Page 53 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO USE

KNOW YOUR EYEPIECES

There are four main types of eyepiece. Adding a Barlow lens will increase their magnifi cationere are four main types of eyep

RADIAN NAGLER
Around £180 From £170 to £440
The Radian is one of The Nagler’s most
PLÖSSL LÖSSL the newer types of impressive attribute
From £20 to £150 m £20 to £150 eyepiece on the market. is its huge fi eld of
With a fi eld of view view. While other
Plössls have a wide fi eld of view (around comparable to a Plössl, manufacturers keep their
an
52°), so they can be used successfully for you may wonder what the eyepieces within the human
planetary as well as deep-sky viewing. The difference is? Well, one is eye’s 50° fi eld of view,
to
only drawback is the short eye relief that the big eye relief – even Naglers go the extra mile to
becomes an issue with focal lengths of 12mm with focal lengths down to develop an ultra-wide 82° °
or less. Eye relief refers to how far your eye 3mm. This is a lifesaver if you fi eld. Imagine the amazing g
bulae
must be from the eyepiece in order for you need to wear glasses while vistas of star fi elds and nebulae
to see the entire fi eld of view. observing, and very user- you get with that! The design incorporates
The internal construction of a Plössl friendly for everyone else. The design suits six or seven elements, all coated with special
eyepiece consists of two back-to-back lens medium and higher magnifi cations in order chemicals to increase the amount of light that
systems. There’s quite a price variation to get plenty of detail when looking at the travels through the eyepiece. The downside to
between the highest quality examples and planets. Internally, there are six or seven lens some of these eyepieces is their weight, which
those produced more cheaply. elements that have very short focal lengths. may require you to rebalance your scope.

ORTHOSCOPICSCOPIC DOUBLE UP WITH A BARLOW LENSITH A BARLOW LENS
From £30 to £7575
It isn’t actually
These were the This is a marvellous bit of kit. It isn’t actually
mainstay for an eyepiece, but has optical elements
elements
o increase
many an amateur eur that work with an eyepiece to increase
eved by a
astronomer the magnifi cation. This is achieved by a
cally slot
until the Plössls very simple process: you basically slot
ens and the
took over, but the eyepiece into the Barlow lens and the
Orthoscopics are still re still whole contraption gets popped into
ed into
rmally
good little eyepieces. They’re made h d where the eyepiece would normally
w,
with a four-element optical system that go. Depending on the Barlow,
provides very good eye relief. The design you can double or triple the
also keeps down the amount of light that is magnifi cation you would get
refracted within the system very effectively. from the eyepiece alone. This
The fi eld of view, at only 40° to 45°, may means that with one Barlow
not be as great as a Plössl, but they are lens you have effectively
still pretty good all-rounders. They come in doubled the number of eyepieces
eces
particularly useful for making observations of – and therefore magnifi cations s
sal.
the Moon and planets. – that you have at your disposal.
FIELD OF VIEW
FOV: 2º7’ FOV: 51’ FOV: 25’
These three diagrams show what
fi eld of view (FOV) is all about.
Needless to say, the wider the fi eld
of view, the more of the sky you
can see. The fi rst view of the Moon
shown here is that seen using just a
25mm eyepiece. In the next image
we take an even closer look, with a
narrower fi eld of view, by changing
to a 10mm eyepiece. Finally, an
even smaller fi eld of view as we
use a Barlow lens with the 10mm
eyepiece. The FOV is given in
degrees (°) and arcminutes (’) 25mm eyepiece with a telescope 10mm eyepiece with a telescope 10mm eyepiece and 2x Barlow with
above each view. focal length of 650mm focal length of 650mm a telescope focal length of 650mm

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