Equatorial Mount: the concept

It took me two full hours to understand the concept of equatorial mount when I first started off as an amateur astronomer. First of all, the diagrams which I had where really terrible, and only mixed things up even more than the text only of the book I had. This is why I’m going to try to explain my best what it does, how it’s useful, and how it works.

An equatorial mount is a device to which you attach a telescope or a camera and who automatically compensates for the earth’s rotation. Sometimes amateurs and professionals alike need to take important pictures of the sky, sometimes for leisure, sometimes for work. When you take pictures of dim objects such as nebula and galaxy, you often need to leave the camera for a very long time taking its picture. The problem is that the Earth rotates and you would get a blury image without an equatorial mount. This is where they come in handy.

It would be pretty easy to compensate for that rotation if the earth didn’t have a tilted axis. The functioning of the equatorial mount The earth’s axis is tilted of about 23°26 to the ecliptic axis, which is the axis on which it rotates around the sun. If the earth wasn’t tilted, and had a plain 0° angle, all you would need is a telescope that had a vertical and horizontal axis, and you would be able to compensate for the earth’s rotation. But because it’s tilted, you have to make your telescope/camera turn around the same axis as the earth to compensate for its rotation. The diagram will show this pretty well. The angle to which you tune your mount is the same one as your latitude, because the latitude indicates, in a way, what’s the difference in degrees between your place and the north pole. So to tune your mount correctly, you want your mount pointing to the north pole, which is the projection of the Earth’s axis in the sky.