Polar Alignment

The earth rotates around the north/south pole axis once each day. If we ignored this fact when imaging the sky, all we would get would be totally blurred images. We therefore need to rotate the telescope in exactly the opposite direction and with correct speed in order to have any hope of capturing sharp images.

The task of adjusting the scope's rotation axis to be perfectly parallel with the earth's rotation axis is called Polar Alignment. A slight error in the alignment will cause the object to drift out of the camera's field of view.

German Equatorial Mount (Vixen Super Polaris)

Achieving perfect polar alignment

This description applies to a German Equatorial Mount as shown above, but the overall principles are the same no matter which equipment is being used. We also here assume you are located in the northern hemisphere.

Step 1: Rough alignment

Set up the mount on a stable tripod or pillar. Notice that an aluminium tripod is not considered stable under any circumstance. For accurate work, a wooden tripod or even a concrete pillar is required. This description is particularly relevant for a permanent setup on a concrete or steel pillar, as it requires some time to complete all steps.

Adjust the altitude and azimuth of the mount so that the Right Ascension (RA) axis points roughly to the North Pole (see image above). The star Polaris is a good approximation in the northern hemisphere for this step.

Step 2: Fine alignment using polar axis scope

Most quality equatorial mounts have a polar axis scope built into the mount. For the mount shown above you simply have to remove the green plastic "butt cup" (shown lower left in image above) and open up the cover on the other side (where the tape is seen). Other mounts may be slightly different. You may have to rotate the declination axis to get a clear view through the polar axis scope.

At this stage you must realise that Polaris does not lie exactly in the position of the north celestial pole. It is located approximately 45 arcminutes (3/4 degree) from the true pole. It is therefore incorrect to place Polaris at the center of the polar axis scope. The polar axis scope includes a convenient Polaris circle indicating the correct distance from the pole, but where on the circle is dependent on date & time.

View through polar axis scope
(NOTE: the placement of Polaris on the circle depends exact time!)

You can always use the instructions that follow you mount, but such instructions are often more complicated than necessary. It is far easier to use a program that will show the correct placement of Polaris on the polar axis circle for any given time. The program PolarFinder.exe by Jason Dale does exactly that. A screenshot is shown below.

The program PolarFinder by Jason Dale.

Simply adjust the view in the polar axis scope until it matches what PolarFinder.exe displays for the current time. Unless your polar axis scope is wildly out of alignment, you will now have a very good polar alignment, good enough for visual and short exposure imaging. The PolarFinder web page was nowhere to be found when I first discovered the PolarFinder program, but since then Jason Dale has found me instead, and has begun making updates to the program :-) Therefore, I have the pleasure of providing this link to Jason Dale's page, where you can download the latest version of PolarFinder.
Update May 2007: Jason's page seems to be gone, so here is a copy: Download
Update May 2012: Jason's page has reappeared with version 2.04 (thanks for the tip Knud Strandbaek). The link above has been updated accordingly.

Step 3: Precise alignment using visual drift method

If you want to image the planets at high magnification or want to do long exposure imaging of deep sky objects, the polar axis scope method will usually not give sufficiently accurate alignment. You will soon notice that the object you are looking at drifts out of view, even if the RA motor you are using is perfect.

You will find more detailed descriptions of the drift method elsewhere on the web (for example http://members.aol.com/ccdastro/drift-align.htm), but here it suffices to say that any error in alignment will cause a drift in declination only, i.e. if you look at a star using high magnification while the motor is running, the star will tend to drift up or down, not left/right. It is also worth noting, that the only possible alignment errors are east/west of north pole (adjusted with azimuth screw) or "over/under" north pole (adjusted with altitude screw).

The drift method takes advantage of these facts. To perform the drift method, you can do as follows:

The polar alignment is now very precise, and good enough for most needs.

Step 4: Perfect alignment using webcam and drift method

Even at this stage, there might be very small remaining alignment errors that the previous method was not able to detect. An even higher accuracy can in fact be achieved simply by replacing the eyepiece with a webcam (keep the barlow to get max resolution though). Use a program with a big preview screen and on-screen crosshairs. I used VEGA version 1.2 for this purpose. Make sure the camera is oriented such that the long image side correspond to RA, and the short (vertical) side correspond to declination.

Use the exact same method as described for Step 3, but place the guide star on the center of the on-screen crosshair (you can also use the mouse indicator symbol as a reference if you can't find a program with crosshairs). Watch the vertical drift, and make tiny adjustments. See screen capture image from VEGA 1.2 below.

Using the Vega 1.2 focus window with crosshairs for extremely accurate polar alignment.

Once this process is complete, you will enjoy perfect polar alignment. No longer will stars drift up or down and out of view. All remaining movements will now be in the RA direction only. Such errors may be caused by the drive system's periodic error or overall drift. Such issues will not be discussed here (considering writing another page for it though).

I welcome comments and tips about this page!