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"Polar Alignment of Telescopes in the
Southern Hemisphere" by R W Jones and S N de Villiers. First published in
MNASSA, 54, Dec 1995, p 108.
"A quick and acurate method of polar alignment, well suited for
use with a portable equatorial telescope, is described." |
OPERATORS OF PORTABLE
telescopes in the southern hemisphere need a quick and accurate
method of aligning the polar axis of their instrument parallel to the earth's
axis. Telescopes with fixed installations require only one polar alignment, but
portables must be aligned every time they are used. A precise polar alignment is
a basic requirement for any equatorially mounted telescope, in finding celestial
objects from their hour-angle and declination co-ordinates.
To minimise the time required for realignment of a portable
instrument, three holes for the tripod feet should be provided in a fixed
surface, such as brick or cement-slab paving. The holes for the tripod can be
located by placing sigma Octans at the cross-hairs of the finder telescope, when
declination is set at 90°. For observing sites not in the backyard,
binoculars are useful for reaching this rough location. Near the south celestial
pole, in the constellation Octans, there are four bright stars arranged in a
large irregular rectangle. Figure 1 [reproduced at the bottom of this
webpage] shows how they appear in a 6-power finder telescope.
THE NEXT STEP
is to determine the size of the field of the main telescope eyepiece. This is
done by timing the transit of a bright star near the celestial equator across
the field of the eyepiece (with drive motor off). Since the earth turns through
15 degrees in 3600 seconds, the size of the field in degrees can be obtained by
multiplying the transit time in seconds by 0.004167. In the case of an 8-inch
telescope using a low-power eyepiece, transit time was 180 seconds, so the field
was 0.75 degrees across, or 45 arc minutes.
Then carefully align the finder with the main telescope, by
sighting the latter on an object at least a mile away in daytime, and adjusting
the finder until it agrees. This method is easier than using a star for
sighting, since terrestrial objects stay put while adjustments are being made.
With the Octans rectangle on the finder scope, and with
declination set at -90°, adjust the azimuth and altitude controls until the
pole, as represented by the center of the finder cross-hairs, is correctly
positioned in relation to the stars sigma Oct and B Oct (see Figure 1). The pole
is on a line through sigma Oct which is at right angles to a line from B Oct to
sigma Oct, and the pole is at a distance from sigma Oct of 2sigmaB (Figure 4).
There will now be a group of three bright stars on or just off
the main telescope field (Figure 3), in the shape of a number 7 (or the Greek
capital letter gamma if a diagonal is used). The magnitudes and relative
locations of the three stars are shown in Figure 2. The distinctive seven or
gamma shape greatly assists in identification. The pole is located towards the
inner side of the space enclosed by the 'seven'.
Now readjust the azimuth and altitude controls so that the two
brighter stars of the 'seven' form an equilateral triangle with the pole (centre
of the eyepiece field), as shown in Figure 3. The optical axis of the telescope
is now very closely aligned with the earth's axis.
IN DAILY USE OF A
portable telescope, this accurate alignment takes only about
two minutes to regain when the telescope is brought out, because the 'seven'
will normally be on the main scope when first looked at, thus eliminating the
preliminary step involving the finder scope.
The position of the south celestial pole relative to the stars
changes slowly with time, due to the effects of proper motion, nutation, and
precession. The first two of these factors can be neglected for a short period
like 20 years, but the precession has a marked effect in that length of time
(33.4 arc minutes per century).
Using a method described by Jean Meeus in his book Astronomical
Algorithms (Willmann-Bell, Inc., 1991) a computer program was written which
enabled calculation of the position of the pole at 10-year intervals between
1950 and 2050.
These results are plotted on Figure 4. It will be seen that the
pole can be expected to travel from its 1995 position of making an equilateral
triangle with the two bright stars of the 'seven', until it passes between them
in about 42 years. |
