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. Lot’s of binoculars that seem fine in the daytime will fail this test.

 

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 astrobinos:

 

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: