Binoculars are a key part of any stargazer’s equipment, and are greatly complement a telescope. The may not give the same high magnification of a telescope, but a wide field of view and both-eye viewing mean incredible views of night sky objects. Whether for a mid-week grab-and-go session, or helping track down elusive objects for further scrutiny with the telescope, a decent pair of bins is essential. When it comes to stargazing, magnification isn’t the be all and end all anyway. In fact, many objects in the night sky are so large that they won’t fit in the field of view if you magnify too much. Binoculars give amazing views of large objects like the moon, the Andromeda galaxy, and large star clusters, such as Messier 44 and 45. They’re a great way to get started with stargazing. If you have any interest in the night sky, get a pair of binoculars, you won’t regret it.
Terminology
Magnification and light gathering: All binoculars are defined by a pair of numbers (8×40, 10×50, 15×70, 20×80, for example). The first number refers to the magnification factor (8 times, 10 times, 15 times, etc). The second number refers to the diameter of the objective lenses (the big ones at the front), and these determine the light gathering power of the binoculars – larger objectives allow in more light, thus allowing you to see fainter objects. Bear in mind that the larger the numbers the bigger and heavier the binoculars will be, and you would have to consider getting a tripod to support them. My Helios Naturesport Plus 10x50s weigh just under 800g, and are just light enough to hold steady. My Celestron Skymaster 20x80s, at just over 2kg, definitely need to be mounted on a tripod.
By far the most common specs for stargazing are 10x50s: 10 times magnification and enough light gathering to view crater detail on the moon, the moons of Jupiter (albeit as tiny pinpricks of light), colours in double stars such as Albireo, and even distant galaxies, such as Messier 81 & 82. I recently spotted comet C/2013 US10 Catalina with mine.
Eye relief: This is the distance you can hold the binoculars from your eyes and still see the whole field of view. Long eye relief binoculars allow you to hold the binoculars further from your face (and are, incidentally, more suitable to glasses wearers). Shorter eye relief binoculars mean you may have to press your eyes into the eyepieces to see the field of view … which would be uncomfortable. Try different binoculars to get a feel for what suits you best.
Exit pupil: This is the diameter of the circle of light that exits the binoculars, and is calculated by dividing the diameter of the objective lens by the magnification figure (my 20x80s have an exit pupil of 4mm, the 10x50s, 5mm). The larger the exit pupil the more light that will pass through to the eyes, and the brighter the object being viewed will appear. However, the diameter of our eyes at night-time would be around 7mm, so, in effect, my eye could actually ‘accommodate’ a larger exit pupil (and hence more light) than my binoculars are delivering. Our dark adapted pupil diameters change as we get older, so an ideal exit pupil for a 25-year-old may not be the same for someone in their sixties. How can you tell your dark adapted pupil diameter? Ask your ophthalmologist, or, for a rough assessment, ask a friend to measure it with a ruler!
Field of view: For astronomy purposes you want a good, wide field of view. Note that there are different definitions of field of view (‘true’ field of view, ‘angular’ field of view, and ‘apparent’ field of view) – be sure you compare like for like … Manufacturers’ field of view specifications usually quote ‘angular’ field of view – this is the actual area of the sky you can view. My 10x50s, for example, have an angular field of view of 6.5 degrees, whereas the angular field of view of the 20x80s is 3.7 degrees. The Helios field of view is large enough to comfortably accommodate Andromeda, or the Hyades, Pleiades, and other large star clusters, such as M44 in Cancer – whereas with the Celestrons, M44, only just fits, albeit at greater magnification.
Evaluating binoculars …
The best advice is to try as many pairs as you can before you buy. Make sure there’s nothing loose and rattling around inside. Check that the inter-pupilliary distance is adjustable to suit you – there’s a central hinge about which the binocular barrels pivot, and you should be able to adjust this so that the images from both barrels form a single, circular view. If you can’t make the separate views come together, walk away. Check that the lenses are clear (not cloudy, dusty or mouldy inside), and that the central focuser is smooth and brings the view to focus (with the right eye closed). The focus of the right barrel is adjusted on the right eyepiece by what’s called the ‘dioptre’: with the left eye closed, the dioptre should be able to focus the view of the right eye.
Check for chromatic aberration (colour fringing on either side of a bright object) caused by different wavelengths of light not being brought to the same convergence point.
Distortion around the edge of the field of view (common on cheaper binoculars, but present to some extent in all but the most expensive models). If you find it’s very obvious, or affects more than the outer quarter of the view, then look for a different pair. With my binoculars, for example, this distortion (in which stars appear as streaks rather than pin-points) is present, but it doesn’t bother me, and I have to actually look for it.
Avoid ‘zoom’ binoculars (these are generally poorly regarded for astronomy), and fancy coloured coatings on the objectives bring nothing to the views. Speaking of objective lenses, avoid anything less than 40mm as the light gathering won’t be adequate for night-time viewing.
There’s no reason to necessarily avoid cheaper/secondhand binoculars, as long as you’ve checked the above points. My son has a pair of £15 8x40s, for example, and they work very well. Ultimately, as long as you can focus them, and both barrels resolve to a single, circular view, they’ll definitely let you see things you wouldn’t see otherwise.
What’s up?
Comet Catalina is ideally positioned for binocular/telescope viewing at the moment; climbing up the left of the Big Dipper. Jupiter is clear of the eastern horizon by about 10PM, and above it you’ll find the Leo galaxy triplet (definitely well worth a look). Best viewed in a telescope, Hind’s Crimson Star in Lepus is a wonderful object, and well worth looking out for: its deep coppery red colour means it really stabds out from the background stars. This first of this year’s evening globular clusters are appearing in the east. Like all globular clusters, M53 and M3 are spectacular objects, and well worth checking out. In the mornings, Mercury (low in the southeast), Venus, Saturn Mars and Jupiter (high in the west) are all visible.
Clear skies!
Kevin Quinn is an amateur astronomer based in Cerne Abbas, he is the proud owner of a ten-inch reflector, a small refractor, a case of eyepieces, and a couple of pairs of binoculars. He tweets via @CerneAstro, blogs via theastroguy.wordpress.com, and his ebook Demystifying Astronomy – A beginner’s guide to telescopes, eyepieces and accessories for visual astronomy is widely available.
©Kevin Quinn
