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Choosing a First Binocular for Astronomy

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I strongly recommend that you read this page in conjunction with Binocular Basics.

1.  What makes a Good First Binocular?
2.  Realistic Expectations and Quality
3.  10×50
4.  15×70
5.  Best Buys
5.  Links


1.  What makes a Good First Binocular?


Binoculars are described by two numbers, magnification and nominal aperture, for example, a 10×50 binocular has a magnification of 10 and an aperture of 50mm.  (See here for more information and explanation).

A larger aperture lets more light into the instrument, enabling youto see fainter objects.  You should therefore consider the greatest practicable aperture (more on "practicable" later).  It also theoretically increases the resolution of the binocular but, since the binocular will not be used at a magnification anything like enough to achieve maximum resolution, this is largely irrelevant.

The magnification is important for two reasons.  Firstly, it enables you to make out detail that you would not normally be able to see.  Secondly, increasing the magnification darkens the sky background, improving the contrast of some objects.  Unless you observe under exceptionally dark skies, you should therefore consider the greatest practicable magnification.

So, what do we mean by "practicable"? If we increase the aperture, we increase the physical size and, more importantly, the weight, of the binocular (if we double the aperture of a lens, its weight will increase by approximately eight times).  This makes it more difficult to hold steadily for long periods.  If we increase the magnification, we magnify our "shake"; this makes it more difficult to observe satisfactorily and actually makes some objects invisible.  We can ameliorate some of these effects by holding the binocular in a more effective manner, or by mounting it.

It varies from individual to individual, but a general rule of thumb is that almost all adults can comfortably hand-hold an 8×40 for sustained periods, for many a 10×50 is the largest size for sustained hand-held observing, and that a 15×70 is hand-holdable for short periods. The shake you get when hand-holding a binocular is not usually obtrusive on large objects (e.g. Orion Nebula, Andromeda Galaxy), but is infuriating on double stars that test the binocular (e.g. Albireo or δ Cephei for a 10×50)

The question of "Porro prism" or "roof prism" inevitably arises (see here for more information).  Briefly, a roof prism needs to be made to a much higher optical standard than does a Porro prism, you will therefore in general get the equivalent optical quality for less money if you opt for a Porro prism binocular.


Left: Porro-Prism, Right: Roof Prism

Other factors that you must consider are general optical and mechanical quality.  Nobody ever complains that the quality of a piece of observing kit is too high!   Once you become aware that your binocular is of a lesser quality, it becomes increasingly difficult to enjoy it.  Do not be tempted with claims of exotic glass or coatings, in budget binoculars, much of it is marketing hype and there are much more important considerations for binocular choice, such as crispness of focus, flatness of field, edge distortion, chromatic aberration, build quality, smoothness of focus ...  I could go on ...  and on.  The first binocular I ever used for astronomy, a 1960s or '70s vintage Zeiss 10×50, had BK7 prisms and single-layer coatings – overall it was optically and mechanically superior, and hence better suited to astronomy, than many of today's budget fully multi-coated efforts with BaK4 prisms. 

In my opinion, the best choice for a first binocular for astronomy is a 10×50.  I'll expand on "Why?" below

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2.  Realistic Expectations and Quality

Where binoculars are concerned, you tend to get what you pay for and, if something looks too good to be true, it probably is.

It is nowadays possible to obtain a 10×50 binocular for as little as £15 and a 15×70 binocular for around £50.  These budget 15×70s, most of which are made by the Chinese manufacturer Kunming Optical (also known as United Optical and Binoculars China), are a remarkable achievement for the price, and have made this class of binocular available to many more people than was the case 20 years ago when you would have needed to pay ten times as much for a 70mm astronomical binocular (albeit of far better quality).  However, these binoculars are over-represented in "how do I solve this problem?" type threads on astronomical forums.  Think about it: a minimally decent astronomical eyepiece costs about £40.  A binocular has two eyepieces, two objectives, two different focusing mechanisms, prisms and housing, and other bits of tubing: realistically, what sort of quality is it reasonable to expect for the cost of one and a quarter eyepieces (in the case of the 15×70)? Many people claim to be satisfied with these binoculars and undoubtedly some are.  It is also the case that we find it psychologically difficult to admit to a poor choice, so we tend to fool ourselves — I have been as guilty of this self-deception as anyone else; it is a human trait.

