Binocular Astronomy
Published on Mar 23, 2009 at 11:12 am.
8 Comments.
Filed under amateur astronomy, skywatching.
I am often asked what sort of telescope that I recommend for beginning amateur astronomers. What often surprises people is that I don’t normally recommend a beginner buying a telescope. Rather, I suggest that a beginner buy a good pair of binoculars. There are several reasons for this. Even seasoned amateur astronomers often use binoculars.
First, using telescopes takes practice. And, in order to see anything, you need to know the sky. The Moon is pretty easy to recognize by an amateur, but if you are not familiar with the sky, you often have a tough time spotting anything else, unless it is something like a particularly bright comet.
Binoculars, on the other hand, are a bit easier to use (granted, it still takes a bit of skill to find things in the sky compared to terrestrial objects). But, binoculars have a much gentler learning curve than telescopes. And, of course, you can use them for looking at other things as well. They are useful for bird watching, catching a ball game, etc. And, binoculars are far more portable than any telescope. I wrote a bit about binoculars for astronomy nearly three years ago, but it is time for an update.
First of all, people ask me just that you can see with binoculars. The answer is that you can see a lot! There are literally hundreds of objects visible with binoculars, if you have good skies, good binoculars, and you know what you are looking for. Most any of the Messier objects, and hundreds of NGC objects are visible. You just need to know the limits of your binoculars, your eyes, and your sky. The more light pollution, the fewer things that you can see with binoculars, just as with telescopes. Not all binoculars are the same, though. Some are better for astronomy than others. If you already have binoculars, then try them out with the sky before buying new ones. If you don’t have binoculars, though, then you may wish to read up on what makes for good astronomical binoculars. There are a lot of different styles and sizes of binoculars on the market. One of the more important things when buying binoculars are two numbers. Binoculars come as 8×20, 7×35, 7×50, 10×50, 20×80, etc. You would read 7×35 as “seven by thirty-five.” The first number, the 7, is the magnification. The second number, the 35, is the diameter of the objective lens (the first lens that light passes through) in millimeters. These binoculars are not all equivalent. They have different objectives and are optimized for different purposes. While any binoculars are likely to be better than looking at celestial objects with the naked eye, they are not all ideal. Some are better than others.
The basic idea of binoculars is simple: you have two optical systems so that you can observe things with both eyes open. This is easier on the eyes. For terrestrial objects, you can also get a bit of depth perception using binoculars, too. That isn’t the case with celestial objects, though, since they are so far away. However, it is still easier on the eyes if you can observe something using both eyes instead of just one. There are several types of binoculars, and some are better for astronomy than others. The two most common types of binoculars are roof prism and porro prism binoculars. The roof prism binoculars tend to be quite compact, with each side of the binocular looking from the outside like a simple tube. Porro prism binoculars have an offset between the eyepiece and the objective lens, as seen in the picture at the top of this post. The objective lens, the first lens that light passes through, inverts the image. The prisms correct for this, giving an upright image. You lose a little light in the prisms, but that is generally considered acceptable since inverted images are often seen as being aesthetically displeasing for terrestrial objects. In principle, it is less of an issue with astronomy, since objects look upside down anyway half of the time; however, a binocular manufacturer would have a tough time selling binoculars that didn’t show erect images.
What are the advantages and disadvantages of roof prism versus porro prism binoculars? Since roof prism binoculars do not have the offset seen above, they tend to be a bit more compact. That makes them easy to carry. But, that is about their biggest advantage. Porro prisms use total internal reflection to reflect the light, as seen in the following diagram. Roof prisms, though, typically use a mirrored surface. There is always light loss in reflection from a mirrored surface. So, roof prism binoculars are less useful for astronomy, because astronomical objects are typically very faint (excluding the Sun and the Moon).
There is another limitation to roof prism binoculars and that is that the objective lenses are closer together, and thus can be only so large before they begin to interfere with one another. Larger aperture binoculars, therefore, tend to be more likely a porro prism design.
