Page 45 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO USE

ALIGNMENT TIME

Four steps to getting your scope lined up on the north celestial pole

THE POLE STAR

In the northern hemisphere, we’re
lucky enough to have a fairly bright
star sitting practically at the point that
the sky appears to rotate around:
the north celestial pole. This star is
Polaris, the Pole Star, in Ursa Minor.
Find it and you’ll have found true
north. What’s more, it never shifts
from that position during the night
STEP 1 Adjust the mount’s altitude setting so STEP 2 As well as being angled up, the while everything else in the sky turns
that it’s the same as your local latitude. In the polar axis needs to be aimed so its highest around it.
UK, this will be between 58º (John O’Groats) end points due north. Some mounts have a Polaris is actually 0.7º away from
and 50º (Land’s End). Release the front and big ‘N’ at the top of the tripod to show which the north celestial pole. This tiny offset
back bolts and tilt the mount head so that the side should face north. You can use a doesn’t matter for visual observations,
pointer lines up with the right number on the compass to fi nd out which direction is north, but to take astro images you’ll need
altitude scale, then do the bolts up again. but remember that this will show magnetic more accuracy: polar aligning
Doing this aligns the mount’s right ascension north and we want true north, which is a few through a polarscope takes that 0.7º
(RA) or polar axis with the Earth’s axis of degrees east. At night, fi nd the star Polaris offset into account. Polaris is easy
rotation, so that the two are parallel. and line up the polar axis with it. to fi nd, courtesy of two stars in Ursa
Major known as the Pointers. Simply
draw a line through them and you’ll
end up at Polaris, as shown below.

DRACO

URSA
URSA MINOR
MAJOR
North celestial pole
STEP 3 The mount should now be polar STEP 4 If you need to make any fi ne
The Pointers
aligned. To check that it is, when the stars are adjustments to get the polar axis aimed at the
out look along the polar axis up at the sky north celestial pole, use the altitude and
and make sure that it is pointing at the star azimuth settings. Make altitude adjustments
Polaris. This kind of visual alignment is fi ne for like those covered in step 1. To make azimuth
making observations through the eyepiece. adjustments, unscrew the two azimuth bolts to
But for more accuracy – if you want to take move the mount head and scope left or right Find Polaris by drawing a line to it
photos through your scope, for instance slightly, parallel to the horizon. This is easier through the two stars in Ursa Major
– you’ll need to polar align looking up than lifting the tripod and the whole setup to known as the Pointers (see page 15)
through a polarscope fi tted in the RA axis. aim the scope due north.

also has to be aligned to this axis of rotation close to the celestial pole. This provides an Once the mount has been lined up on the
to track the stars’ movement. Equatorial instant ‘marker’ – and the good news is that celestial pole, your scope will track the stars
mounts are designed specifi cally to be polar for visual observations, you don’t even need with ease and you’ll fi nd it simple to keep
aligned – if you don’t bother, you might as to be overly accurate in your polar alignment. objects in your eyepiece for longer. You only
well have saved your money and bought a It’s simply a case of adjusting the altitude need to adjust the RA or polar axis with its
cheaper altazimuth mount. setting so it’s the same as your local latitude slow-motion control to do this. It’s unlike a
(fi nd this at http://itouchmap.com/latlong. camera-type altazimuth mount, which needs
POLE POSITION html), then pointing the polar axis north so its two axes to be adjusted to track objects.
When it comes to getting your mount’s polar it’s lined up on Polaris. If you’re intending to But remember that even an equatorial mount
axis pointing in the right direction, those of do any astrophotography, though, then you’ll will need both its axes adjusted when you
us in the northern hemisphere have a helping need to be more accurate, and you should want to move the scope so that it points at
hand because the bright star Polaris sits very polar align using the mount’s polarscope. another star.
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