For most people a telescope is the logical choice for astronomy and that is what this site is mainly about. But Binoculars can offer great views of the Moon, planets, comets, Milky Way star fields, and some large deep-sky objects. And they are portable and relatively inexpensive compared to Telescopes and provide a very good way of learning about the constellations.
Thus for some people starting out in astronomy, particularly for parents with youngsters for whom it may be just a fad, binoculars can be preferable to a telescope. They have a wide field of view and provide right-side-up images, making objects easy to find. They don't require any setup and are very portable, ideal for those clear or partly clear nights when you don't have the time or inclination to get a telescope out. And for most people, observing with two eyes open rather than one seems more natural and comfortable.
However, they do have a few disadvantages compared to telescopes. If they are knocked they may go out of alignment and the cost of repair for the cheaper models will be more than they are worth. They are also dew magnets and it is not easy to protect them in the same way as telescopes. But the biggest disadvantage is the difficulty in holding them steady enough to make out any detail. So unless you pay a lot more money for an image-stabilised binocular (see later) the only really sensible option is a tripod on which to mount it. But if you are tall or suffer back or neck problems it can still be difficult to get one sufficiently tall to comfortably view objects high in the sky.
Mirror mount. This is another option which completely eliminates all the neck bending and discomfort associated with looking up into the sky with binoculars. It quite literally turns things upside down by using a high quality surface aluminised mirror to reflect the image upward before viewing at the binocular. Unfortunately this does result in an upside down image so making it a bit more difficult to find your way around. But sitting down is much more comfortable than standing and, most important of all, you have a steady image.
As is discussed later under Telescopes types an important feature is the Binocular Aperture. This is the diameter of the front lenses and the larger the better - if you can hold them steady. It is reckoned that astronomical binoculars should have a diameter of at least 40 mm, unless you use the expensive image stabilised ones. Those with apertures of 35mm or less are ok for daytime viewing, but they don't gather enough light to give good views of most night-time objects.
Binocular specifications, 8x45, 10x50, etc., are displayed on the side of every binocular. These give the magnification and the aperture or diameter (in mm) of each of the front lenses. Thus an 8x45 has a magnification of 8 and an aperture of 45mm and 10x50s have a magnification of 10 and a 50mm diameter. For astronomical purposes go for binoculars that magnify at least 7 times. As regards aperture the highest you'll want for hand-held binoculars is usually about 50mm. Binoculars with magnifications above 10x and apertures greater than 50mm need a tripod to deliver good views.
Also important is field of view. This is usually displayed somewhere on the binoculars and expressed either in degrees or in metres at 1,000 metres (for example, 105m at 1,000m). To convert to degrees, divide the number of metres by 17.5 (e.g. 105/17.5 = 6°). Most binoculars have fields of view from about 5° to 7° whereas many telescopes make do with 1/2° or less! The Moon and Sun are around 1° in diameter and the Andromeda Galaxy stretches to about 3° so you can see all of these in binoculars.
And then there is the exit pupil. This describes the width of the focussed beam of light you look at. The exit pupil is the aperture divided by the magnification and all 10x50 binoculars have an exit pupil of 5mm and 7x50s have exit pupils of about 7mm. Why is this important? Because of the size of the pupils of our eyes - or how wide open they can be.
We all know that the human eye adapts to darkness - our pupils 'become' larger. Unfortunately, as we age our eyes' adaptation ability decreases. For youngsters the pupils can dilate to 7 or even 8mm, but by the time we reach our 40's it will typically be around 6mm, whilst 5mm may well be the maximum in our later years. If you use binoculars with an exit pupil wider than your pupil size, some of the incoming light won't get into your eyes and images will appear dimmer.
There are different types of binoculars according to the arrangement of the prisms. For astronomical use the Porro-prism binoculars are generally preferred because they provide total internal reflection. But they usually are heavier and bulkier than roof-prism models. Most better quality binoculars are made of barium crown glass (BaK-4) instead of borosilicate glass (BK-7). They also have coated optics which will give brighter and higher contrasting images.
Most binoculars have a central focusing knob that moves both eyepieces at once as well as one eyepiece that can be focused individually - usually the right one. Thus first use the central knob to focus the eyepiece that can't be individually adjusted, then focus the other eyepiece. On other binoculars, which tend to be more rugged and better sealed against moisture, both eyepieces can be focused individually.
Tripods come in a wide variety of sizes and shapes and this site isn't the place to discuss them in detail. Suffice to say that, like telescope mounts they should be strong enough to support the binocular and stable enough to ensure that they don't wobble at the slightest breeze. There are also the monopods which attach to tables and the mirror mounts as previously mentioned.
Image Stabilised Binoculars. Canon make a number of these which range in price from the cheapest 8x25 at around £230 all the way up to 10x42L at about £1630. For astronomy the cheapest practical ones are probably the 10x30 at around £330. Are they worth it?
Well all the reviews say yes, if you can afford it and absolutely great for wildlife. To quote from one reviewer
'The IS feature is simply incredible. Within a second the view stabilizes as if mounted on a tripod. So much more detail can be seen when the image doesn't look like an earthquake. Once engaged the IS view difference is dramatic. Much finer detail can be perceived without the wiggle. To use image stabilization on the 10X30's a button on top has to be depressed and held... the optics and color although reasonable are not "the best'. Bright targets have some false color and stars near the edges did not appear flat. However, with IS kicked on, I could actually see and study craters on the moon. I found myself studying the Orion Nebula with them '
And if you can afford more the Canon 10x42L IS and 15x50 are really fantastic. But they are quite a bit heavier and very much more expensive.
Finally zoom binoculars: these offer a range of magnification from maybe 10 to 30 times. But you are probably better off with a 'proper' Telescope or Image-Stabilised Binoculars. An excellent guide to binoculars is Stargazing with Binoculars by Robin Scagell and David Frydman.
Praktica 10x50P Porro Binocular
Basic table top tripod
Bresser field tripod
Vanguard VS-92 Pod-clamp
ScopeTeknix Binoflex ST50 mirror binocular mounting
Canon 10x30 Image Stablisers Binoculars