Orion 9877 guide

The further that one moves across the sky from Polaris, the apparent motion of the stars becomes more evident and their Altitudes and Azimuths will be continually changing. Taking the star labelled ‘AA’ in Fig. 1, at the instant of the observation its Altitude was 60° and its Azimuth bearing was also 60°. It can be seen that Azimuths are measured in degrees from due north (0°) through east (90°), south (180°), west (270°) and back to north (360˚ or 0°). Altitudes are measured in degrees over a maximum range of 90 — objects exactly on the horizon are at 0° and those overhead are at 90° (it is possible to have Altitudes of neg- ative sign, but this means that the object is below the horizon and therefore invisible).

The continual changing of Altitude and Azimuth as a celestial body rises in the east, traverses the sky and sets to the west makes tracking an object at high magnifications quite a challenge, but it is surprising how soon one becomes proficient at doing so. However, should the observer wish to attempt any form of time exposure with the telescope to photo - graph a faint galaxy, for example, then a dif- ferent type of instrument mounting known as an Equatorial is required.

The Equatorial mount: the equatorial mount consists of two axes that lie perpendicular to one another (as per the Alt-azimuth system), but one is tilted such that it is aligned parallel to the Earth’s axis, which means for observers in the northern hemisphere one axis will always point close to Polaris in the northern sky — not surprisingly, this is termed the Polar Axis. As depicted in Fig. 2 on page 11, the Equatorial is the mounting of choice if any form of astrophotography is envisaged. It also makes the process of prolonged tracking so much easier since the telescope can be motorised about the Polar Axis such as to automatically follow the Moon, planets and

stars in their diurnal paths across the sky.The so-called declination axis can remain locked once the desired object has been located. Unlike the Alt-azimuth system, the coordi- nates of objects remain (relatively) fixed and a slightly different convention has to be used.

Equatorial conventions: the coordinate sys- tem is based on projections of the Earth’s gridwork of latitude and longitude projected onto the Celestial Sphere. Consequently, a star such as Polaris that lies very close to the northern celestial pole would be always over- head for an observer on the North Pole, whereas a star such as that labelled ‘BA’ which lies 90° away from Polaris will be over- head at some point for an observer on the Earth’s equator. This is known as the star’s Declination and varies from +90° near Polaris to -90° at the opposite celestial pole. The other coordinate is termed Right Ascension and is measured in hours from 0 to 24. Thus, star BA has a Right Ascension of 2h and a declination of 0°.

All telescopes in the Helios range designed for prolonged and serious use are mounted in the Equatorial fashion, which as has been described makes for far more convenient viewing. Motorised tracking is available for most models which makes for prolonged exposures for astrophotography or lengthy observations of the Moon and planets at high magnifications.