“ I became interested in binocular astronomy when I first used my Dad's binoculars to see the moons around Jupiter. When I bought my first pair of astronomical binoculars I saw the Andromeda Galaxy and other deep sky objects I was hooked!” - Joel Quimpo

Have you ever taken a moment and looked up at the night sky filled with stars and wondered what they really look like? Perhaps you are fortunate to live in however a place that is still relatively dark at night and the Milky Way can still be seen in the Summer time. Consider yourself very blessed!

Many of us live in sprawling urban areas where light pollution has washed the sky out and we see only the brightest stars and planets from home. Regardless where you live if you own a set of binoculars, and have a smart phone such as an Android or Apple, you may already have what it takes to enjoy binocular astronomy.

If you are just getting into binocular astronomy or you have been an amateur astronomer for years using astronomical binoculars will help you improve your observing skills and allows you to get out more often without the heavy lifting of using a telescope for casual observing.

If you are new to binocular astronomy we will introduce you to the science of astronomy without breaking the bank. Using astronomical binoculars has many advantages:

• Good astronomical binoculars are generally less expensive than a good astronomical telescope
• Its easier to locate many deep sky objects with astronomical binoculars than with a telescope
• Astronomical binoculars are easier to use and its easier to learn to operate than a telescope
• Astronomical binoculars have daytime applications making them a flexible and fun to use
• Finally astronomical binoculars have superior optical qualities that do not distort images or wash even the faintest colors out of an object

Understanding binocular types

Most binoculars sold today have built in prisms that direct the light through the objective lens to the eyepiece. Old fashion Field Glasses are binoculars that do not have a prism in them and really two small telescopes molded together. Today we see these in opera glasses or in toys.

There are two types of binoculars with optical prisms. They are:

• Porro prism
• Roof prism

Roof prism binoculars are very compact and can be very expensive. The prism itself is coated with a highly polished aluminum or silver surface to transfer light through the prism. They sacrifice brightness to be compact. As a result they are better suited for bright daylight conditions. While most roof prism models are not suitable as astronomical binoculars there are a few standouts like a Carl Zeiss Conquest Binocular .

Porro prism binoculars come in two groups of their own but the construction is similar. The first is the German style with two-piece body with separate barrels holding the prism and eyepieces together. If they are dropped or badly jarred they can knocked out of alignment. This model is also prone to dust, water, and fogging as moisture can penetrate the joints.

The American style is a one-piece construction with a solid body with no central joint. This makes it better at handling being jarred and keeps dust and water out thus not prone to internal fogging and moisture. Porro prism binoculars in the American style are the better suited of the two for binocular astronomy.

Magnification factor

Next is the magnification factor of the binoculars. In astronomy high magnification does not always yield the best results. The higher the magnification, the heavier the binoculars become. Eventually you will need a tripod to help you steady them. Anything above 10x is considered “giant binoculars”.

My favorite set of astronomical binoculars is my 25 year old 11x80 made by Parks. They are also my heaviest and I need a tripod to use them for extended periods of time. I hunt comets, Messier and other deep sky objects, and do observations of planets like Jupiter to see the Galilean Moons with them, as well as some of the moons around Saturn and even the International Space Station.

Many new binoculars have zoom magnification that allows you to “zoom in” on objects. The problem with zoom magnification eyepieces is that you can literally magnify an object out of existence. As you increase the zoom magnification the exit pupil decreases and light is lost thus faint objects such as nebulae, and deep sky objects simply appear to vanish. They are better suited for daylight applications than binocular astronomy.

Objective lens

The “80” on my 11x80 astronomical binoculars is the size of the objective lens in front of the binoculars in millimeters. The larger the objective lens the more light is able to be collected. Sizes of objective lenses you will find range from 30 millimeters to over 100 millimeters. Combining the magnification factor and objective lens size affects your exit pupil through the eyepiece.

Larger objective lens in light polluted areas can actually wash out images compared to smaller ones. It is a good idea to have a set of 7x50, 10x50, or 15x70 binoculars especially for viewing in cities.

Exit pupil and eye relief

The human eye is able to dilate to 7 millimeters in the dark. A set of 11x80 astronomical binoculars have an exit pupil of 7 millimeters. As you age you individual exit pupil may not open as wide. Too much exit pupil may cause light to fall beyond your pupil and make you have to look around in the field of view to see everything. Other combinations of binoculars with 7 millimeter exit pupil are 7x50, 10x70, and 11x80. These sizes are optimal for astronomy.

