Binoculars for Astronomy
Published on Sep 10, 2006 at 5:24 pm.
2 Comments.
Filed under astronomy, observing.
I give lots of public lectures, and I often hold star parties. Frequently at such events, people ask me about purchasing telescopes. They want to know what telescope they should buy as their first astronomical tool. I ask them how well they know the sky, how many telescopes they have looked through, what they like to look at, etc. Frequently, I determine that they have never really looked at the sky. In that case, I normally suggest that instead of a telescope, they might want to consider purchasing an inexpensive star chart or sky guide and a good pair of binoculars. Often people are surprised by the suggestion. I always bring binoculars to star parties, and I or one of my students can show them things. We have lots of binoculars at the college, and we use them in some of the labs. Really, they are wonderful. You can see a lot with a pair of binoculars if you know what to look for. The Astronomical League, a confederation of amateur astronomy clubs, has several observing programs (that they call “observing clubs”) to help beginners know what to look for. This is a wonderful guide.
But what binoculars are best for astronomy? As it turns out, just about any binoculars of even modest quality will show you more than you can see with the naked eye. But, higher quality binoculars will generally show you a lot more.  If you don’t know much about binoculars, there is a bewildering array at the store to chose from. Some online astronomy retailers have binocular guides, but can you trust them? Actually, I have found most of those guides to be quite good, factual, and very useful, particularly from the more well known online retailers such as Orion or Astronomics.
So, what do you really want in astronomical binoculars? It depends a bit on the individual preferences of the observer. There are some general guidelines, though.
First, you want to have good quality optics. As a rule of thumb, with binoculars, the more you pay, the better they are. That isn’t always, true, of course. So, what do you look for? How do you tell that standing in the store you are holding good quality binoculars? There are several quick rule-of-thumb tests for the novice. First, you can point them towards a bright field (like out a window), hold the binoculars at arm’s length and look at the eyepieces. The image should look round. For inferior binoculars, the image looks square, oval, or some other shape. Next, look through the binoculars. By adjusting the interoccular distance, the images you see in each eye should superimpose without having to strain your eyes to make them overlap. (Note: for a typical person, each eye tends to focus differently, so you really need to be able to adjust the focus for each eye. Normally, this is done by having one eyepiece with a separate fine adjust focus. You focus with the other eye, and then adjust the variable focus so that both eyes are in focus.) Also, the image should be equally in focus throughout, not just at the center of the field of view.
To get the best and brightest images, you need good quality glass and anti-reflection coatings. The best anti-reflection coatings are the “multi-coated” optics. They visible surfaces of such normally appear sort of bluish or violet. The best kind of glass is BAK-4 glass. Don’t be confused if you see something marked BK7. That isn’t the same thing, and the bigger number does not mean better. Go with the BAK-4. It is more expensive, but yields far superior images.
So, you find good quality binoculars, is that all? No, because there are still considerations. Binoculars come in all sizes and magnifications. What is best for you? Again, that depends a bit on the observer, but there are some general guidelines. Binoculars are marked as MxD, as in 7×35 or 10×50. You read 7×35 as 7 by 35. The first number is the magnification, and the second number is the diameter of the front lens in millimeters. As you can expect, the higher the magnification, the bigger that it makes things appear. But, that isn’t always what you want. If the magnification is much over 10 power, then it becomes difficult to hold the binoculars steady, and the image will shake too much to really be able to see much. I know some people who can hold 20 power binoculars steady, but they are the exception. The second number, the objective diameter is also important.  The bigger that is, the more light that the binoculars gather, and the dimmer object that you can see. