A Basic Guide to Telescope Mounts
Mounts come in two basic types – alt-az and equatorial. Although they sound different, they’re actually very closely related, an equatorial mount being essentially an alt-az mount with one axis tilted over so that it points at your local celestial pole. In the UK for example, as we’re in the Northern Hemisphere, our local celestial pole is the North Celestial Pole (NPC) which is roughly marked by the star Polaris. From the latitude of the UK, all of the stars and constellations appear to rotate around the NCP.
An alt-az mount gets its name because it allows you to move something attached to it up-down and left-right. The up-down movement changes the instrument’s altitude while the left-right motion changes what’s known as its azimuth. As altitude-azimuth is a bit of a mouthful, this mount type is typically shortened to just alt-az. A simple example of an alt-az mount is a standard camera tripod.
To understand why you need an equatorial mount at all, put your winter woollies on and imagine you’re at the North Pole. If you imagine a line drawn from the Earth’s South Pole through to its North Pole, where you’re now standing, this defines the rotational axis of the Earth. Keep the line going so that it emerges from the ground under your feet and extend it up into the sky for an infinite distance. Eventually, it’ll point at the NCP. So the NCP isn’t a tangible entity in its own right, it’s simply a by-product of the Earth’s rotation and how that rotation manifests itself in the apparent daily movement of the stars.
At the Earth’s North Pole the NCP sits directly overhead, a point in the sky known as your zenith. All the stars move in circles around the NCP, completing one revolution in slightly less than 24 hours. At the North Pole this motion is seen as the stars slowly moving around the sky parallel to the horizon. Here, a basic alt-az mount comes into its own because its motion naturally mimics the motion of the stars. If you locate a star in the finder of an alt-az mounted scope at the North Pole, all you have to do to keep it on the cross hairs is to change the scope’s azimuth. The star’s altitude remains fixed.
Okay, let’s return home again. As you move away from the North Pole, the location of the NCP in the sky appears to change. As you head further south (something you can’t fail to do when moving away from the North Pole!) the position of the NCP appears to get lower in the northern part of the sky. In fact if you travelled all the way down to the equator, the NCP would appear on the north point on the horizon with its Southern Hemisphere counterpart, the South Celestial Pole (SCP) visible on the southern point on the horizon. In the UK, the NCP is located just over half way up the sky in the north. Its actual height (its altitude) is defined by your latitude on Earth. From my home town of Selsey on the south coast which has a latitude of 51°, the NCP is located due north with an altitude of 51°. Just to press the point home, from the North Pole, the NCP is directly overhead with an altitude of 90° which is the latitude of the Earth’s North Pole. At the equator the NCP’s altitude is 0° which again, is the latitude of the equator.
As you move away from the North Pole, the motion of the stars across the sky appears to change. At the Pole, they appear to describe circles parallel to the horizon. Moving further south, these circles are still there but they start to tilt with respect to the horizon. Consequently the stars now appear to describe arcs in the sky. This is bad news for alt-az mounts which don’t naturally move in this way. From the UK’s latitude, using a telescope on an alt-az mount such as a camera tripod, requires you to constantly move the telescope in azimuth and altitude to keep up with the object you’re viewing.
A Simple Solution
Fortunately there is a very simple solution to this. By tilting an alt-az mount over so that its left-right, azimuthal axis keeps pointing at the NCP, the motion of the mount mimics that of the stars. This is what an equatorial mount is – it’s nothing more than an alt-az mount which has its azimuthal axis tilted over to point at the NCP. Terminology changes when this happens and the tilted azimuth axis becomes known as the polar or right ascension axis, while the altitude axis becomes known as the declination axis. Movement of the scope in a left-right fashion changes its right-ascension, while moving it in an up-down fashion changes its declination. You can think of right-ascension and declination as being analogous to a sort of longitude and latitude in the sky if you like.
Equatorial mounts also tend to be beefier than you’d get by simply titling a camera tripod over. Applying magnification through a telescope requires a sturdy mounting platform in order to prevent the wobbles upsetting the view. The general rule of thumb is that the heftier the mount, the better the view will be. Heavy mounts are also desired by imagers who need the most stable platform they can get.
