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.
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