Page 35 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO USE

Light exits
HOW DO
Eyepiece
Ocular
lens BINOCULARS
Ocular
lens
WORK?
Porro
prism
A pair of binoculars is basically a device to gather light. You
could think of them as two identical telescopes strapped side
by side to point in the same direction.
Light from the object you’re looking at has to pass through
Porro the objective lens fi rst. This is a convex piece of glass that
prism concentrates the light into a beam that converges on the fi rst
Porro prism. These prisms optically fold the light path inside
the binoculars, so there’s a longer focal length and more
magnifi cation than you’d expect from the compact size. They
also turn the image the right way round. After that, the light
travels through the focusing lenses to end up at your eyes.

FINDING FOCUS

To get the best view through
binoculars, adjust the
eyepieces to fi t the distance
between your eyes and
Objective focus them both to give
lens sharp views.
• ADJUSTING EYEPIECES To
get the distance between
the eyepieces to match the
Light enters distance between your eyes,
called the interpupillary
distance, move the two
< From left to right, halves of the binoculars
the view through around the central hinge
binoculars of the until the view through both
crescent Moon, eyepieces is a clear circle.
the Orion Nebula, • ADJUSTING FOCUS Close
the Pleiades and your right eye and look
Jupiter with its four through the left eyepiece,
largest satellites then adjust the central
focusing wheel to sharpen
the view. Now close your
left eye and look through the
The Moon is a must. The eye shows light is a stellar nursery made up of dust and gas, where right eyepiece, which has its
and dark areas, and several splats of impacts more than 1,000 stars are currently being created. own focus adjustment. Turn
around craters, but binoculars reveal the craters Next, fi nd the Pleiades in Taurus, one of the this until you get a sharp
themselves. Then there’s the mountainous terrain, fi nest star clusters in the sky. To the eye, this looks focus. The view through both
eyepieces will be sharp.
all brought into great relief by highlights and like a close family of six or seven stars (though some
shadows created by the Sun’s light. The best time to people can see 12 or more), but with binoculars this
look at the Moon is between the crescent phase and number increases to around 40.
the gibbous phase, when the Moon is more than half Finally, would you believe you can see another Try out a few
but less than fully lit up. planet’s moons? Jupiter’s four main moons, Io, WHAT pairs of binoculars
Europa, Ganymede and Callisto, will look like four NEXT to compare
SEEING STAR BIRTH little points of light through binoculars. Because the them. Your local
If you look at the Orion Nebula with your eyes, all inner moons orbit in just a few days, it is possible to astronomy society is a good
place to go. Exploring The
you’ll see is a small smudge below the three belt see some movement in the system over the course
Night Sky With Binoculars by
stars of Orion (and if light pollution is bad where of a few hours. As for the planet itself, this just looks
Patrick Moore (Cambridge
you live, sometimes not even that). But train your like a star when you see it with your eyes, but with University Press, 2000) is also
binoculars on it, and it will look like a delicate semi- binoculars you’ll be able to see it as a disc of light well worth reading.
circular curving structure with a bright centre. This – very exciting the fi rst time you see it!
skyatnightmagazine.com 2012 35