Choosing Binoculars for Astronomy
Introduction
When I was
a child many popular astronomy books of the time recommended starting off with
binoculars. I tried it, but quickly moved on to a telescope, so it’s only in
recent years that I’ve started to really enjoy using handheld binoculars for
astronomy.
There’s no
doubt that under a dark starry sky, astronomy with a simple pair of handheld
binoculars can be one of the most pleasurable ways of enjoying this hobby: it’s
simple and immediate. Swapping between naked eye and binoculars you can
appreciate the beauty of the whole sky and I know of no better way to learn
your way around the constellations. You can make an evening of it, seeking DSOs
with a red torch and star charts to guide you, or just pop out for fifteen
minutes to catch a few highlights.
So which
binoculars should you choose? In some ways this is quite a difficult question
to answer and much has been written about it in the online forums, some of it
good advice, some of it partisan rubbish from people with a pet make, model or
type. In the following paragraphs I’ll try to take an objective (excuse the
pun) look at buying hand-held binoculars for astronomy.
Binoculars
for Astronomy
Most
hand-held binoculars are designed for birding, not astronomy. Mostly a good
birding binocular makes a good astronomy binocular as well, but in some cases
the requirements do differ. For example, many birding binoculars are
waterproof, but this isn’t exactly necessary for astronomy (although if you
accidentally leave them on the lawn overnight, it might be)! A good birding
binocular needs quick focusing, but this isn’t essential for astronomy either.
Then again, for daytime use you rarely need objectives bigger than 30-40mm: any
more is just wasted because your pupil contracts and effectively masks off (‘vignettes’)
a larger objective. For astronomy, though, your pupil is bigger in the dark, so
you can benefit from the extra light gathered by bigger objectives. So what
characteristics should you look for in binoculars for astronomy?
Objective
size
To start
with, let’s consider the two main binocular characteristics: objective size and
magnification. Binocular sizes are generally quoted as magnification x
objective lens size in millimetres, so that 10x50 means 50mm objectives with a
ten times magnification. Hand-held binoculars come in numerous sizes between
8x20 and 20x60.
Three
of many possible binocular sizes: 15x56, 12x50 and 7x42. These three pairs all
work well for astronomy.
Apart from
looking at the Moon, no binocular with lenses smaller than 30mm are much use
for astronomy and 40-50mm are better. Lenses in the 50-60mm range may make the
binoculars too heavy to hold for long, but may just be worth it in terms of the
brighter images you get and the fainter stars and nebulae you can see.
Light
gathering power is a function of the area of the lens, so 56mm objectives
(about the largest you get in a hand-held binocular) gather twice as much light
as 40mm objectives.
Binocular
Objectives: 42mm, 50mm and 56mm
Power
In terms of
magnification, 7x used to be the recommended power for
astronomy and a good 7x binocular does give a wide, steady view. But in modern
light-polluted skies, 7x and even 8x are too low, tending
to accentuate sky glow from city lights, unless you live under very dark skies.
Such a low power will also make it hard to find smaller deep sky objects like
nebulae and globular clusters, which are easy to find with say a 12x
magnification. Magnifications of 10x and above also give
a much more involving view of the Moon.
About 12x is the comfortable maximum that most people can hold
without shakes blurring the view too much, but you might just get away with 15x
if you have a steady hand or lean on a convenient wall (or bush, car etc!) for extra support. Anything above 15x
would need some kind of mounting.
In short, 10x50 is an ideal
binocular size ideal for a beginner,
but you might eventually find that the effort of using say 12x50s or 15x56s is
worth it for the more involving views you get.
The effect of exit pupil for older
users
The size of
the exit pupil of a binocular – the little round bright image of the objective
you see in the eyepiece – is simply the objective size divided by the power.
For a 7x50 (the size often recommended in older books) the exit pupil is thus
about 7mm in diameter. But this is a problem. Only younger peoples’ pupil can
open that wide; older people can manage perhaps 5-6mm. So if you’re over forty and buy 7x50s, you are effectively wasting
aperture and 7x40s will seem just as bright!
