Telescopes 7 - The Mount

A telescope is not much use unless we have something sturdy to mount it on. I would be quite a wally if I tried to hold my Helios 200mm scope in my arms and aim it at something. It's just not going to work!

The mount must allow the telescope to move in at least two directions at once. The telescope must be able to point from the horizon all the way up to the zenith (right over head) and be pointed from east to west and back to east again. It must be strong enough to cope with the weight of the scope, some telescopes can weigh up wards of 40 kilograms or more. The mount must also be sturdy and not wobble or sway at all.

One of the simplest telescope mounts around has to be the "Dobsonian", so named after its inventor who specialised in "side walk astronomy", showing the delights of the night sky to the passing public. The Dobsonian mount consists of an azimuth bearing, which allows movement from side to side and an altitude bearing that allows the telescope to be angled up or down. The beauty of this mount is in its simplicity. The user simply points the telescope in what ever direction is required.

The Dobsonian is best suited to large Newtonian reflectors. Although it could be used with any telescope.

The next mount is just as simple. The Alt Azimuth mount is basically a scaled down version of the Dobsonian and designed to be positioned on a tripod. It is the most popular mount for budget telescopes. Again it has two bearings allowing movement in an arc from side to side as well as the required up and down movement. This type of mount is only suitable for small lightweight telescopes.

The third mount I want to introduce you to is the "equatorial mount". As the name suggest this type of mount is designed to keep one axis in parallel with the Earth's equator. The other axis is in parallel with the Earth's axis and normally points towards the north (in the northern hemisphere). Why is that I hear you ask?

If you have paid attention to the night sky you will have noticed that the stars, Moon, Sun and the other objects move across the sky. For us here in the northern hemisphere they travel from east to west. This movement is due to the rotation of the Earth, we are standing on a rotating platform. If we could draw lines across the sky that show the track objects follow, we would see arcs across the sky.

The equatorial mount is designed so that one of its bearings or axes accurately follows these arcs. The axis of the mount that follows this track is called the "right ascension bearing" and must aligned so that it is angled, to the horizontal, by the same amount as the observer's latitude on Earth.

The other axis of the mount is known as the "declination bearing". This bearing is at right angles to the right ascension bearing, so the mount looks like the letter "T", but tilted over.

If we are using a telescope or binoculars the movement of these objects across the sky is magnified quite a lot. To keep the telescope concentrated on one object, using either a Dobsonian mount or the Alt Azimuth mount, we would require the observer to move both axes' at the same time. This can prove quite awkward, especially if the object is something you want to study or photograph, say one of the planets.

Because the equatorial mount has one axis that tracks the paths objects follow across the sky, it is only necessary to move this one axis. With this type of mount it is quite simple to install a motor that causes the telescope to move, slowly but in sync with the movement of the objects in the sky (by objects I mean celestial objects, not man made).

Equatorial mounts also allow the user to make use of setting circles on the mount. These setting circles, once set up, will allow the astronomer to quickly find objects using coordinates given in books and magazines. Their use takes practice and is beyond the scope of this article

Recap ...

Telescope mounts come in three main varieties, the Alt Azimuth, the Dobsonian and the Equatorial mounts. The Alt Azimuth is mainly restricted to small light weight telescopes. It has two axes, to allow side to side movement and up and down movement. This mount is normally placed on a tripod.

The Dobsonian is very similar to the Alt Azimuth mount, in that it has the same arrangement of bearings. But it is generally designed for larger Newtonian telescopes. It also tends be be floor mounted, not on a tripod. Both the Alt Azimuth and the Dobsonian mounts require the observer to adjust both axes together to follow an objects movement across the sky.

The equatorial mount also has two axes, but has one of them, the right ascension bearing, tilted over. This tilt is at an angle that equates to the observers latitude on Earth. This mount will allow the telescope to follow or track objects as they move across the sky by only adjusting one bearing. The movement of objects across the sky is due to the rotation of the Earth.

Well I hope my Beginners guide to 'how telescopes work' has proved both informative and useful to you.

Good luck with your hobby and clear skies!

Telescopes Part 7The Mount - What the Telescopes sits on

Recap ...

Telescopes Part 7
The Mount - What the Telescopes sits on