Fitting a drive to the polar axis of an equatorial mount which moves the telescope at the same rate as the Earth rotates but in the opposite direction, will allow the mount to keep pointing at the same object without having to constantly adjust its position. This is called a right ascension drive and such a mount is described as being driven. For convenience and to allow for pointing corrections in declination (up-down), it’s common to fit another drive on the declination axis. Unlike the right-ascension drive however, the declination drive isn’t constantly active, only being used when you want to make adjustments to the declination of the scope.
An alt-az mount can also be driven but here there’s a problem because unless you’re observing from the North Pole, driving the azimuth axis of such a mount will not naturally follow the stars. It’s therefore uncommon to have a plain vanilla alt-az mount which is driven.
Everything I’ve described so far is non-Goto. These are basic mounting systems with no intelligence added. For a polar aligned driven equatorial mount, after you point it at your chosen target, it then blindly moves following that target. Goto adds a further level of sophistication to this arrangement by the introduction of a computer which can take control of both drives. By supplying a Goto computer with your location, the current date and the time, and by telling the system where the telescope is initially pointing, the Goto computer has all the information it needs to take the strain out of finding objects in the sky. Add a database of objects with accurate, up to date positions, and the Goto computer can, under your control, point to any object you select from its database at the touch of a few buttons. This is great if you don’t know the sky very well because it’ll find things for you. However, it’s not a great way to learn the sky because, as the saying goes, “there’s no gain without pain”. Pressing buttons to locate objects in the sky removes the need to star hop and hunt; the basic process which causes you to learn your way around the night sky.
Having said this, if you’re in a hurry and don’t have the time or inclination to learn the night sky, Goto is a great way to see the sights. Another big advantage Goto has for imaging is that it allows you to point your telescope at objects which may not be visible given a low brightness or poor sky conditions. In this respect, Goto can be a godsend.
Another interesting option that becomes available by adding a computer to a mount is that the computer can be made to make an alt-az mount emulate the motion of an equatorial one. This has a number of advantages and some disadvantages. The advantages are that it’s cheaper to create a left-right/up-down style Goto mount to hold a heavy telescope than it is to engineer a sturdy equatorial platform. It’s also easier to set up an alt-az mount as you don’t have to point any of its axes at any particular location in the sky. In contrast, an equatorial mount doesn’t work too well if you don’t align its polar axis with the celestial pole properly.
You still have to let the Goto computer know which way is north for example, but advances in technology and the use of GPS receivers built into some Goto mounts now means that initialising the mount is pretty much automatic. You simply need to confirm that the telescope is pointing at a test star the Goto computer thinks it’s pointing at.
On the negative side, the drive system for a Goto-enabled alt-az system needs to be well designed and robust to work effectively. Apart from the fact that a fast slew to an object can really put a lot of strain on a drive train, the motors driving such a mount must both be constantly active in order to keep up with the object you’re tracking in the sky. The biggest disadvantage though is the fact that an alt-az system suffers from an effect called field rotation which isn’t great if you’re into long exposure imaging. Field rotation manifests itself as the stars you’re imaging slowly appearing to rotate in the field of view. Rather than appearing as points as they should do, a long exposure image taken on an alt-az platform will result in the stars appearing as arcs. This effect can be overcome by adding a field de-rotator but these can be pricey. Certain imaging systems can also cope with field rotation by taking lots of short exposure images and, applying the de-rotation correction through software manipulation.
All this sophistication does come at a cost though and for the same amount of money, you may get more scope in a non-Goto system than you would in a Goto-system, where a significant amount of your money has to go towards paying for the on-board computer system.
So in summary, there are two basic types of mount, alt-az and equatorial and these can be either driven or undriven. A driven alt-az mount is of little use unless you’re at one of the Earth’s poles but can be made to work if you introduce a computer to make its motion emulate that of an equatorial mount.
Goto is the term used to describe any mount which has an onboard computer with a database of objects. With basic information about where and when it is, the computer can point the telescope at any of its stored objects automatically, assuming that object is currently visible.
It's important to distinguish between Goto and mount types. Goto isn't a mount type at all but rather a system which can be added to a mount, alt-az or equatorial, to make it easier to locate objects in the sky.
Using a computer to emulate equatorial motion in an alt-az mount is not Goto, but does tend to find itself being offered in many Goto enabled systems due to the fact that there’s already a computer on board.
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