Why does aperture matter? Well, for starters, the bigger the aperture, the better the resolution (the smaller things that you can see). Also, the bigger the aperture, the more light that it gathers, and thus the dimmer objects that you can see (sky conditions permitting). Based on this information, your first impulse may be to go out and try to buy the biggest aperture binoculars that you can afford. That is a mistake! First of all, there are other things that go into how good the image is besides aperture, and secondly the larger the binoculars, the heavier they are, and the harder they are to hold. About 50mm aperture is typically the largest aperture that most people find useful. Even that aperture, though, is tough to handle for many people. You need good steady and strong arms. It is possible to mount many binoculars (particularly the larger ones) to a tripod or a binocular stand, but it is far easier to just be able to hold them. So, I tell people not to buy anything larger than 35mm aperture unless they can try the binoculars out and can hold them steady pointing at some object high in above the horizon for at least ten seconds. It is tougher to point at something high than at something level. If you cannot hold the binoculars steady, then they are of little use without a tripod or binocular stand. And, though larger binoculars allow you to see dimmer objects, the gain that you get going from 35mm to 50mm aperture objective is less than one magnitude. 35mm binoculars have a limiting magnitude of dimmer than 10th magnitude and 50mm binoculars have a limiting magnitude of about 11th magnitude (assuming clear and dark skies). The 50mm binoculars, though, do give a bit better resolution (2.3″, compared with 3.3″ for the 35mm binoculars). But, if you can’t hold them steady, then the better resolution and limiting magnitude doesn’t help much.
Another factor to consider with binoculars is the exit pupil of the binoculars. The exit pupil is the image formed by the eyepiece. The eyepiece lens is typically larger than the exit pupil, meaning that you are not really using the entire lens. An example of exit pupils can be seen in the image above. The approximate size of the exit pupil can be found by dividing the objective aperture in mm by the magnification. So, a pair of 10×50 binoculars has an exit pupil of 5mm. Likewise, a pair of 7×35 binoculars also has an exit pupil of 5mm. But, a pair of 7×50 binoculars has an exit pupil of 7.1mm. So, what exit pupil is best?
The best performance occurs when the exit pupil of the binoculars is equal to or a little smaller than the pupils of the observer’s eyes. If the binocular exit pupil is larger than the eye’s pupil, then not all of the light gathered by the binocular objective is used. If the binocular exit pupil is exactly the same as the eye’s pupil, then alignment with the eye becomes critical in order to yield the best image. For smaller exit pupils, alignment with the eye is easier to achieve. The pupils of the human eye change, though. As you become dark adapted, the pupils get larger. In brighter light, the pupils contract. However, not everyone has the same size pupils. The older you get, the less your eyes dilate. Also, the brighter the environment, the less your eyes dilate. So, how do you know how big your eyes pupils are? Well, one way is to have a friend measure them at night (using a red flashlight so that you don’t lose dark adaption). Or, you could just estimate. For young adults, about age 20, the pupils dilate to about 7mm. By the time you reach age 50 though, your pupils dilate to only a shade larger than 6mm. The pupils of a 70 year old only dilate to about 5mm. Now, these are rough estimates, and individuals vary greatly. Furthermore, some medications can also affect the degree to which your eyes dilate. Also, if you do most of your observing from city or suburban sites, then the ambient light might cause your eyes to not fully dilate. You may be better off buying binoculars to fit where you’ll observe the most rather than buying them for optimal observing conditions if you rarely have those conditions.
So, what do I recommend? I usually tell people to try 10×50 binoculars for astronomical work. If the 10×50’s are too heavy, as they are for many people, then I suggest 7×35 binoculars. The 7×50 binoculars might technically be better for astronomy, but most people that I talk to seldom get out to perfectly dark skies where they would be most useful. I also tend to suggest 7×35 and 10×50 binoculars because they are the most common sizes sold in the USA. There are plenty of other sizes, and, of course, you can order binoculars online if you don’t see what you want in person. And, you can do what I do, go look at things in person and then check to see if you can buy the same thing for less online. But, is there more to look for than just the size of the binoculars? Yes!
Binoculars come in an extremely wide range of quality. Typically, you get what you pay for. Expect to pay over $100 dollars for good binoculars. Earlier in this posting, I mentioned the prisms. It is important that the prisms be of good quality. The better ones use BAK-4 glass. Look for that to be in the specs for the binoculars that you are considering purchasing. Typically, the BAK-4 prisms will give better and brighter images. If it doesn’t say BAK-4 prisms, then that particular pair of binoculars probably doesn’t have them. Another good test is to hold the binoculars at arm’s length and look at the exit pupils. They should be round circles, like the ones in the photograph above. Most binoculars with BAK-4 prisms do this. If the exit pupil is square or spiked, then the binoculars will not give as clear of an image. Now, having said that, however, I should point out that my first pair of binoculars was not very good, but they still served me quite well until I got a good pair. So, if you cannot afford the really good binoculars, even the lesser quality ones will benefit you for scanning the sky.
