Tuesday, June 1, 2010

Binoculars in Astronomy

Portable, Cheap, wide field viewing and easy!, you say why not. although they have their limits, you can always enjoy observing by them a good number of sky gems.

Why do you need one?

Amateur astronomers usually view medium aperture (50–70mm) binoculars either as an inexpensive “entry instrument” to the hobby, or as a useful accessory to a more experienced observer’s “main” instrument, a telescope.
There is a great deal of justification for this. Binoculars indeed make excellent starter instruments for new observers, especially those of limited financial means. Not only is a medium aperture binocular of reasonable quality less expensive than the cheapest useful astronomical telescopes, but it is also more intuitive to use, easier to set up,more portable, and has more obvious uses outside astronomy,for example, bird watching or horse racing.

Binoculars can be thought of as two small telescopes fixed together. Inside the binocular, two prisms fold up the light path, making them more compact than a normal telescope. Every binocular is marked with two numbers, such as 7 x 40 or 10 x 50. The first figure is the magnification, which is the enlargement relative to the naked-eye view. The second figure is the aperture (that is, the width) of the front lenses measured in millimeters. 
Wider apertures let in more light, making fainter objects visible. However, larger-aperture binoculars are also inevitably heavier and more expensive than smaller ones. Binoculars with zoom magnification are available but generally produce poorer images.

It also enables the new observer to engage in useful observing programs, such as the Society for Popular Astronomy’svariable star program. Where the more experienced observer is concerned,the wider field of a binocular is ideal for having a preliminary scan around the sky in order to evaluate it at the beginning of an observing session, and is also useful in conjunction with the telescope’s finder as an aid to hunting the objects to be observed.Additionally, there are large objects with low surface brightness, such as the Pinwheel Galaxy (M33, NGC 598), that are distinctly easier to see in such binoculars than they are in most telescopes of even twice the aperture.

What an increasing number of experienced observers are coming to realize is that the binocular is not limited to being an adjunct to a telescope, but is an exceptionally valuable astronomical instrument in its own right.Many of the advantages of the binocular,when used for its “beginner” or “adjunct” purpose, translate to its advanced use.

Binoculars come in all different shapes and sizes, though all share one of two basic designs.
Silhouettes (a) through (c) show typical Porro prism binoculars, while (d) through (g) illustrate the roof prism design.


There are two sides to the portability of binoculars; The first side is the compactness and weight of the instrument itself. A 10×50 binocular is possibly the most common starting binocular and adjunct binocular. It is typically about 18 cm (7in) long and about the same width, and usually weighs a kilogram (2.2 lb) or less,considerably less in the case of lightweight models.
Second, binoculars of this size and weight can easily be hand-held for moderate periods of time, so they do not need a mount to be carried with them. Even 15×70 or 16×70 binoculars, which are typically about 28 cm (11 in) long and around 2 kg (4.4 lb) in weight may be handheld for short periods.

Of course, all binoculars will benefit from being mounted. If a mount is to be carried with this size of binocular, a reasonably sturdy photographic mono pod or tripod with a pan/tilt head will suffice for binoculars up to 80 mm in aperture, or100mm if they are the lighter weight ones. However, it does need to be stated at the outset that the photographic tripod with pan/tilt head, although commonly used, is far from ideal as a binocular mount for astronomy.


Binoculars of 100-mm aperture or smaller are usually trivially easy to set up. If they are to be hand-held (usually 50mm or smaller), all that is required is that the interpupillary distance and focus are set. They do not normally require time to reach thermal equilibrium,so they can literally be regarded as “grab and go” instruments,with observations being made within a minute or so of the decision to observe! Even larger binoculars are generally considerably simpler to set up than many telescopes. 

Binoculars are not generally equatorially mounted on account of the awkward position that such mounting would require of the observer’s head! For this reason, binoculars are usually mounted on some form of altazimuth mount,often a photographic tripod and head. Even with 6 kg (13.5 lb) binoculars on a sophisticated parallelogram mount, I routinely find that I am observing in less than10 minutes of having made the decision to observe.

There are many aspects of telescope astronomy that are unavailable to the binocular user. Binoculars are used almost exclusively for visual astronomy, but for sheer 
enjoyment of the sky, they are unparalleled!

"Field of view,,

The field of view (also simply called the "field" or "FOV") of binoculars is typically 3° to 5° across, equivalent to six to ten Moon diameters. This is a good deal larger than the field of most telescopes, allowing you to view larger and wider areas in a sky. Some objects are ideally suited to observation with binoculars, such as comets with long tails, some scattered star clusters, and Milky Way star fields, and of course casual viewing of the Moon.

"Choosing a Binocular,,

When buying binoculars, take into account who will be using them- a child, for example, will need as smaller, lighter pair. And do not be tempted by binoculars that provide high magnification combined with a small aperture, as the resulting images will be faint and indistinct. Ideally, for astronomical use the aperture of a binocular in millimeters should beat least five times greater than the magnification figure. Binoculars with a smaller ratio are fine for daytime but will produce darker images at night.

Compact Binoculars, Roof prisms allow a "straight through" light path, resulting in a compact design with straight barrels. However, the lenses are relatively small, so they are not the best choice for astronomy.
Standard Binoculars, In these binoculars, the beam of light(yellow line) is folded by two prisms before it exits at the eyepiece, as shown here. The type of prisms in these binoculars are called "Porro prisms". Some binoculars have rubber eye cups on the eyepieces to keep out stray light. Glasses-wearers can push these back to get their eyes closer to the eyepieces.

Standard and roof prism binoculars.

large Binoculars, The more a pair of binoculars magnify, the more the image will seem to shake because of the movements of your hand - even the smallest vibrations will shake the image making it very hard to view. The solution is to mount the binoculars on a camera tripod. Better still, although more expensive, is a specialist binocular mount, available from telescope suppliers.
Newer, image-stabilizing binoculars sense movement and adjust the prisms accordingly. These cut out hand shake very well, but not slower movements. Although these binoculars have the advantage of portability, they are heavier and more expensive than standard types.

When using large, heavy binoculars with magnifications greater than 10 times, a tripod mount is desirable.

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