AstorPearls - Astronomy Software
 Polar Alignment Part 1

So, what do we want to achieve?

We would like to keep the star (or any other object) centered in the field of view (FOV) of our telescope for as long as possible.

Isn’t this the case?

No, due to earth rotation towards the east, sky appear to rotate from east to west - no wonder sun rises near the east and set near the west! As a result, stars and other objects once centered in your FOV will drift slowly and you will eventually lose them. This happen especially at high magnification.

How to solve this problem?

Well, let’s rotate the telescope (or to be precise, your OTA Optical Tube Assembly) at exact same speed and same direction with the sky hence anything we center in the field of view will stay there.

Is this possible?

Indeed, that’s what Equatorial mounts are good at - rotating! But for this to work, we need to make sure the rotation axis of the telescope mount (RA-axis) is exactly aligned with the rotation access of the sky.

But where is this axis for the sky?

This axis goes from the earth center towards the north celestial pole (NCP) - or SCP if you live in the southern hemisphere. See Figure 1. So, the RA-axis of your Equatorial Mount must be precisely pointing to the North Celestial Pole NCP (or SCP if you are located in the southern hemisphere).

How about the rotation axis of the mount?

Well, this is the Right Ascension (RA) axis of your equatorial mount. Equatorial telescope mounts are designed such that RA-axis can be easily tilted allowing you to point it towards NCP.

Tilted, how?

Yes, good catch, I should’ve said tilted relative to what. It is tilted relative to the horizontal level - the Horizon. And if you carefully analyze Figure 1, you’ll see that the angle the axis makes with YOUR horizon is equal to YOUR latitude on earth. Note also that naturally, to look towards the NCP is to look towards the north of your horizon - This is where a compass comes to the picture. You must examine an Equatorial mount to get a feeling of how it works. Join a star party if you don’t have one, you’ll find a lot of people with Equatorial mount who will be happy to show you how it works.

Figure 1
                                                Figure 1

Now let’s summarize:

All you need to do is the following:

  • Make sure mount pointing towards the north - a compass is handy now.
  • Level your tripod to ensure you are measuring the angle relative to the horizon - oil drop is helpful here.
  • You might want to repeat the previous two steps until you get it right. It will save you time later.
  • Tilt your mount by an angle equal to your latitude - now you know why equatorial mounts come with latitude scale.

Note about the use of a compass
If you use a compass you need to remember that the compass gives you the direction of the magnetic north - that depends on the earth magnetic field and does not exactly match the geographic north. To workaround this small problem, a slight correction (called the magnetic declination) is needed and you can get its value form the USGS web site.

How good is this alignment process?
This simple process described here is good for casual observation. It will ensure that your object stay in your field of view long enough for you and bunch of your friends to watch the object before it drift outside your view. That said, it is not accurate enough for astrophotography (unless you are taking a picture of bright object at very short exposure). To get better accuracy, you need to jump to next level, using Polaris!