Finding the Right Astronomy Binoculars
While telescopes are the most obvious tool of the astronomer, they are not the only things which can be used to study the sky. Binoculars designed for astronomy exist, as well, with their own advantages and disadvantages compared to telescopes.
In general, the flaws of a pair of astronomy binoculars are that they will not be able to reach the lens apertures or magnification levels of telescopes – at least, not without being far too large for a normal person to carry. Binoculars also generally have fixed magnifications, variable only by adjusting or replacing the eyepieces. However, they do have a set of advantages as well. Binoculars will tend to be lighter and more portable than telescopes. They will have a larger field of vision than telescopes (which is very helpful when scanning the night sky for something). Binoculars also alleviate the need to have to squint constantly in order to look through a telescope’s single eyepiece.
So – what should you know to select the right set of binoculars? The first thing you should consider is the magnification and the aperture (the size of the big (objective) lens, which gathers light). These will usually be expressed in the format of AxB, where A is the magnification and B is the aperture of the binocular. For example, a 7x50 binocular will have a magnification of 7x and a lens aperture of 50mm. A bigger aperture permits more light into the binoculars and provides a brighter and clearer picture, but it also makes the binoculars heavier.
Then, you should consider the exit pupil – or the diameter of the light beam that leaves the binoculars and travels to your eye. The exit pupil can be calculated by dividing the aperture by the magnification. This usually gives a number between 4 and 8. Try to get a pair of binoculars that will produce an exit pupil about the same size as your dark-adapted iris, which is around 7mm.
The next thing to be considered is eye relief, or the distance from the eyepiece to your eye. If they are too near of too far, the image will be blurred. This is especially important for eyeglass wearers, because the spectacles will affect how close you can put your eyes to the binoculars.
Finally, you should take note of the interpupillary distance, or the space between the eyes. While most binoculars can be adjusted to accommodate most interpupillary distances, some women and children are too small for most binoculars, and so they must use compacts instead.
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