Polar Aligning a Telescope

Whether you have a portable or observatory-mounted equatorial telescope, accurate polar alignment is a must if you plan to do any long-exposure photography.  Here’s one basic procedure you can use.

If you have a fork-mounted Schmidt-Cassegrain telescope, you can begin your polar alignment process during the day.  First, using a bubble level, make sure your telescope base is completely level.  Next, adjust the equatorial wedge so that it is set to the latitude of your observing location.  Then, point the telescope at the zenith and adjust both the right ascension and declination motions until a bubble level atop the telescope end cap reads completely level.  Then set your declination setting circle so that the declination reads the same number as the latitude of your observing location.

As soon as it is dark enough to see a star, align your finderscope and main scope so that the star is at the center of both fields.  When it is dark enough to see Polaris, set your telescope’s declination to 90° and adjust the azimuth of the equatorial wedge until Polaris is as near as possible to the center of the finderscope’s field of view.

With your unaided eyes, note the location of the 2nd brightest star in the Little Dipper, Kochab, relative to Polaris.  Kochab is the bowl star at the opposite end from Polaris that is closest to the bowl of the Big Dipper.  Presently, the North Celestial Pole is located 40 arcminutes (⅔ degree = about one-and-a-quarter moon-widths) away from Polaris in the direction of Kochab.  Adjust the altitude and azimuth of the equatorial wedge so that the center of the finder field is located ⅔° from Polaris in the direction of Kochab.  This may be quite difficult, so just do the best you can.

Now, pick a star on or very near the celestial meridian and the celestial equator (declination 0°).  Center the star in the main scope and make sure the clock drive is on.  If the star drifts south, make a slight adjustment to the equatorial wedge towards the west (counterclockwise).  If the star drifts north, make a slight adjustment to the equatorial wedge towards the east (clockwise).  Ignore any east-west drift.  Keep making adjustments until you have eliminated all drift.

Next, center a star in the main scope that is about 20° above the eastern horizon, and again very near the celestial equator.  If the star drifts south, adjust the altitude of the equatorial wedge so it points slightly higher in the sky.  If the star drifts north, adjust the altitude of the equatorial wedge so it points slightly lower in the sky.  Ignore any east-west drift.  Keep making adjustments until you have eliminated all drift.  (You can also use an equatorial star about 20° above the western horizon, but if the star drifts south you’ll need to lower the equatorial wedge, and if the star drifts north, you’ll need to raise the equatorial wedge.)

Now, pick another equatorial star on the meridian and repeat the procedure outlined in the two paragraphs above until no more adjustments are needed.

Your telescope is now precisely polar aligned.

Constant as the Northern Star

There are frequent astronomical references in the plays of William Shakespeare (1564?-1616).  One famous example is in the tragedy Julius Caesar, written around 1599, where Julius Caesar states,

“I am constant as the northern star,
Of whose true-fix’d and resting quality
There is no fellow in the firmament.”

Little did Shakespeare know that Ejnar Hertzsprung (1873-1967) would discover some 312 years later in 1911 that Polaris, the North Star, actually varies in brightness.  Of course, Shakespeare was referring to Polaris’ proximity to the north celestial pole, but there are multiple ironies in that Polaris varies in brightness—albeit a tiny amount—and it will not always be the “pole star”, thanks to the precession of the Earth’s axis.

Polaris is a classical Cepheid pulsating variable star, with a visual magnitude that has historically ranged as much as 1.9 – 2.1 over a period of about 4 days.

At a distance between 426 and 439 ly, Polaris is the nearest and brightest Cepheid variable star in our night sky. Polaris is a supergiant star (F7Ib) weighing in at about 5.4 solar masses. Polaris and its nearest companion star (F6V, 1.3 solar masses) enjoy a complete orbital pas de deux every 30 years.

Currently, Polaris lies only 40 arcminutes from the north celestial pole (declination +89° 20′).  As with all stars, the Earth’s rotation causes the stars to wheel around the celestial poles, although in the case of Polaris the angular speed is exceedingly slow, making it a great target for a telescope without a clock drive.

Let’s figure out how fast glacial Polaris moves. It traverses a tiny circle around the north celestial pole every sidereal day (23h56m04s), so what is its angular speed?  We need only divide the path length (the circumference of a circle of radius 40′) in arcseconds by the number of seconds in a sidereal day to get the angular speed in arcseconds per second of time. The circumference of a circle is 2πr, so plugging and chugging we get [(2)(3.141592654)(40*60)] / 86164 = 0.18 arcsecond per second of time. Sound like a lot, or a little?  This angular speed means that Polaris moves an arcsecond every 5.7 seconds, or 11 arcseconds every minute, or 11 arcminutes every hour. That’s just 4.2° per day.

Not quite a perfect pole star, but it will certainly deux.