Polar Alignment in Several Easy Steps
Developed by Fr. Lucian J Kemble, OFM and Dr. Peter Bergbusch - 1975
Page last updated: - 2000-01-23
Polar Alignment of a Telescope on Equatorial Mount
Fr. Lucian J. Kemble, OFM
Preliminary remarks:
I know I sometimes tend to get long-winded, so I will try to
keep this as brief but as comprehensible as possible.
This method is designed for clock-driven Fork or German mounts
and might not be of much use for Poncet or so-called barn-door
types.
This method is designed for telescopes fitted with fairly large
and
accurate setting circles. It will be useless on
small, badly-functional types, and I have no experience with
automated or electronic circles
VERY IMPORTANT: The telescope tube must be aligned with the
mount's polar axis, so that the SCOPE and AXIS point in such a way
that when the scope is pointed at the North Celestial Pole or [NCP]
then the Dec. circle itself will read +90. I will not go into
details of adjusting the Dec. Circle. User's manuals usually
explain the procedure. Off-alignment in this feature can lead to
some strange field-rotation streaks, etc., on photographs. It can
also lead to sometimes fairly large mis-readings when it comes to searching for
faint objects by coords alone. But the use of setting circles is another issue I
won't go into here except for use in polar alignment.
The method by no means minimizes the use of other methods, e.g. the
drift method which is used successfully by top-notch photographers
such as John Mirtle of Calgary. I have just found it quicker, very
accurate and, for my particular interests, applicable even in the
daytime. It was worked out in 1975 by myself and Dr. Peter
Bergbusch of Univ. of Regina. It's success was tested during the
eclipse of 1979 at Estevan, and successful, steady photography and
movie of the event were achieved with several scopes. Dennis di
Cicco gave an abbreviated description of a similar method in Sky and Telescope magazine,
Dec/86, p.570, using different reference stars. I will append the
reference stars' coords at the end of this explanation.
All coordinates used are those for 2000.0, since we are close enough to that
standard epoch. Originally I had to work out the coordinates, local
sidereal time, etc. by hand, later from The Astronomical Almanac
and now from a computer programs such as ECU. Invaluable
experience.
Principle behind the method:
The principle involved relies on the positions of four fairly
bright stars, one or two of which is always in one's observable sky
near the Celestial Equator: two of them along or very near the
hour-line joining Polaris and the true NCP; the other two on the
hour-lines at right angles to this line. Since Polaris is at RA 02
31 50 the reference stars I use are then roughly on the Equator at
the ~2.5, 8.5, 14.5 and 20.5 hour lines.
Keep in mind that the mount moves in Azimuth and Altitude; the
scope in Right Ascension and Declination.
Procedure:
- Scope on mount and leveled tripod or pier, positioned in
Azimuth and Altitude such that the RA axis is roughly pointing
towards Polaris.
- Scope, preferably without diagonal, with medium power eyepiece
pointed visually to one of the four reference stars closest to meridian;
locked in RA and Dec.
- RA setting circle adjusted to correct RA of the reference
star. Ignore Dec for the moment.
- Using ONLY RA and Dec circle readings, the scope is set to
correct RA and Dec of Polaris. Locked on.
- Mount or wedge only is now moved in Azimuth and Altitude
until Polaris is found and centered in finder. IMPORTANT: do NOT
use RA and Dec slow motion controls for this part of procedure.
Just move the whole assembly of wedge and tripod.
- Scope returned freely to center the reference star again and
locked on. RA setting circle again re-adjusted to correct RA of
this star, again ignoring Dec reading.
- Steps 4 and 5 repeated and, if further minor adjustments are
necessary, step 6 is repeated, using a higher power eyepiece.
- Check accuracy. By now Polaris should easily be found in the
eyepiece, using RA and Dec settings alone. For greater refinement,
using ONLY RA and Dec coords , not using slow motion controls nor
finder, seek out several other bright stars, preferably to the East
or West of the meridian and equator.
