The Bizarre and Confusing World of Telescope Mounts – Part One

We are putting together a course on telescopes and in the process I spend quite a bit of time talking about telescopes as they come in all shapes and sizes. This piece is all about the basics of telescope mounts and the general pros and cons of them. Often when people go and buy their first telescope they don’t think much about the mount, they tend to focus more on the capabilities of the telescope itself. These are important as the telescope is what you’re going to look through so you want to make sure you get the right tube. Unless you’re super human and can hold the telescope in you hands very still, then you’re also going need to put the telescope tube on something. I’ll refer to the telescope from now on as a tube, that’s because the mount’s job is to hold the tube steady and allow it to be pointed in the right direction.

A mount that is tracking compensates for Earth’s rotation. For the non tracking mounts, if you still want to be able to see that object for longer, then you will have to compensate manually.

There are basically two fundamental type of mounts in the world, ones that sit parallel to the ground and ones that are inclined to the celestial equator. Both have their benefits and their pitfalls. Most of the variations are to do with how to balance the mount and the tube so the whole lot doesn’t fall on the ground. John Dobson is the person responsible for the famous Dobsonian mount that many Newtonian reflectors are stuck on. He came up with the design to give a cheaper option for people to get into astronomy, and it worked, as the mount compared to the cost of the telescope, for Dobsonian mounted Newtonians, is significantly less than if a more complex mount is used.

On the left is the mount sitting parallel to the ground, If it’s inclined to be parallel with the celestial equator (the mount on the right) then the dashed arrow will point to the South Celestial Pole rather than straight up (North Celestial Pole if you’re in the Northern hemisphere) (Credit: me).

John Dobson’s design is basically a fork mount made out of plywood on a turn table on the ground that uses the height of the Newtonian’s design to make them easy to use, win win for everyone.

John Dobson (Credit: Wikipedia)

The benefit of having the base of the mount flat on the ground is that it doesn’t matter which way you point it. The negative is that the tube can only track the movement of the sky if you move the telescope on both axes, not a problem if you are just observing and not doing astrophotography of very dim and distant objects and don’t mind moving the tube every 20 seconds or so. The mounts that have the rotating base parallel to the ground are called altazimuth mounts (alt/az).

A Dobsonian mounted Newtonian – the base is parallel to the ground and the rotation axis is pointing straight up. (Credit: Me)

For the other configuration where the mount’s base is parallel to the celestial equator, then to keep up with the moving sky you only have to move it on one axis – if you put a motor on it then it will do it by itself (at the right speed of course). The mounts with the rotating base inclined to be parallel to the celestial equator are called equatorial mounts. These mounts have their bases inclined at an angle that is the same as the latitude of your location and the axis that was pointing straight up now must point to the celestial pole that is visible above the horizon. In the Southern Hemisphere, the South Celestial Pole and in the Northern Hemisphere, the North Celestial Pole.

An Equatorial mount, the brass disk to the lower right is inclined so the axis of rotation points to the South Celestial Pole. The amount it has been rotated from being parallel with the ground is the same as the latitude where the telescope is. (Credit: Me)

If you happen to be looking at Mars through your telescope then you’ve probably have got, in the focuser, a small eyepiece at 10mm or less, and/or maybe a Barlow lens to try and see the planet’s disk. In altazimuth mounts that have no motors, then you will have to adjust both the side to side axis (azimuth) and the up and down axis (altitude) to keep the planet in the centre of the field of view. On the equatorial mount to keep Mars in the field of view, you only have to move the side to side axis, called Right Ascension. The up and down axis stays in place as long as the mount is perfectly parallel to the celestial equator so there’s no need to adjust that to keep Mars in the field of view. The up and down axis in this configuration is called declination.

To summarise:

Altazimuth mounts – the base is parallel to the ground and the axis of rotation points straight up, azimuth. The axis that moves the telescope up and down is parallel to the ground, altitude. Side to side – azimuth, up and down – altitude. To keep an object in the field of view you have to move the tube in both axes.

Equatorial mounts – The base of the mount is inclined at an angle, that is the same as the latitude you’re on and pointed to the celestial pole that is above the horizon at your location. The side to side rotation, that is now inclined is called right ascension. The up and down motion is called declination. To keep an object in the field of view you only have to move the telescope in right ascension.