Binoculars have some great advantages over telescopes when doing astronomical observation. I discussed this in my article Astronomy With Binoculars.
But how do you go about choosing a suitable pair of binoculars for astronomy?
I would say that the key advantage of binoculars is their portability and ease of use. Consequently, you are going to want a reasonably lightweight and compact pair. This will rule out binoculars having particularly large lenses, but you will want to get a full-sized pair, rather than ultra-lightweight pocket versions, which usually have inferior image quality.
Let’s get specific and look at the numbers.
Binoculars are usually designated by a pair of numbers such as 8×40 or 10×50.
The first number in the pair is the magnification (or “power”), the second number is the diameter of each main lens in millimeters. So a pair of 8 x 40 binoculars will give a magnification of 8 times and each of the two main lens will be 40mm in diameter.
The bigger the lenses, the more light will be gathered, so enabling you to see fainter objects in the sky. But bigger lenses, mean heavier and larger binoculars.
You can get specialist binoculars having with lenses as large as 100mm, but these are like monsters! For general astronomy use, 50mm lenses are usually considered to be the best compromise.
With magnification, the higher the number the smaller the area of sky will be in the field of view.
You might think that a high magnification is good, but it means the binoculars become much more susceptable to movement in hands or arms, making the image shake. Also, the image degrades if magnification is too high for a given size of main lens. Consequently a magnification of 7 times or 10 times, is usually best.
So we come down to 7×50 or 10×50 binoculars, as being the best choice for general astronomy.
As regards price, you can get a very acceptable pair of binoculars for as little as $60 or £40. I have done this and have been pleased with the results. However you can of course, spend a lot more and get better quality.
The coating on the lenses is important. Higher quality lenses will have better “multicoated” lens. The coatings are intended to reduce internal reflection of light, within the binoculars as this degrades image quality.
Another factor to look for is field of view (FOV). More expensive binoculars may have “wide-angle” eyepieces. These give a wider field of view (for example, 8 degrees rather than 6 degrees), enabling you to see more sky at one time. This is good and makes it easier to see whole star constellations and so learn areas of the sky.
Another important advantage of wide-angle eyepieces is better “eye relief”. This means that your eye does not have to be as close to the eyepiece, to be able to see the full field of view. This is especially useful for spectacle wearers.
If you have the money, you can now get “image stabilized” binoculars. These have a built-in gyroscope mechanism that compensates for wobble and shake, by adjusting the internal optics in real time.
So you can enjoy a steady image, even if the binoculars are moving about somewhat. Image stabilized binoculars also allow higher magnifications, say 12 or 15 times to be used, while still being unmounted and handheld.