But don't take my word for this quality, read what the manufacturer says:
For years, the international markets are flooded with unbelievably low-price Chinese binoculars and some of the users have been complaining or bashing loudly about the quality control consistency of Chinese binoculars for a while.  Actually, it's quite simple to improve the quality consistency: spending much more time in grinding and selecting glass, spending much more time in training the workers for assembling the binoculars, and spending much more time in the final quality check - then, a much better quality binocular will be made, however, at the trade-off of much higher production cost.


Two 15x70 Binoculars   Left: Kunming BA1 (Budget)   Right: Kunming BA8 (Premium)

This confirms a long-held suspicion that Kunming finds it more cost effective to let the customer do the quality control on these budget offerings.  An unknowledgeable customer is more likely to accept a binocular of such poor quality that any self-respecting quality controller would have to reject, and the company only effectively pays for the quality control of rejected binoculars.

A common feature of these budget offerings is that they lose collimation very easily.  A well-made binocular will hold collimation for decades unless it is severely abused (usually to the extent that the outer casing is dented or shows other signs of abuse); a high proportion of the budget binoculars reach the customer already out of collimation.  Poor collimation may result in double images or, if it is of lesser severity, eye-strain that can lead to headaches and/or nausea.

It is therefore of utmost importance that you learn to evaluate a budget binocular if that is what you are considering.  For this reason, I publish this guide to evaluating binoculars.

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3.  10×50

In my opinion, the best value for money all-rounder is a Porro-prism 10×50 binocular of the best quality you can afford.  The reasons for this are:


A selection of inexpensive 10×50 Binoculars. From L-R:
Zenith: An old fashioned style. Available very cheaply second hand.
Swift Newport Mk2: A robust 1-piece body with wide-angle eyepieces.
Helios Naturesport: A modern, lightweight binocular; a good all-rounder.
Strathspey Marine: Very robust. Has individually focusing eyepieces, which is good for astronomy.


Some authorities recommend a 7×50 – There is a slight increase in brightness of extended objects (e.g. nebulae and galaxies), but this is matched by a corresponding increase in background sky brightness, so there is little real advantage except on perhaps half a dozen very large objects (e.g. M33, NGC 7000).  However, it does have the advantage that it is easier to hold steadily. One thing to be aware of is that internal vignetting (common in budget binoculars) can reduce the effective aperture to around 42mm, so you may not get the full 50mm aperture. 

With that last point in mind, if you are concerned about your ability to hold a 10×50 steadily, you may prefer to choose a 8×40 Porro prism, or a 8×42 or 10×42 roof prism, provided the stated aperture is correct. You should also be aware that the prisms of many budget binoculars are very easy to dislodge, resulting in poor collimation. It is often possible to restore them to conditional alignment using the collimation screws; this may be a skill worth learning.

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4.  15×70


The 15×70 is an increasingly popular choice, and the increased aperture over a 10x50 (nearly twice as much light throughput to the eye when compared to a 10×50) makes it attractive (but note that this is less than a magnitude of brightness!). The budget ones weigh around 1.3kg, and the good quality ones (such as the Helios Apollo) nearly twice this.  You must therefore consider some form of support if you are going to use it for anything other than very short observations. This could range from the arm-rests on a picnic chair, through an upturned broom as a jury-rigged monopod, to a good parallelogram mount. 

There are many brands of binocular in the budget class, many of which look cosmetically identical.  Kunming permits the supplier to specify various options, so this outer similarity can, and often does, hide subtle differences such as coating quality, lens quality, prism coatings, eyepiece types, internal baffling and the degree of quality control.  You should also be aware that internal vignetting (common in budget binoculars) can reduce the effective aperture to around 63mm, so this would reduce the light gathering advantage over a 10×50 to about 1 ½ times.  The sensitivity of the prisms to accidental miscollimation (mentioned above) also applies here.

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5.  Best Buys

My intention in these pages is to enable you to make your own informed decisions.  However, if you are interested in my opinion, you may find these suggestions to be useful.

If you are considering budget models, please note the caveats above relating to miscollimation and stopped-down apertures.

Around £50

Around £100

Around £150

Around £200

Over £200

Mounting Hardware

There is much more comprehensive coverage of this at Mounting Binoculars for Astronomy (opens in new tab).

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5.  Links

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