AstorPearls - Astronomy Software
 Know Your Equatorial

Motion of the stars

The truth is that earth rotates around its axis towards the east, and so the sky appears to rotate in the opposite direction around the same axis. Nevertheless, the motion of the stars in the sky is very real, very useful, and…sometimes annoying!

Before you proceed with this article, it is important to be familiar with terms like celestial equator, Earth rotation, North/South Celestial poles…etc. If you are not, you are encouraged to go back to learning center and read the article titled: The Celestial Sphere.

So, why this is a problem?

Well, if you pointed your telescope towards some star, you’ll notice that the star doesn’t stay in the telescope’s field of view for long, it soon drifts towards the west and the only way to keep it locked is to continuously move your telescope with the star. This problem gets more severe as you increase your magnification - compare this to the motion of a tree far from your car as you drive on the highway and the motion of a tree right behind the shoulder!

A simple solution!

The obvious and logical solution to this problem is to rotate the telescope’s optical tube (OT) in the same rotation direction of the celestial sphere and around the same axis. Equatorial mount - also known as German Mount - makes this very easy.

Equatorial Mount

Our goal is to rotate the OT, so the telescope mount must have a block that can rotate freely. I’ll call this the RA-Block (for reasons to be revealed later). Figure 1 shows a sketch of this block. Mounts vary in their designs but share the same concept. Now we must ensure that the RA-Block rotate around the rotation axis of the sky (and earth). So, equatorial mounts allow you to point the whole RA-Block towards the NCP (or SCP if you are in the southern earth hemisphere).

Equatorial Mount
                                                Figure 1

Now we can’t attach the OT (not shown) directly to the RA-Block. Doing this we won’t be able to point it to any arbitrary direction, in fact we need to be able to rotate it around another axis that is perpendicular to the earth rotation axis - think of your favorite joystick, you need two axes to cover all possible directions. Another block exists for this reason. I’ll call it the DEC-Block (again name will be explained later).

Look carefully at figure 1 and notice the following:

  • Rotation in the directions indicated by the green arrows allow you to point the RA-Block towards NCP/SCP ensuring that this block will rotate with the sky as we wish.
  • Rotating RA-Block (direction indicated by red arrows) will rotate the DEC-Block along with the attached OT. This will draw circles on the celestial sphere centered on the NCP. See figure 2.
  • Rotating DEC-Block (blue arrows) allow you to point the OT to any star starting from NP up to the equator. Increasing the radius R of the circle C from 0 to 90 degree. (Yes, we can use degrees).

Position of Optical Tube
                                                Figure 2

Ok, how this solved the problem? Simply use a motor to rotate the RA-Block one rotation per day following the sky, that’s all!

That was easy?!

Not really, many things have to be right for this to work as expected. Briefly:

  • Mount have to be level (tripod adjustment)
  • Mount must be properly aligned with rotation axis.
  • Rotation speed must exactly match the rotation speed of the sky. Detailed explanation of different alignment methods are explained in the Polar Alignment articles in the learning center.

So, what about these RA and DEC enchantments?!

RA refers to the Right Ascension, the first coordinate of an object as given in the equatorial coordinate system. The second coordinate is Declination - hence the DEC! To learn about these, refer to the Equatorial Coordinate System article in the learning center.