Page 40 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

KNOW YOUR

TELESCOPE

MOUNTS A LOOK AT THAT CURIOUS

STRUCTURE UNDER YOUR
TELESCOPE: THE MOUNT

The fi rst question you might ask about a telescope, if you’re new and right. Many telescopes come with this variety of mount because
to astronomy is: by how much can it magnify? The idea that you’d they are easy to make. When it comes to observing you just plonk
look at the mount to see how sturdy it is might seem absurd. It’s as them on the ground and away you go.
though the telescope is the amazing bit; the rest – the tripod that Unsurprisingly, though, there are drawbacks to this simple
keeps the scope at the right height to look through, and the mount solution. One is that to follow an object as it appears to move across
that fi xes the scope to the tripod – is incidental. the sky from east to west, you’ll need to move your telescope in
But don’t be fooled: tripods, mounts and telescopes go hand in both directions at once to keep up with it: both upwards, and to the
hand. One is no good without the other two. The telescope shouldn’t right. Another is that the view in the eyepiece rotates as the Earth
wobble too much, or knock you off-target when you give the mount turns on its axis. In other words, the orientation of the object you are
and tripod a light tap. Generally, the cheaper the telescope the looking at changes very slowly. This means that without expensive
cheaper the mount, so it is always best to see any potential scope equipment, an altazimuth mount is not suited to taking astrophotos
setup in the fl esh to make sure that everything is good and sturdy. that need the camera shutter to stay open for any length of time.
The two most common types of mount are the altazimuth mount
and the equatorial mount. Which one is best for you depends on what THE EQUATORIAL ADVANTAGE
you want to do with them. Altazimuth mounts are the simplest. They There are no such problems with equatorial mounts. These also have
move in altitude (that’s up and down) and in azimuth, which is left two axes of movement, but instead of an azimuth axis that’s parallel
to the horizon, equatorial mounts have a polar
axis tilted so that it’s parallel to the Earth’s axis
HOW IT WORKS of rotation. This means that when you’re setting it
up, you’ll need to fi nd the latitude you’re viewing
POLAR ALIGNMENT from and set the polar axis to the same angle. If
you don’t know your latitude, you can fi nd it from
The closer your equatorial mount is a local Ordnance Survey map, or using a website
aligned to the Pole Star the better, such as http://iTouchMap.com.
but you don’t have to be exact. If Before observing with an equatorial mount,
it’s close enough to the Pole Star,
objects you’re viewing won’t drift you need to ‘polar align’ your scope. This means
north or south very much over that the mount’s polar axis, the one you aligned
time. Once you’ve got the knack, to your latitude, must be pointing very close to
you’ll need less fi ne adjustment the Pole Star, so that it is aligned to the axis of
of the declination axis to keep a Earth’s rotation. You’ll fi nd the Pole Star (also
celestial target centred. Equatorial called Polaris) on star charts, in the constellation
mounts sometimes have a motor Ursa Minor. Line your mount’s polar scope up on
that drives the polar axis. This
ILLUSTRATION BY PAUL WOOTTON, PAUL WHITFIELD X 2 same speed that the sky appears will show you how to use the polar scope.
the Pole Star, and then use it to align the axis to
makes observing easier, because
the motor drives the mount at the
the north celestial pole. Your telescope’s manual
to rotate, so you only need to
The beauty of going through all this is that
adjust the declination axis to keep
once it’s done, you only need to twiddle one dial
an object centred in the eyepiece.
to follow the sky. Also, the view does not rotate,
because the scope turns with the sky. With this
type of mount, even if it’s only roughly aligned,
With an equatorial mount you
can polar align your scope with
you’ll be able to keep stars and other objects in
the north celestial pole
celestial bodies you’re looking at.
40 skyatnightmagazine.com 2012 view for longer, and even take pictures of the