Ken Hough's Website
Astronomy

Binoculars

Binoculars can provide an easy and relatively inexpensive means to view the night sky. Little or no technical knowledge is required to use them. As with most kinds of equipment, not all binoculars are suited for astronomical work.

In this section some of the most basic aspects of choice are discussed, sufficient I hope, for you to do further research.

There are two basic types of binoculars. ie those that use 'porro' prisms and those that use 'roof' prisms.


Binoculars that use 'porro' prisms look like traditional binos where the distance between the objective lenses are greater than the distance between the eyepiece lenses.

Binoculars that use 'roof' prisms are more compact with the distance between the objective lenses and that between the eyepiece lenses being the same.

For comparable quality, binos that use roof prisms are typically more expensive than those that use porro prisms. Some folks prefer the more compact feel of roof prism binos, although personally I prefer the chunky feel of porro types.

What size and magnification is best?

Binoculars are specified in terms of magnifying power and diameter of the objective lens in millimetres. So a pair of 10 x 50 binos would provide a magnification of ten times and would have objective lenses of 50 millimetres in diameter.

The brightness of images as seen through the eyepieces will depend on both magnification and diameter of the objective lenses and is proportional to the ratio of objective diameter divided by magnification. In the case above this would be 50 / 10 = 5. This is a useful value to be able to compare binos of different objective sizes and magnifications. For example a pair of 7 x 35 binos, while providing a magnification of only 7 times would still produce a brightness of 35 / 7 = 5, exactly the same as the larger 10 x 50 binos.

For astronomical use, 10 x 50 binos are a good balance of magnification and overall size. Higher magnifiation makes it difficult to hold the binos steady enough to view properly. Larger objective lens size makes for heavy binos which can be tiring to hold steady over extended periods.

It's unlikely that compact binoculars of around 10 x 25 would be of much practical benefit. The lenses are just too small to gather sufficient light for night time use.

What about quality?

Beware of cheap binoculars! These typically use inferior quality optics and focusers can be poor. Check that all optical surfaces (including the prisms) are fully multi-coated, that the focusing mechanism is smooth, and that the eyepieces are adequately supported and remain steady during use.

Try before you buy!

It is VERY important that you get binoculars that will suit your particular needs. This will depend on your own eyesight and whether or not you wear glasses. I strongly recommend that you buy binoculars from a stockist that will allow you to try them out before committing to them. Some binoculars provide only limited adjustment for focusing which can be a problem if you wear glasses but wish to view without them. In some cases, there can be very little 'eye relief' which prevents use while wearing glasses or at least can make this difficult.

When trying out binos, include tests such as focusing on high contrast subjects (eg. a church spire that is silhouetted against the sky). Look for scattered light which spoils contrast and for 'purple fringing' which is typical of serious chromatic aberration.

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19/02/13