The Equatorial Mounting; Setting Up And Using An Equatorial Mount; Polar Axis Alignment - Helios 9877 User Manual

The equatorial mounting...

Setting up and using an
equatorial mount
T he Earth's eastward rotation causes
stars to apparently rotate around a point
in the sky which is a projection of the
Earth's axis, called the celestial pole. A star's
path t h rough the sky from an eart h b o u n d
o b s e rv e r ' s viewpoint is therefore an arc of this
rotation; it rises in the east towards its highest
altitude (on the local meridian - directly s o u t h )
and then sets in the west. It does not travel i n
a line parallel to the horizon. (Except at the
north pole, from where the celestial pole is at
the zenith!)
Telescopes are generaly provided with one of
two basic designs of mountings. The so called
alt-az allows a telescope to move in altitude
(up/down) and azimuth (left/right). This does
not allow easy tracking of a star's curved path
t h rough the sky since an alt-az has to move in a
series of vertical and horizontal steps, which for
the purpose of steady observation, are cumber-
s o m e .
In the Equatorial, the alt-az mounting is in eff e c t
tipped at an angle, provision being made for the
azimuth axis to be aligned to the Earth's ro t a t i o n a l
axis (towards the celestial pole). This allows a tele-
s c o p e to follow a star by simply countering the
E a rth's rotation in one motion, rather than the
combination of two movements which would be
n e c e s s a ry with an alt-az. A motive force, either
manual or motorised, applied to what we must
now call the 'polar' axis, is all that is necessary to
track a star through the sky.
In order for this action to follow a star without
d e v i a t i o n the polar axis of the equatorial mounting
must be very accurately aligned on the celestial
pole. The better this is done, the less re q u i re m e nt
there will be for minor adjustments during
observation.

Polar axis alignment

(approximate setting)
Try to carry out polar axis alignment on a level
s u rface, where the location of the tripod (or pillar)
feet can be permanently re c o rded to allow the
telescope to be placed in exactly the same
position for subsequent observations. If a
choice of o b s e rving location is limited, try to
position the telescope for the best view of the
southern sky as possible.
Adjust the angle on the latitude scale of the
polar axis, shown in f i g u re 1, until this
equates to your latitude (previously deter-
mined from an OS map).
For the sake of ease and safety, it is recom-
mend that the telescope and its counter-
weights be removed from the mounting for
this exercise.
The polar axis itself should then be aligned
north-south, with its upper end facing true
north. (Not magnetic north!) An approximate
heading can be obtained by sighting along the
polar axis so that it points towards Polaris.
With such approximate alignment, the tele-
scope should be able to track objects with
only occasional corrections on the declination
axis.
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