Polar Aligning Techniques

There are several ways to polar align any telescope. Which method you use depends on several factors. For the ETX, a one-time alignment on Polaris and then mounting a magnetic compass at the location where you most often use the ETX (or on a table or tripod used by the ETX) will probably work best. For more accurate aligning, the use of the "Star Drift" Method will yield the best results. Each of these are described here.

To do the initial polar alignment, first adjust the ETX for your Latitude by setting the Latitude on the ETX front leg (or adjusting your tripod/wedge tilt). Next, locate the "North Star", known as Polaris. In the Southern Hemisphere life is more difficult since there is no bright star along the earth's axis. But the next steps can be used in both hemispheres. Set up the ETX with the front leg (or what would be the front leg if you have the ETX mounted on a tripod/wedge combination) pointing opposite to the alignment direction (i.e., pointing South in the Northern Hemisphere). Swing the ETX to 90 degrees of declination according to its Declination Setting Circle. Rotate the ETX in Right Ascension until you can look through the Finderscope and Eyepiece. Now physically rotate the whole ETX, including the base, left or right until you can see Polaris in the Finderscope and then centered in a low powered eyepiece. If necessary make minor adjustments in the declination using the Declination control. You are now roughly polar aligned. The ETX manual discusses these steps in fairly good detail.

Once you have this rough polar alignment, it is nice to have an easier way to repeat it. This is especially true if you have to move the ETX from where you did the aligning. Adding a magnetic compass to the ETX (table or tripod/wedge) will assist in the repeatability. Place the compass in a position such that it can be used to point the ETX in the proper direction by using the compass alone. Note that in most locations on Earth, magnetic north has some "variation" from true north. If you can not locate true north (or south) because you can not see Polaris (perhaps due to a building or trees), using a compass may be your only solution. In this case, you must know the "magnetic variation" for your location. Check with your local airport, harbor, or library. Since magnetic navigation is important to both planes and ships, the airport or harbor may be your best source. The next time you set up the ETX, just position it such that the compass is pointed in the same direction as it was when you did the real polar alignment or is pointed correctly adjusting for magnetic variation.

For observational use, the polar alignment you get by doing the above is generally acceptable. However, if you want to try long duration astrophotography you will need a more accurate polar alignment. The best way to get this is by using the "Star Drift" Method. You can also use this method in place of the above two methods but it is faster if you have a rough alignment already made. Tom Price (tompr@csi.com) originally described the "Star Drift" Method in the July 1997 Feedback page and his instructions are repeated here:

[1] Do a rough polar alignment using the compass/level method or Polaris, whichever is convenient. This puts you in the ballpark.

[2] Put in your highest power eyepiece and aim the ETX at a star as close to South as you can find and with a declination between -10 and +10 degrees. Center the star in the eyepiece and turn on the RA drive so that the ETX will follow the star. Ignoring any East-West (RA) drift, watch to see if the star drifts North or South in the eyepiece, which will indicate an error in the azimuth of your polar alignment. If the drift is South, the azimuth is too far East of North and if it drifts North, the polar axis is pointing too far West of North. Make small adjustments of the azimuth until the star no longer drifts North or South over a period of several minutes.

[3] Now find a star near either the Eastern or Western horizon, again with a declination within 10 degrees or so of 0. Center the star in the eyepiece and again check for drift either North or South, again ignoring and drift in RA. If the ETX is pointing East, a drift South indicates that the polar axis is pointing too low. If the star drifts North, the polar axis is pointing too high. These directions will be reversed if you're using a star near the Western horizon or if you happen to be in the Southern hemisphere. Adjust the altitude of of the polar axis until the star no longer has any North-South drift for several minutes.

[4] If you had to make large corrections (several degrees) in either steps [2] or [3], repeat them, since they do interact somewhat. If only minor corrections were needed you're done and you have a very accurate polar alignment for your ETX.

If you happened to know the identity of the star you're using in step [2], compare its declination with what the ETX DEC setting circle shows. If there is close agreement you can probably eliminate step [3] altogether, since that indicates that the polar axis altitude is already set pretty accurately.

Douglas Cann (cann@axionet.com) offers the following tip:

I have been reading all of the concerns and questions regarding polar alignment and the ETX and unless one is doing long exposure photography, which I am sure 99% of ETX's aren't, exact alignment is not really required. I hope that my own experiences will help the growing number of ETX owners out there.

Over the past few nights I have been experimenting with polar alignment error and the effects on visual observing. My observing site does not have a north view and therefore, in two years, I have never used the pole star to assist with alignment. I made up a triangle of stiff card that is about 6 inches on the short sides. One of the angles is a right angle, one is the angle of my latitude and the third one comes automatically. (basic tri...) On level ground, I set up my tripod and with the triangle and a small spirit level set on one edge of the triangle, I set the tripod head angle so that the 1/4 inch screw was pointing up at the correct angle. I locked the head and made a mark on both sides of the head bearing so that if I move the head to the horizontal, I can go back to the polar setting without using the card. All you need to know now is where north or south is and when you set up your tripod (on the level), point the tripod screw to the north and that's it. I did this head setup two years ago and it still works. Even if you move location, you just make up a new card for the new latitude.

Once your clock drive is on, find your object and lock the RA knob. Ideally, the object should be to the west in the eyepiece so that as the slack takes up, the object ends up in the centre. Even if the alignment is not exact, which it won't be, you can usually observe or study an object for several minutes, up to 15 usually even at higher powers without having to adjust the DEC or RA. Now when the object starts to slide to one of the edges of the eyepiece, either tweek the DEC knob or slightly rotate the tripod head in azimuth to re-centre the object. Do not unlock the RA knob or you will have to wait for the drive lag to catch up again. With very minor adjustments, you can keep an object 'centered' for a long time. Even without the tweeking, I find that the objects stay in view for some time. If you find that you have to rotate the tripod head slightly a couple of times during observing, don't forget to bring the head back to the north when you go another object. Your polar alignment has to be off by several degrees for any major rotating of the tripod head. As I say, without having been able to see the pole star or even to the north, I can obtain very good alignment after having originally set up the tripod head at the correct angle.

In a nutshell therefore, don't get hung up on polar alignment and I would suggest buying another eyepiece instead of fancy bore scopes and other polar alignment scopes and devices.

Return to the top of this page.

Go back to the Buyer/New User Tips page.

Go back to the ETX Home Page.