Once you get binoculars, you need to learn how to use them. The first thing is to learn to adjust the interocular distance (the spacing between the eyepieces). Different people have different spacing between their eyes. Roof prism binoculars often can be compacted enough to fit anyone’s eyes. The porro prism binoculars, though, frequently have a minimum interocular distance. Unfortunately, that distance may still be wider than the eye separation of some people. Children often have eyes set a bit closer together than many binoculars’ interocular distances. Some adults also have rather narrow set eyes. I am one of those whose eyes are a bit narrower than many standard binoculars adjust for. Unless the interocular distance matches the eye separation, though, the binoculars are useless. You will be unable to see through both sides at once. Since the porro prism binoculars tend to have wider interocular distances, they are often a problem for those with narrow set eyes. In order to make my pair of binoculars work, I took a grinding tool and ground down the stops of my binoculars so that they would close enough for me to use them properly. If you don’t want to do that yourself, then the solution may be to use roof prism binoculars. Though you lose a little light in the roof prism binoculars, that is better than buying binoculars that don’t match your eye separation if you are not willing or able to modify the binoculars (and many people are understandably hesitant to file or grind away at several hundred dollar binoculars).
The next step in using binoculars is learning to focus them. There is a focus knob, wheel, or lever typically somewhere between the barrels of the binoculars. This adjusts both eyepieces. However, it is rare for a person’s eyes to both need exactly the same focus adjustment. That means that almost always when using binoculars, adjusting both eyepieces will get either the left eye or the right eye side in focus, but not both at the same time. That can be annoying, and it limits the usefulness of the binoculars. However, almost all pairs of binoculars, and all decent ones, have a remedy for this problem. One eyepiece has a separate fine focus adjustment. So, you focus the binoculars so that the non-adjustable side is in focus for you, and then you use the eyepiece adjustment to get the other side in focus. If you do it right, then both sides are in focus at the same time, and you are ready to go out and enjoy the sky! I tend to do this by putting a hand over the front of the adjustable side and focusing the other side, and then putting a hand over the non-adjustable side and adjusting the focus on the adjustable eyepiece. That works better than just closing one eye or the other, since that often slightly changes the focus on the other eye.
Next, you need to practice finding things in the sky. That is easier said than done. There are few landmarks in the sky. I often see people put the binoculars up to their eyes and then turn their heads towards the sky. That doesn’t work well. Much better is to look at the point in the sky that you want to observe, and then hold your head as still as possible, looking steadily in the same direction as you bring the binoculars up in front of your eyes. Then, you don’t have to scan around as much trying to find what you want to see.
Finally, you need to know what to look for. There are a number of good books on binocular astronomy. Some books just list objects that are good to look at, and others have maps and charts in them. If you need a star chart, then by all means buy one! My favorite star chart for beginners is the Edmund Mag 5 Star Atlas, available from Edmund Scientific. So, with a pair of binoculars, a star chart, and perhaps a binocular astronomy guide, you are ready to start observing things in the sky!
-Astroprof
Astrogeek on March 23, 2009 at 12:58 pm: 1
Thanks! I’ve linked this post to my Beginner’s Guide page. I’m always telling people to spring for binocs before they get a scope.
Haplo on March 23, 2009 at 2:44 pm: 2
Nice guide, just a note, you completely forgot about coatings.
Ed Davies on March 23, 2009 at 3:07 pm: 3
Another point worth considering is the focal range. If you are reasonably short sighted (or, I presume, long sighted) then you are likely to be using the binoculars close to the end of their focus range. I have a pair which seemed fine when I tried them in daylight - close to the end of the range but not passed it for my eyes - but in the dark with pupils dilated the don’t fully come in to focus. I assume it’s because the dilation reduces the depth of field available.
Astroprof on March 23, 2009 at 3:17 pm: 4
Haplo, oops. I had meant to mention coatings! Actually, I did write a little about them, but my internet connection timed out and I lost those sentences. I guess that I’ll have to do a follow-up posting.
Ed, good point. I usually wear my glasses with my binoculars, and that helps out, but if you don’t wear glasses, then the focal range becomes a big issue.
Ed Davies on March 25, 2009 at 4:25 pm: 5
Being able to wear glasses with binoculars is very valuable from the point of view (literally) of being able to switch from unmagnified to binocular viewing directly - it helps with keeping fixed on the line of sight to an object. It’s easy to lose the exact position in the blurry bit involving flipping the glasses up and getting the bins in place.
However, with many (particularly cheaper) binoculars the eye relief is not enough to allow this. If you use them whilst wearing glasses the field of view is very seriously restricted.
I have to admit, though, that eye relief is something I’ve not really been able to understand. If a scope of some sort if focused for somebody with normal eyesight then aren’t they rays focused to infinity so parallel so why is any particular distance special?
By the way - I’m away in the Highlands of Scotland and wishing I’d brought my binoculars along even though there’s almost zero chance of clear skies this week (there were some last week). I read your post last night then, having binoculars in mind, realised there were a couple of things I’d have liked a closer look at today.
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