Other articles in this series
THE EQUATORIAL MOUNTING
The Alt-Azimuth mounting is
a simple design usually with a means of elevating and rotating the
telescope. In this article we will be
moving on to consider how to use an ‘EQUATORIAL’ mounted telescope to observe
or take images using an attached camera.
The Dobsonian
mount is a very simple version of the wider range of telescope mounts known as
the Alt-Azimuth. This simply means the
mount has one vertical axis that allows rotation around the horizon known as
the ‘AZIMUTH’. It also has a horizontal
axis that provides ‘ALTITUDE’ of the tube assembly from the horizontal position
to vertical. The typical example of this
type of mount is the seaside telescope.
An
simple Alt - Azimuth Mounting A Dobsonian Telescope
An Alt-Azimuth Telescope can
have a telescope tube assembly very similar to the Newtonian usually used with
a Dobsonian mount.
There is however no reason why any one of the many telescope tube
assembly designs can’t be used with any of the Alt-Azimuth or Equatorial Mount
designs.
THE
EQUATORIAL MOUNTING
The Alt-Azimuth Mount is a
simple design with its most simplified version in the form of the Dobsonian. It does
however have one major drawback in that it is quite difficult to use with a
powered drive. Astronomical targets such
as stars, planets and deep sky objects appear to move across the sky in an arc
from east to west. This arced track is
caused by the rotation of Earth on its 23.4 degree tilted axis.
As Earth rotates, the tilt
causes the stars to rise in an arc from the east. The highest point is reached as the stars are
in the south. They then follow the arc
down into the west. By setting the
‘Azimuth’ axis of the telescope mounting at an angle equal to the tilt of the
Earth, the telescope can be made to trace the same arc across the sky as the
stars follow. This tilted ‘Azimuth’
axis, which is known as the Right Ascension (RA) axis, will now be pointed
towards the same point in the sky as does the north axis of Earth; this point
is known as the Celestial Pole. We in
the northern hemisphere are very lucky to have a fairly bright star close to
the north celestial pole. This is the
star Polaris (The North Star or Pole Star) in the constellation of Ursa Minor (The Little Bear).
The major advantage of the
Equatorial mounting is in its ability to be driven on just one axis to track
objects across the sky. By driving the
axis with a motor that will give one rotation every 24 hours the telescope will
rotate at the same speed as Earth. This
type of drive is essential if the telescope is going to be used for photographs
or digital imaging with exposure times exceeding 30 seconds.
There are a number of
different designs of equatorial type mountings.
The beautiful hand drawn pictures below are taken from a very famous
book written in 1937 by Rev. Albert G. Ingalls called Amateur Telescope Making.
The
German Equatorial Mounting design is the most common equatorial mounting used
by modern amateur astronomers. The 120mm
Skywatcher refractor is mounted on a typical mounting
of this type.
This
design is often used to mount large professional telescopes. A similar set-up is achieved by fitting a
Schmidt Cassegrain Telescope (SCT) such as a Meade on
an Equatorial Wedge. The Newbury
Astronomical Society’s SCT (Daren’s telescope) is shown mounted on an
equatorial wedge.
This
design is also used to mount many large professional telescopes. There are a number of variations on this
theme that will allow access to the area of sky around the poles. The upper yoke is often greatly enlarged into
a ‘horse shoe’ shape so that the telescope tube can be pointed through the
upper bearing to gain access to the sky close to the pole. The picture of the 200inch Hale Telescope
shown in the next column shows the upper yoke bearing enlarged and cut away as
described above. The Hale was for many
years the largest telescope in the world.
Setting up an equatorial
mounted telescope is more difficult than an Alt-Azimuth but it is not too
difficult. In both types of mounting it
is necessary to ensure that the mounting feet are stable and level. The levelling becomes more important with the
degree of accuracy that is required for the observing task to be
undertaken. If just visual observing is
planned then an Alt-Azimuth may not need to be levelled at all. However the equatorial really needs to be
levelled for every session to enable it to track objects.
Some Alt-Azimuth mounts have
setting circles to enable objects to be found using the positions given by
charts or computer programmes. To do
this the mount needs to be levelled and aligned with a set position, normally
north. The equatorial mount requires the
same alignment but needs the additional alignment of the Right Ascension (RA)
axis to the Celestial Pole. We are
fortunate to have a star very close to the North Celestial Pole. This star is Polaris (The North Star also
called The Pole Star) located in the constellation of Ursa
Minor (The Little Bear).
Some
equatorial mounts have a small telescope fitted into the RA bearing that can
used to align the axis to the pole star.
If not the telescope needs to be positioned to zero ‘0’ on the RA
setting circle and locked. The
Declination setting circle needs to be set to 90° and locked. The mounting then has to be adjusted so that
the RA axis is aligned to the north and is elevated so that the telescope
points towards Polaris. The accuracy of
the alignment can be checked by rotating the telescope around on the RA axis. Looking through the finder it should describe
a small circle around Polaris. To
accurately align the RA for imaging work requires further alignment beyond the
scope of this article.