Binocular
exit pupil
Field of view
Nikon
12x50 SEs - 5 degree true field of view
A wide apparent
field of view is preferable (I like 60 degrees or more, to avoid that “looking
through a straw” feeling), but avoid very wide fields as you often get a lot of
distortion at the edges. Some birding binoculars deliberately leave in some
field curvature for comfortable panning, but this isn’t ideal for astronomy,
where a flat field makes viewing extended objects like star fields more
satisfying. A bright flat narrow field is actually better for astronomy than a
wide, dim curved one. But consider that a high magnification and a small true
field may leave you looking at such a tiny patch of sky that it’s difficult to
find things!
All-in-all a moderate field of view
may work best for astronomy – something like 6 degrees in a 10x binocular. However, some premium designs,
like Swarovski’s EL 12x50s offer a bit more whilst still having a wide,
well-corrected view across the whole field.
Eye Relief
Eye relief
is the distance behind the eyepiece lens at which the image is formed. Short
eye relief means you have to press your eye right up to the eye lens.
This is an
area where a lot of older binos with simple (Kellner type) eyepieces fall short. If you wear glasses then you’ll want good eye relief – 14mm is about minimum for glasses
wearers and 16mm is better.
I recommend
trying before you buy, not least because as far as eye relief is concerned the
size of a millimetre seems to vary widely between manufacturers!
Binoculars
with a lot of eye relief need eye cups to allow for people who wear glasses
(eye cups in) and those who don’t (eye cups out). Click-stop adjustable
eye-cups are much easier to use than the older rubber fold-down type.
Eye
cups extended for use without specs.
Eye
cups folded back for specs wearers
Optical Quality
Good
optical quality is very important. A good easy test of overall optical quality,
as with telescopes, is focus snap – best focus should be easy to obtain, crisp
and definite. If you find yourself fiddling around trying to get best focus,
buy a different pair.
Modern
binoculars should have multi-coatings. These mean that they transmit a lot more
light to your eye and this makes a big difference for astronomy. Reflect a
bright light in the objectives: the reflection should look dim purplish or
greenish and the best coatings make the lenses almost disappear. Whilst you are
at it, look at the interior of the barrels – the best will have ridge baffles
or very matt black paint to cut out unwanted reflections.
These
Nikons have excellent coatings and barrels with ridge baffles
Point the
binocular to one side of a strong light source – a streetlamp or the Moon, NOT
the Sun - to check for internal reflections which are annoying and reduce
contrast: if the view becomes washed out or strong reflections (“ghosts”)
appear, don’t buy. The “Jupiter” test is another good one. Focus on a bright
planet and you should see a well-defined tiny disk with little smearing or
flare of the light; anything else means the binoculars are of poor quality.
Chromatic
aberration (“CA” - colour fringing around bright objects) occurs because the
lenses don’t bring all colours to the same focus. During the day you can check
this by focusing on something with high contrast (tree branches against a
bright sky, for example) and look for purple or green fringes. Most binoculars
show modest CA, and this isn’t so much of a problem for astronomy, unless it is
severe or you like looking at the moon, but very bright false colours are bad
news.
Finally,
avoid zoom binoculars as these are typically poor optically (Leica’s expensive
dual-power Duovids excepted).
Roofs vs Porros
Roof
and porro-prism binoculars
There are
numerous different makes and designs of binocular, but they fall into one of
two main types: porro prism and roof prism (all
binoculars apart from the simplest opera glasses contain prisms to turn the
image the right way up). Roof prism binoculars are the more “modern” type with
straight barrels, porros being the traditional type
with “shoulders”.
Roof prism
binoculars (“roofs” from now on) have the advantage of being generally smaller
and easy to hold, what’s more they have internal
focusing so it’s easier to make them waterproof. Hold on! Before you rush out
to buy a pair of roofs, know this: for a
given level of optical performance, roofs are about twice as expensive as porros (if not three times) and no cheap roof prism
binoculars are much good.
So, for astronomy you don’t need
roofs!
That’s not
to say that a top-quality pair of roofs from the likes of Zeiss, Leica, Nikon,
Swarovski et al won’t be good for astronomy, it’s just that high quality porros may be just as good, but much cheaper. The problem is that very few
high quality porro prism binoculars are made any
more.
Which
Make and Model: Cheap ‘n’ Chearful or High-End
Quality?
