faster it will drift. Ignoring the drift
in RA, note which way the star drifts. If it drifts North, your tripod is aligned too far
to the West. If it drifts South, you are too far East. Turn the tripod a very small amount
and try again. You can get a feel for how much drift there is by paying attention to how
long you observe the drift at each stage. If the star passes the meridian, choose another.
Once you get the
drift to a minimum, choose another bright star close to the equator but near the Eastern
horizon, about 30 degrees elevation. Center it in the eyepiece and watch for drift. If it
drifts North, you are aimed too high. If it drifts South, you are aimed too low. Use
different bricks to raise or lower accordingly. Find another star and repeat the process.
Of course, you may not need to do this in the field if all you are after is rough alignment
for a nights worth, but it will help if you want to set up something more permanent.
When I started out and got my tripod aligned, I put down some cement blocks in
the ground where my tripod
would go and marked them with nails so I could just go out and plop down my scope with a
minimum of fuss and be close to aligned.
That should do it for alignment. What
you need to do now is to set your RA setting circle. Follow the link at the bottom of this
text to get a printout of bright stars and their celestial coordinates. Choose one that's
close to the meridian and use its coordinates to set your circle. In the Northern hemisphere,
the numbers to use are closest to the center of the fork mount. The numbers mark hours
of right ascension, the long marks denote 15 minute intervals and the short marks denote
5 minutes of RA. You'll have to fudge a little for values between 5 minutes, but you'll
get the hang of it with practice.
After that, it's just a matter of dialing in
the coordinates for an object you want to see and surfing around a little until you find
it! One way to check the accuracy of your alignment is to calibrate your circles on one
star, then point to another star on the list and note the difference in declination. This
will give an idea of how far off you are.
If you have any
questions at all, please don't hesitate to email me!
9/16/99: A number of people have asked for a little more
explanation of exactly why polar alignment is important, and especially what each
step of this procedure is accomplishing. Please click here
for a little more detail.
Steve Dodder aka Astroman firstname.lastname@example.org
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ARVAL Guide Star Catalog
Page URL: http://www.stonehavenobservatory.com/Alignme.html
Copyright 1999, Steve Dodder
: Steve Dodder