Absolute Beginners No. 4: Mounting your telescope
2016 January 26
In the last ‘Absolute Beginners’ we considered the various types of telescope which are available to the amateur. However, it is often said that a telescope is only as good as its mount, and this is very true. You can have a first class telescope of excellent optical quality, but if it quivers like a jelly in the slightest breeze it will be completely useless.
In general there are two main types of telescope mount: ‘alt-azimuth’ and ‘equatorial’. There are several different examples of both types of mount – some are simple, others are complex and expensive. It is common today for a mount to be driven so that it keeps ‘sidereal’ (stellar) time. Sidereal time is the time it takes for the Earth to turn round once on its axis, so it is the length of a real day, which is 23 hours 56 minutes (not 24 hours). A telescope fitted with a clockwork or electric drive will track or follow an object in the sky, a great help to both imagers and visual observers alike.
In addition, some mounts also have a ‘goto’ facility. In this case the mount has a computer which contains a database of celestial objects, and a complex algorithm enabling these objects to be located at any given time or place. By selecting the object you want, the telescope will (in theory) then move (or slew) to the object you have selected. The type of telescope mount you want should really be dictated by what type of observing you wish to do. For example, an experienced variable star observer may find a Dobsonian helpful as you can move quickly from one star to another. Supernovae hunters may need a ‘go-to’ mount so a large number of galaxies can be found and examined (often automatically) in a single observing session. Most importantly, whatever mount you choose, it must be able to support the weight of your telescope and be easy enough to use. The most expensive mount is not necessarily always the best.
The altazimuth is the simplest type of telescope mount. It is called ‘altazimuth’ because a telescope on this mount can move up and down in altitude, and also rotates about the vertical axis. In Figure 1, imagine you are at position X, and north is directly in front of you – your overhead point is called the ‘zenith’. If you draw a line from you to a star in the sky, the angle between the horizon and the line to the star is called the altitude (i.e. the height of the star above the horizon). This is measured in degrees. The azimuth is the angle you would have to turn away from the north (clockwise) so that you were facing the star, and it is also measured in degrees.
The simplest type of altazimuth mounting is an ordinary tripod, but a far more common type is the Dobsonian mount primarily used for Newtonian reflectors, and invented by amateur astronomer John Dobson. The base of the Dobsonian mount can turn clockwise or anticlockwise, and a pivot on the telescope tube allows the telescope to move up and down in altitude.
The Dobsonian mount is popular as it can be quite inexpensive, and very portable. It is also not uncommon to see very large telescopes on this type of mount, since it is easy to operate – low friction bearings mean that the telescope doesn’t have to be locked into position, and you can move from one object to another very quickly. Driving this type of mount however is much harder. The drive has to move the telescope in both the azimuth and altitude directions at the same rate as the Earth’s rotation, so when a drive and ‘goto’ is added to an altazimuth mount, it becomes much more elaborate and expensive.
The equatorial mount is a little more complex than a simple altazimuth. Similarly it has two axes of rotation, but these two axes utilise the coordinate system used by astronomers to track objects in the night sky: Right Ascension (abbreviated to R.A. or often the greek letter alpha) and Declination (abbreviated to ‘dec’ or the greek letter delta). I will cover this coordinate system in detail in the next article, but for now it is enough to say that, if we imagine the sky is situated on a sphere surrounding the Earth (we call this the celestial sphere) then declination is equivalent to latitude on this sphere and right ascension is equivalent to longitude (although it is measured in hours, minutes and seconds, rather than degrees).
In the northern hemisphere, the Earth’s rotational axis points towards Polaris (the north pole star) and Polaris marks the position of the North Celestial Pole (Figure 3(a)) On a clear night, you will notice all of the stars appear to rotate about the north celestial pole during the course of a night – this is why if you take an exposure of the night sky with an undriven camera for more than a few minutes, you get star trails (in fact Polaris is not quite at the pole, and due to precession the location of the north celestial pole changes over many thousands of years).
The main point about an equatorial mount is that the RA axis points towards the pole star just as the Earth’s axis does. As a result, objects in the night sky rotate around the axis of your telescope in precisely the same way as they do around the north celestial pole. This means that once your mount is aligned with the pole star you only have to take account of east-west motion (unlike the alt-az mount where you have to move your telescope constantly up and down, and east to west to keep track of objects).
Having only to worry about east-west motion of objects in the sky means that driving an equatorial mount is much easier and cheaper than doing so for an alt-az mount. A small drive can be fitted to the RA axis so that the telescope rotates at the same rate as the Earth (i.e. your telescope keeps sidereal time). These drives are fairly cheap and easy to fit, and when added and switched on, you’ll find objects should remain stationary in the field of view rather than move quickly out of the eyepiece. This is essential if you want to carry out long exposure photography since the object being imaged must remain stationary in the eyepiece as any motion will blur it.
Aligning an equatorial mount isn’t difficult. Today, many come with ‘polar scopes’ fitted – a small extra telescope located inside the RA axis of the main telescope. If you look up through the polar scope in the daytime, you will see a small black circle – this is where you have to put Polaris rather than at the centre of the field, since Polaris is not quite situated at the north celestial pole. By adjusting the altitude on the mount, you can bring the Pole star into view and then set it onto the black circle. The altitude of the pole star above your horizon is the same as your latitude on Earth. So if your latitude is 51°, you’d need to set the angle on your mount to 51°, and Polaris should be in view in your polar scope. Even a rough alignment makes observing easier as you only have to move the slow motion controls in the east-west direction.
In the examples given here I have been describing the German equatorial, which is the most common type of equatorial mount. There are other systems – one can have a fork type of equatorial mount for example. The late Sir Patrick Moore’s 15-inch (380 mm) Newtonian was situated on a fork-type equatorial mounting.
Once you have your equatorial mount correctly aligned, you can use the setting circles to find any object in your night sky, but before we can do that we shall need to understand the coordinate system of the sky: Right Ascension and Declination. These will be covered in my next article.
Paul G. Abel
|The British Astronomical Association supports amateur astronomers around the UK and the rest of the world. Find out more about the BAA or join us.|