Aperture Magnification and Field Diameter

Having ensured that our binoculars are of decent optical standard, the next points to consider are aperture and magnification. These factors are important because they will determine whether or not a double star can be resolved into two separate sources of light.

Binoculars such as the popular 7 x 50 range (denoting a magnification of seven times and an objective lens diameter of fifty millimetres) are reasonably priced, lightweight and will provide good views of many double stars plus a host of other interesting celestial objects. They are also suitable for general daytime use. Larger instruments with a higher magnification will divide much closer pairs and show greater detail, but are more expensive and bulky. It may be advisable for beginners to invest in a fairly modest pair of binoculars before progressing to an instrument of greater power and aperture, should a deeper interest in astronomy develop.

The field diameter of a pair of binoculars is a numerical value expressed in degrees and fractions of a degree. It is directly related to magnification and objective lens diameter. For a given aperture, field diameter diminishes as magnification increases. As might be expected, it is easier to locate an object through binoculars with a wide field of view, because the area of sky represented is proportionately larger.

To obtain the field diameter of a pair of binoculars, if this value is not known, we need to note the length of time taken for a star near the celestial equator to drift centrally across the field of view from one edge to another (it is necessary to secure the binoculars to a tripod or some other means of support for the test).

Suitable bright stars include 5 Orionis (in the belt of Orion), Z Virginis and a Aquarii. The elapsed time, recorded in minutes and seconds, is multiplied by 15 to give the field diameter in minutes and seconds of arc. This method can also be used for determining the field diameter of a telescope eyepiece.

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