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Frequently Asked Questions 

 

Polar Alignment 
of an Equatorially Mounted Telescope

I. The Celestial-Coordinate System

The celestial-coordinate system is an imaginary projection of the Earth's geographical coordinate system onto the starry sphere which seems to turn overhead at night. This celestial grid is complete with equator, latitudes, longitudes and poles, and it remains fixed with respect to the stars.

(Actually, the celestial-coordinate system is being displaced very slowly with respect to the stars, because the Earth's axis is very slowly changing the direction of its point. This effect is slight, however, and in any case is being continually accounted for as new star atlases are published.)

The celestial equator is a full 360 degree circle bisecting the celestial sphere into the Northern Celestial Hemisphere and the Southern Celestial Hemisphere. Like the Earth's equator, it is the prime parallel of latitude and is designated 0 Degrees. The celestial equator passes through the constellations Orion, Aquila, Virgo and Hydra.

The celestial parallels of latitude are called "coordinates of declination (Dec.)," and like the Earth's latitudes they are named for their angular distance from the equator. These distances are measured in degrees, minutes and seconds of arc. Declinations north of the celestial equator are "+," and declinations south are "-." The poles are at 90 degrees, with the equator at 0 degrees

The celestial parallels of longitude are called "coordinates of right ascension (R.A.)," and like the Earth's longitudes they extend from pole to pole. There are 24 major R.A. coordinates, evenly spaced around the equator, one every 15 minutes.

Like the Earth's longitudes, R.A. coordinates are a measure of time as well as angular distance. We speak, for example, of the Earth's major longitudes as being separated by one hour of time because the Earth rotates once every 24 hours. The same principle applies to celestial longitudes since the celestial sphere appears to rotate once every 24 hours.

Astronomers prefer the time designation for R.A. coordinates, even though the coordinates denote locations on the celestial sphere, because this makes it easier to tell how long it will be before a particular star will cross a particular north-south line in the sky.

So, R.A. coordinates are marked off in units of time eastward from an arbitrary point in the constellation Pisces. The prime R.A. coordinate which passes through this point is designated "0 hours 0 minutes 0 seconds." All other coordinates are named for the number of hours, minutes and seconds that they lag behind this coordinate after it passes overhead moving westward.

Given the celestial-coordinate system, it now becomes possible to find celestial objects by translating their celestial coordinates into telescope point. For this purpose, your telescope may come equipped with setting circles. One dial is the setting circle for R.A. The other dial is your setting circle for Dec. You can use these circles to acquire celestial objects once you have properly mounted your telescope on its equatorial mount and pointed the polar axis of your telescope toward the North Celestial Pole. If you have an altitude-azimuth (Alt-Az) type mounting, such as a Dobsonian, you will not be able to use setting circles.

II. Lining Up on the Pole

The celestial pole is that imaginary point on the celestial sphere toward which the Earth's axis of rotation points. It is around this point that the stars appear to move nightly - their paths being concentric circles with the celestial pole at the center. If the polar axis of your telescope points directly at the celestial pole, then a star at any declination may be kept centered in the field of your telescope simply by rotating the telescope in right ascension, or by letting the electric clock drive rotate your telescope in right ascension.

For casual visual observing, a simple polar alignment on the north star, Polaris, is adequate. Polaris, which is within 1-degree of the true north celestial pole, is easy to find. The pointer stars in the bowl of the Big Dipper point straight to Polaris (see the diagram of the Celestial Polar Region).

Tilt the telescope tube until the declination axis of your telescope points towards Polaris, or the declination circle on your telescope reads 90-degrees. Then, if possible, lock the Dec. clamp, and then move the tripod and adjust the altitude until Polaris is in the center of the field of view. The telescope is now ready to be used. Your setting circles should read to within approximately one-degree accuracy and the drive should keep an object in the field of view for a considerable period of time.

III. Finding the Big Dipper

Because the Big Dipper appears to revolve around Polaris, it will be found in different locations depending upon the season and the time of the night. Face north and look for the Big Dipper (and Polaris) in the positions shown above in early evening, local time. It takes 6 hours for the Big Dipper to revolve the 90' from one of the indicated positions to the next.

IV. Polar Aligning on Polaris

When the declination pointer reads 90-degrees, the optical axis of your telescope should be parallel to the polar axis. At this setting, if Polaris is visible through the telescope, the polar axis of your telescope is aligned on Polaris.

If you would like to achieve a more accurate polar alignment after aligning on Polaris, repoint the telescope at a bright star near the celestial equator. Look up that star's right ascension in a star atlas (or use the Alphabetical Listing of Bright Stars at the back of this manual) and move the R.A. setting circle until the R.A. pointer is indicating the right ascension of the star you have chosen. Now turn your telescope in R.A. until it indicates the R.A. of Polaris (this ' is currently 2 hr. 10 min.) and lock the R.A ' clamp. Now move the tube (only in declination) until the declination pointer indicates 90'. From this point, continue moving the tube in the direction away from the Big Dipper (i.e., toward Cassiopeia) until the declination reads +89.2', the declination of Polaris. Lock the declination clamp. Now move the tripod and adjust the wedge until Polaris is centered in the field of view.

The telescope will now be aligned well enough for you to try deep sky photography using exposure times of up to 15 minutes or so without significant mistracking.

The Celestial Polar Region

The two stars in the front of the bowl of the Big Dipper point straight to Polaris. Polaris is less than 11 from the true North Celestial Pole (N.C.P.). Cassiopeia is the "W" shaped constellation on the side of the pole opposite the Big Dipper. See "Lining Up on the Pole".

V. Precise Polar Alignment for Astrophotography

This precise alignment method is desirable only if you intend to try long exposure, guided astrophotography. The advantages are that there will be no image drift in declination, there will be no star trailing caused by field rotation. the tracking will be more accurate, and your setting circles will read very accurately. Because it eliminates the need to make corrections in declination during long exposure astrophotography, it allows you to concentrate on R.A. corrections.

After the quick alignment methods described previously, you will need an illurninated reticle eyepiece for this more precise method. A Barlow lens will also speed the procedure considerably.

Insert the illuminated reticle (and Barlow if used) and repoint the telescope at a fairly bright star near where the meridian and the celestial equator intersect (preferably within ± 1/2 hour R.A. of the meridian and ± 5 degrees' of the celestial equator) and monitor the declination drift (ignore any drift in R.A.).

a. If the star drifts south, the polar axis points too far east.
b. If the star drifts north, the polar axis points too far west.

Move the telescope's polar axis in the appropriate direction until the north or south drift stops. Accuracy of this adjustment will be increased if you use the highest possible magnification and allow the telescope to track for a period of time.

Now repoint the telescope at a fairly bright star near the eastern horizon and near the celestial equator (the star should be at least 20 minutes' above the horizon and ± 5 degrees' from the celestial equator).

a. If the star drifts south, the polar axis points too low.
b. If the star drifts north, the polar axis points too high
.

Again, monitor only the declination drift using high magnification over a period of time. After you have made the necessary adjustments to stop the declination drift, you will have achieved a highly accurate polar alignment.

The same procedure may also be employed by Southern Hemisphere observers, but the directions of drift will be reversed.



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