Binoculars may not immediately seem the most obvious tool when it comes to astronomy but in many ways a pair of binoculars has advantages over a telescope, especially for the beginner. Binoculars are portable, durable and are easier in general to use when compared to a telescope.
Binoculars come in two main sorts, Roof Prism binoculars and Porro Prism. Porro Prisms are the more common sort and probably the best for astronomy. The different design of the two has to do with the use of prisms and the passage of light. In both cases though, the use of the objective lens, ocular lens and prism, allows for the observed image to be flipped, ensuring that it looks the correct way up.
Binoculars come in a range of magnifications, a magnification that is normally indicated by the first of two digits in a sequence, a sequence that looks like 7×40 or similar. It may be tempting to get as great a magnification as possible, but although binoculars will often go up to 15x magnification there are limiting factors. The greater the magnification the greater the need for a steady hand, or tripod, otherwise objects can be continually go in and out of view as the hand shakes. People who wear glasses also often suffer eye strain from the greater magnification
The second number in the sequence relates to lens size. The higher the number the greater the lens size in millimetres, and the greater the lens size the greater the amount of light that is let into the binoculars. This is an important factor when it comes to observing the overall darkness of the night sky.
The magnification and the lens size are also important when it comes to focusing on observed objects, and in particular the exit pupil size of the binoculars. Dividing the lens size by the magnification gives an exit pupil in millimetres. During the day the human eye has a pupil size of about 2mm, which expands as it gets dark to about 6mm, although this figure depends on age. The older a person is, generally the smaller the dilation of the pupil. If the exit pupil of the binoculars is wide than the pupil size then images will appear dimmer as the eye wont be able to take in all of the available incoming light.
Magnification will also have an impact upon the ability of the eye to focus, and relates to the distance that the eyes are away from the secondary lens. Most binoculars can be focused, either both lenses being focused together or individually, but if insufficient focusing is achieved then it can lead to an uncomfortable time viewing through the binoculars.
Larger lenses and greater magnification leads to a larger pair of binoculars, and thus greater weight to carry around as well.
When it comes to choosing binoculars for astronomy the type of glass used, and its coating can also be important considerations. Barium crown glass is better than borosilicate glass, but there are a number of other types of glass used in the manufacture of binoculars. The glass is also often coated to help with the transmission of light though the lenses. This coating can be important, as although some coatings will help when looking at the night sky, a ruby coating will aversely affect the viewing experience, as it is a coating designed for the binoculars used during the brightest part of the day.
A good pair of binoculars can be stored in a car, and so the magnificence of the night sky can be observed when the fancy takes you. Thus choosing a good pair of binoculars for astronomy is something that is well worth spending time over.