If you live in a highly light polluted area like the Los Angeles area then having a smaller exit pupil on your binoculars may not be a bad thing. Binoculars that are 10x50,and 15x70 produce a 5 millimeter exit pupil. I use the Celestron SkyMaster Giant 15x70 Binoculars and take them with me when I travel. The Celestron SkyMaster 15x70 is perhaps the best value in large astronomical binoculars on the market today. Not every Celestron SkyMaster is recommended and I have more information about the Celestron SkyMaster Binoculars on my Celestron page.

If you do not have enough exit pupil with your binoculars it will cause a condition called, “vignetting.” This create a dark border around everything like tunnel vision and dim objects in view. It is cause by having too small an exit pupil compared to how large your eye is dilated.

Eye relief is the distance you need to have the eyepiece away from the eye. You really do not want your eye directly on the eyepiece itself or so close that it might as well be resting on your eyeball. Good eye relief is at least 2.5 millimeter from the eyepiece. This will allow you some room and reduce the amount of eye strain from looking through the binoculars itself.

Field of view

The field of view (FOV) is usually stamped on the back of the binoculars itself. It can be expressed in two ways – either by yards (or meters) or by its angular field. So binoculars with “1000 yards at 325 feet.” This means you see 325 feet or a little over 108 yards of around an object 1000 yards away.

The apparent angular field is measured in degrees arc. It is found by dividing the magnification factor being used of the eyepiece into the apparent field of view in degrees. Every 52.5 feet FOV at 1000 yards equals 1° degree arc. So 325 feet FOV is around 6° degrees of angular view.

There are binoculars that are advertised as “wide angle” with angular field of view reaching 10° to 12° degrees. They use the same optical combinations like other binoculars but the huge drawback is they after a certain point the edges are so badly distorted that starlight will look at very elongated streaks. This is called, “coma.” Even with such wide FOV's much of the image will be wasted. These types of binoculars should be avoided. They are expensive and not suitable for binocular astronomy.

Optical Coating

When you compare astronomical binoculars check with the manufacturer that the objective lens and eyepieces are coated. Plain optical glass will actually reflect around 5% of the light coming in. Having the optics coated will greatly reduce the amount of light that is reflected and as well as cut down on lens flares and other aberrations. The coating on the lenses is made of magnesium fluoride which appears purple when turn edge wise on the exterior of the lenses.

To be a proper astronomical binocular all of the lenses need to be coated. Some binoculars may actually have plastic internal lenses that are not coated. Really good astronomical binoculars will have optical glass lenses through and through and all lenses will be coated inside and outside. The amount of coating is also critical. If the lenses are lightly coated the glass will appear more pink than purple. Lenses properly coated lose less than 0.5% of the light captured. It is an investment that is well worth the money. Fully multicoated lenses will appear greenish when facing the light. Beware of some less than reputable dealers that may try to pass off heavily coated external lens to make them appear properly coated. If something is really cheap chances are there is a reason for it.

Making the final selection of astronomical binoculars

When you pick up a set of binoculars and check the alignment by look at an object that is far away. Keep both eyes open but cover up one side of the binoculars then quickly uncover it. If you suddenly see two of the same objects for a moment your brain will attempt to put them back together again. It is a sign that the optical alignment may be off. Looking through the binoculars out of alignment can cause headaches and eye strain.

Next hold the binoculars out arms length. Do you see a round circle or a diamond shape? If it is round your prism is make of BAK-4 optical glass. If however you see more of a diamond shape the prism is made of the cheaper BK-7 glass and the light from the image is literally falling out of the prism. The prism itself will have an aluminum coating but will reduce the overall amount of light coming through the binocular itself. Reject any binocular using BK-7 prism.

Now check the mechanics of the focus. Old binoculars have individual focus for each eye. Newer styles have knurled focus wheel between the barrels. Either one is fine but check them to be working smoothly. There are some models with “fast focus” with a paddle that zips the focus for fact actions. The problem is they are also not accurate and they get knocked out of focus easily.

Turn the binoculars objective lenses towards you and examine the inside of the barrels for dust, metal filings, or loose objects. If you see any of the above items reject the binoculars. Check that you can adjust the distance between the two barrels to your eyes. There is no sense having a set of astronomical binoculars that you cannot adjust to the distance of your eyes.

Astronomical binoculars will have a place to mount a camera tripod to it so that you don't have to hold them the entire time. The mount should be near the front of the binoculars and not attached directly to one of the barrels which can cause alignment problems for you.

Most all SLR camera tripods are too light for astronomical binoculars. You'll need a more heavy duty tripod with a fluid moving 3-way pan head that is capable to pointing your astronomical binoculars straight up without tipping over! My tripod originally supported my old VHS camcorder so it was built to handle the weight. VHS Camcorders are long gone but my tripod is still in great shape after almost 30 years of observing work. Expect to pay more than $200 for a good tripod. It will last for many years to come.