What to Look for in a Pair of Binoculars?
Almost any pair of binoculars will help you see more of the night sky. You do want to stay with something that has at least a magnification of 7x to about 10x, anything more tends to be too much magnification, making it difficult to hold them steady and requiring a tripod. You also want to get something with a large objective lens to gather as much light as possible.
When looking for binoculars you will notice several numbers associated with them. The most prominent will be: Magnification x Aperture. This is typically a set of numbers like: 7x35 or 10x50. The first number is the magnification provided by the lenses in the eyepiece, while the second number is the diameter of the aperture, the objective lens in millimeters. For the two sets of numbers shown above, the 10x50 binoculars will have higher magnification and more light gathering ability (about 2 times) than the 7x35 pair.
The next number you may see associated with the binoculars is the field of view. This basically is how wide the area will be that you can see when you look through the instrument. This can be represented in degrees or possibly as a number of feet seen at a distance of 1000 yards, like 360 feet at 1000 yards (360feet/1000yards). There is a simple way to convert this number to degrees by simply dividing the number of feet by 52.5. Usually, binoculars will give about a 7.5 degree field of view but you can buy wide-angle versions that can give 8 to 10 degrees.
There are two other numbers that are of interest: the exit pupil diameter and the eye relief number. The exit pupil diameter is basically the diameter of the image that will come to your eye from the lens. This has bearing on how much light actually hits your retina, which directly correlates to how bright the object appears. As we age the diameter of one’s dark-adapted pupil gets smaller. When we are young, it can be about 7mm dropping to 5mm by the time we are 50 and older. If the eyepiece has an exit diameter of 7mm and your eyes can only dilate to 5mm then you are losing almost 50% of the available light and the image will appear significantly dimmer.
The eye relief number is the distance in millimeters that your pupil can be from the eyepiece and still see the entire field of view of the image. This is important if you wear eyeglasses. Long eye relief allows the observer with eyeglasses the luxury of not having to take their eyeglasses off or put their glasses right on the eyepieces to view the entire field of view. These numbers will vary depending on the construction of the binoculars.
Speaking of construction you will notice two different types of binoculars available: ones with roof-prisms and ones with porro-prisms. The porro-prism binoculars have an offset in the optical path or a stepped shape to the barrel, while the roof-prisms provide a smaller, straighter overall shape. Roof-prism models are lighter, tend to be more expensive and due to internal reflections they are somewhat dimmer than the porro-prism models and therefore less desirable for astronomical use.
You will often see coated optics as another feature offered on binoculars. The optical surfaces are coated with anti-reflecting films to maximize the amount of light getting to your eyes. Well made instruments will have all the lens surfaces coated as well as a treatment on the internal barrels to prevent glare and improve contrast. To check if all the surfaces are coated you can turn the binoculars around and look inside from the objective lens side with the light coming from above or behind you (repeat this from the eyepiece side too). You should see colored reflections of the internal surfaces. If you see any white, that is an indication that the lenses are not all coated.