epending on the type of telescope Dmount that you possess, there are two ways in which you can move the instru- ment in order to locate and track objects in the sky. I stress ‘track’ here since, unlike viewing stationary terrestrial objects, the rota- tion of the Earth on it’s axis once in 24 hours from west to east causes the sky to make one revolution about the celestial poles in the same period (incidentally, if you have not yet familiarised yourself with the ‘Get to know the sky’ section of this booklet, now might be a good time to familiarise yourself with some of
the concepts contained within it).
The Alt-azimuth mount: this is the the sim- plest type of telescope mounting to under- stand and, in some senses, to use. There are variations that I’ll discuss in a moment, but all share the common characteristic that there are two axes about which the telescope can be moved which are perpendicular to one another (see Fig.1, page 10). The first axis permits the telescope tube to be moved from horizontal to vertical and is known as the ‘alti- tude’ axis. the second allows the instrument to be moved in an arc parallel to the horizon through a complete 360° circuit of the com- pass; this is the ‘azimuth’ axis. So, a mount- ing permitting motion about both such axes is called an ‘alt-azimuth’.
In its most basic form (as provided with the Astrolux, Lunar-Cadet 1, Mercury-607 & Mercury-707) there is usually a ‘slow-motion’ control in the form of a threaded rod that is operated by a thumbwheel permitting precise control of the telescope’s tube in altitude. On more sophisticated mounts (such as provided with the Capricorn 70-2 and the Evostar 90-3)
there is provision for slow motion controls in both altitude and azimuth — this makes for much finer control when tracking celestial objects at high power.
Alt-azimuth conventions: As has been dis- cussed elsewhere, looking up at the night sky gives the impression that the observer is at the centre of a vast hemisphere — the so- called ‘Celestial Sphere’. The stars, Moon and planets all appear to lie on the inside surface of this hemisphere an infinite distance from the observer. This is, of course, an illusion since the Moon and stars are in reality at greatly differing distances away from us. However, the Celestial Sphere concept has its advantages in that it makes it easy to define coordinates for objects in the sky and to pre- dict where a given star or planet will be at any given time.
Looking at Fig. 1 once more, note that the portion of the Celestial Sphere shown in the diagram has been divided up by lines and arcs in much the same way as the surface of the Earth has been divided up into latitude and longitude. By careful observation you will note that the star Polaris which resides in the constellation of Ursa Minor (the Little Bear) always appears stationary above the northern horizon at an angle very close to that of the observer’s latitude. This is because Polaris is very close to the northern celestial pole and all the other stars appear to circle around it in a counter-clockwise direction once every 24 hours. Lying close to due north means that Polaris will always have an Azimuth bearing close to zero, or 0°. For an observer in the British Isles its Altitude bearing will lie between 50˚ and 55°.
