Page 51 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO USE

An eyepiece sits in a
telescope’s focuser,
held there tightly by
a little screw

eyepieces. Like most that come with small, child- At least I got into good habits, though. I still take
friendly refractors, or indeed refl ectors, they were care of my eyepieces: the last thing you want to do JARGON BUSTER
not of the greatest quality. But nevertheless, I used with these seemingly insignifi cant, yet important,
to put them back into their little boxes after every things that you pop into the end of your telescope is • EXIT PUPIL This is the size
of the image that comes out
observing session and I made sure they stayed to get them scratched or damaged.
of the eyepiece. Ideally it
scrupulously clean. If I hadn’t, perhaps I’d have had That’s because an eyepiece is just as important
should be close to the size
to replace them sooner – and so would have realised as the scope’s main lens or mirror. It takes the of your dark-adapted pupil
earlier how much I was missing! light that’s captured and focused by the scope and – around 5mm to 7mm.
magnifi es the image that goes into your eye. It • EYE RELIEF This tells you
sounds simple, but the eyepiece needs to do this how far your eye must be
from the eyepiece in order
effectively if you’re to get a really good view.
to see the entire fi eld of
view. A bigger distance
COST AND QUALITY
(called longer eye relief) is
Another reason to keep your eyepieces in the best useful if you wear glasses.
possible condition – and possibly one reason why
they are not always a major consideration when
you’re buying observing equipment – is the cost
of replacing them. The better little cylindrical
eyepieces are manufactured to an exceptionally
high standard. Some have multiple glass lenses
inside that fi t together to give you a beautifully
crafted accessory that will last and last. You can
pay anything from around £30 up to £400 for a • FIELD OF VIEW
good eyepiece – and an item with that kind of price This is sometimes
tag is something you’ll defi nitely want to look after. abbreviated to FOV, and
The diameter of an eyepiece gives some
ULTRA-WIDE ANGLE indication of how well it’s built. If the barrel is the fi gure that lets you
know how much of the sky
you can see through your
As the name suggests, this provides you with measures just under an inch in diameter (and most eyepiece. This measurement
an ultra-wide 82° or so fi eld of view, which is eyepieces are described in imperial units) then it’s is given in degrees.
just gigantic. There is also a super-wide angle most likely been given away with one of the cheaper • POWER This is just
version with a 67° fi eld of view, but the scene telescopes. But in truth, neither the telescope nor another name for
through an ultra is something else. If you magnifi cation. Remember,
took one apart (though this is certainly not the eyepiece will be with you for the long-term. a telescope just captures
recommended) you would fi nd six or seven Most decent telescopes for beginners have a 1.25- the light – it is the eyepiece
elements, all coated to provide you with the inch eyepiece barrel; when you get up to the really that magnifi es the image.
best light-gathering possible. good stuff, though, it’s two inches all the way.

skyatnightmagazine.com 2012 51