With practice, the whole procedure can be done in about 5 to 10
minutes. When looking for an elusive deep-sky object, use a low
power eyepiece first. If the polar alignment has been properly
done, the object should be in the field of the eyepiece. I have
most often achieved a 15' accuracy or better anywhere in the
visible sky. I am fortunate in having a solid pier, a Byers
Mount with its huge, accurate circles [with a hand-made vernier my
Dec circle is accurate to within 15"], and a very accurate
alignment, I seldom use my 12x80 finder, as I've often mentioned.
Great, great fun..
Astrophotography:
With this method I have had great success with piggy-back shots,
using anything from a 50mm to a 210mm lens, for as much as 10
minutes' exposure. I have even had some good 5-minute, unguided
shots through prime focus or with a focal reducer. There are, of
course, limitations, e.g. longer exposure on very high power, where
some kind of guiding is necessary.
Daytime Set-up for Observing:
Since, in step #1, Polaris cannot be seen [yet] just aim the mount
and scope as best you can, from known daytime true North in Azimuth
and Altitude. Use the CURRENT Sun's accurate position in RA and
Dec, [and then Venus, if visible] for steps #2,3,4,5. With care
and luck you should even see Polaris and several other bright
stars, refining the setting of RA each time. With daytime set-up I
have seen so many wonderful things, e.g. Venus near superior
conjunction only 2.5 degrees from the Sun, Mercury and all the other
bright planets. Saturn only 12 degrees away at the time Voyager II
was passing by, all the bright stars of the Pleiades,
all four components of the 'Double-Double', Epsilon Lyrae,
faintest at 6 mag, etc.; Hyakutake and Hale-Bopp within
minutes of sunrise or sunset. Great, great fun.
On a final note. I have used Vega as a test star so many times as
a check after a good polar alignment in the daytime that I know its
coords by heart. So that when, in the movie "Contact", I heard the
heroine read off the coords of her target star on the radio, I knew
exactly what that star was. Great, great fun...
Reference Stars:
In the 20+ years that I have used the method described above,
celestial coords have changed considerably, especially Polaris.
When I began I used the then-common 1950 coords and some
calculation to find current coords. This even involved determining
the exact sidereal time, using data from the annual Astronomical
Almanac. Great, great fun. I have a list of a number of good
reference stars for the current epoch stuck to my observatory wall,
from either the RASC Handbook list of bright stars, or the current
Astronomical Almanac, or ECU, or MegaStar, or ECU.
Here, then, are the reference stars I use - all for 2000.0
| Alpha UMi [Polaris] |
02 31 50.5 |
+89 15 51 |
| Xi 1[65] Cet |
02 12 59.9 |
+08 50 48 |
| Beta Cnc |
08 16 30.9 |
+09 11 08 |
| Alpha Boo [Arcturus] |
14 15 39.6 |
+19 10 57 |
| Theta Aql |
20 11 18.2 |
- 00 49 17 |
| Also Delta Ori |
05 32 00.3 |
- 00 17 57 |
For me this is just a tool to enable better observing and
photography. I do hope some of you may find it useful and it is
with that expectation and the joy of sharing the wonderful night and
daytime skies with you that I present it.
Respectfully submitted, and good luck. Lamplighter
Fr. Lucian was born in 1922 on a small farm in southern Alberta.
After spending four years during WW2 as a radio operator, he
entered the Franciscan Friars in 1946. Ordained in 1953 after 7
years' study in Philosophy and Theology, he spent almost all of
his priestly life in teaching and preaching. A member of the RASC
for over 27 years, he worked with a Celestron 11 on Byers Mount
in a shelter at St. Michael's Retreat, Lumsden, Saskatchewan. His
main interest in astronomy was searching out deep-sky objects, of
which he had over 5550 observed, drawn and noted on file. His
other main interests were photography, music and reading. Lucian
passed away on February 26, 1999. He is sadly missed.
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