The first
thing to understand is that pretty much nobody makes top-quality hand-held binoculars
specifically for astronomers. Smaller binoculars are designed for nature
viewing and birding, larger ones almost exclusively for hunting.
With
binoculars, you mostly do get what you pay for and whilst the more powerful roofs
(10x plus) from any of the top manufacturers (Zeiss, Nikon, Leica, Swarovski)
will be great for casual astronomy, they will all set you back the thick end of
two thousand pounds new. However, with these binos
you will get pretty much everything: light weight, brilliant, sharp images with
a reasonably wide field of view, complete waterproofing and plenty of eye
relief together with twist-and-click eye cups for easy use with glasses. In
fact, if you are used to cheaper binos and don’t find
them very interesting, the fabulous images from these models may really
surprise you!
But what if
you don’t have a thousand pounds plus spare for a pair of binoculars and don’t
need the ruggedness and waterproofing of top roofs? The answer, of course, is a
good pair of porros. In theory and in practice even
the best roofs can’t quite match the clarity and light transmission of the best
porros (Zeiss’ abbe-prism models possibly being an
exception), so you don’t have to be losing out in optical terms. Trouble is
there aren’t that many really good porro prism
binoculars out there, because most birders and hunters go for the waterproof
ruggedness and slim design of roofs.
Probably the
best small porro binoculars are Nikon’s Superior E
line, which are superb optically and mechanically, but are not waterproof.
These can be had in 8x32, 10x42 and 12x50 sizes. Both larger sizes work well
for astronomy. The Superior E line has been discontinued, but as I write (December
2014) you can still get the 12x50s.
Alternatively,
Fujinon’s 10x50 FMT-SX and FMTR-SX (the “R” means
rubber-covered, otherwise they are the same) are excellent optically and
waterproof as well, but much heavier than the Nikons.
Image
Stabilising (IS) Binoculars
Canon’s
12x36 Image Stabiliser Binoculars
The biggest
limitation for hand-held binos is the shakes that
your body will always impart, however steadily you
hold them. Enter, from the mid-nineties, image stabilising (I.S.) binoculars.
These come in various different designs, from passive ones using suspended prisms
and needing no batteries (Zeiss 20x60), to
gyro-stabilised designs (Fujinon) and ones using a
computer to control special prisms which adjust the light path every
millisecond (Canons). By far the most popular are the Canons and I have
personally owned and reviewed the 10x30s and 12x36s (reviews elsewhere on this
site).
I really
liked the Canon 10x30s for astronomy – basically you just push a button on top
and seconds later the image steadies and the resolution dramatically improves;
it’s really as simple as that. Even the 10x30s will resolve more detail than
non-stabilised binos (albeit with less brightness
than good 10x42s) and I used them as just about my only astronomy tool for 18
months when I was living abroad. However, the 18x50s I tried suffered from some
strange effects in use, like changing focus very slightly from moment to
moment, although the level of detail they offered was amazing. Try before you
buy.
Summary
Ten top tips
for selecting hand-held astro’ binos:
1) Power:
10-12 (15-18 with some support or I.S.)
2) Aperture
40-60mm.
3) Weight: Under 1000g is best unless you’re built like The Terminator.
Take 1500g as the absolute maximum.
4) Eye
relief: minimum of about 14mm for glasses wearers; 16mm is better.
5)
Click-stop eye cups are more convenient than the fold down type for glasses
wearers.
6) Always go
for full multi coatings as they yield a much brighter image. Avoid binos made before the mid Eighties,
as they will have single coatings.
7) Make sure any roofs you buy have ‘phase coatings’; porros don’t need them.
8) Roofs
will always be much more expensive than porros for
the same optical quality, so don’t buy cheap ones: buy porros
if your budget is limited.
9) Don’t
buy zooms (Leica Duovids – dual powers, not zooms – excepted).
10) Always
try before you buy if you can, as comfort when holding and eye relief
requirements can vary a lot from person to person.
Best
Buy
Finally, if
you want my specific recommendation for an astronomy binocular, ScopeViews’ best buy is Nikon’s 12x50 SE. These have (very
sadly in our view) been discontinued, but you may still be able to buy them
here: