(This article originally appeared in the Q1 1987 STAR Newsletter and was written by Jerry Watson)


Finding Directions in Your Telescope

When making observations of double stars, especially newly found ones, I like to estimate the position angle (PA). A more formal definition of PA will be given shortly, but you are essentially determining the direction of the fainter member (B) relative to the brighter star (A). Is B south, north-northeast, etc.,of the primary A? To make such an estimate, you must first determine the direction ‘north’ in your telescope’s field of view. Once you have done so, and then estimated the PA, you can compare your PA with that given in your reference book for the double star. In a few cases over my years of observing, there was sufficient discrepancy to indicate that I was not observing the double star sought, but rather a neighbor that otherwise fit the general description. In a recent case involving the double star Castor (alpha) in Gemini, a large discrepancy between my PA estimate and that measured only 20 years ago was real, and due to orbital motion of this famous stellar pair!  Position angle is defined as the angle measured from north (through east) to a line passing through the two stars. In the illustration below, the circle can be thought to represent your telescope’s field of view, which contains a double star. The brighter member, A, is placed in the center. The direction north is ‘up’ and east is to the right. The circle is graduated in degrees from 0 degrees (or 360) at the north point; east is 90 degrees; south is 180 degrees; and west is 270.


Our double star has a PA of 135 degrees. Less formally, and analogous to the terminology of the 16-point compass used to indicate wind directions on the earth, one could say that B is about southeast (SE) of A.  When viewing through the telescope, north is likely to be at any position but ‘up’ in your field of view.

Directions in the telescope are rotated and/or inverted by the interplay of the various mirrors and lenses, not to mention your viewing angle when looking through the telescope. One thing is for sure, an astronomical object allowed to drift across the field of view of an undriven telescope will move exactly from east to west in response to the earth’s west to east rotation. Hence the line joining PA’s 90 and 270 is indicated in “the minds eye” by watching (for a minute or two) the drift of a double star across the field. If the “comes” (pronounced ko’meez, another name for the fainter B member) exactly precedes the primary A as they drift together, then B is obviously west of A (PA 270). If however, B exactly follows A along this east-west line, then B must be east of A (PA 90). Normally, however, B will lie left or right of the exact east-west line, so, the remaining problem is to determine whether north is on the left or right side of the drift line.

Which side is north depends on the type of telescope you are using and whether you are looking “straight through” or through a star diagonal. The following table gives the direction north relative to the drift direction (east towards west) for various types of observing equipment.  Relative to the drift

Type of instrument                                  North is
—————————–                       ————
Binoculars                                             Left
Refractor (straight through)                        Left
Refractor (with star diagonal)                         Right
Newtonian reflector                                    Left
Catadioptric (straight through)                   Left
Catadioptric (with star diagonal)                      Right

(Catadioptric telescopes are the lens-mirror optical systems such as the Celestron-8)

It’s interesting to note that an astronomical star chart is normally oriented with north ‘up’ and east to the left. Therefore, if the chart is rotated to coincide with east-west drift line, the view in the eyepiece will match the star chart. The exception to this is when an instrument is using a star diagonal, in which case the image will then be inverted.

The above “drift method” of field direction determination can be used with any telescope on any type of mounting. If, however, you have an equatorially mounted telescope with the polar axis even approximately aligned to the Pole Star (Polaris), you do not have to wait for star drift to orient the field. East-west is indicated by the path of objects moving across the field as you rotate the telescope around the polar axis. North-south is revealed when the telescope is rotated around the declination axis. As an example, suppose that as you move the telescope towards the west (be careful not to allow motion around the declination axis – some telescopes allow you to lock this axis) the comes B follows the brighter star A across the field. Thus B lies westward of A. Now move the double star back to the center of the field. Without allowing movement around the polar axis, suppose that as you move the telescope northward, B appears to follow A. Thus, B lies northward of A, and combined with our previous determination we finally conclude that B is northwest of A. We repeat the experiment and refine our PA estimate when we note that B more nearly follows A when the telescope is moved northward then when it is moved westward. We thus estimate the PA as north northwest or approximately 330 degrees.

Either the above method, or the drift method, can with some practice provide PA estimates with an accuracy of plus or minus 10-20 degrees. This is adequate for our purpose of a rough check of the professional PA’s given in the double star lists.  As a practice double star, try one of the most famous; Alcor and Mizar (Zeta) in the handle of the Big Dipper in Ursa Major. This star is well placed in the northeastern sky in the early evening, and will remain easily viewable in the night sky for the next several months. These two stars can be seen by eye in sufficiently dark skies, and in the telescope they are widely separated (11.8 minutes of arc) with Alcor almost directly east (PA 72) of the brighter Mizar. The former star should appear to trail behind Mizar, but slightly on the north side of the east to west drift line. Even more interesting is the duplicity of Mizar itself. A magnification of 60x in my 8 inch Newtonian nicely reveals Mizar to consist of two components separated by 14.4 seconds of arc; A is magnitude 2.4 and B is 4.0.  This is an easy double star in small telescopes, and was the first to be discovered telescopically (1650), and the first to be photographed (1857). The comes B is in PA 151 degrees or about south-southeast of A. See if you can confirm this with your telescope. (For more on Alcor and Mizar, see Volume Three of Burnhams’s Celestial Handbook.)

To conclude this discussion, I want to point out that your ability to find directions in the telescope’s field of view has many more uses than just double star position angles. You can estimate the orientation of the major axis of an elliptical galaxy; indicate that a prominent red star is east-southeast of the center of a star cluster; locate a particular faint galaxy which is northeast of some star; report with confidence that your newly discovered comet has moved directly east from its first sighted position; record in your observing notebook that Saturn’s moon Titan is three ring diameters east-southeast of the planet; etc. Field directions included with a sketch of an astronomical object adds value. Directions are a necessity if the sketch is to be compared to an astrophoto or with someone else’s sketch, or perhaps with another sketch of your own made at another time.  I would be most interested to know of your results in applying the above. If you have questions bring your experiences to club meetings so all can learn.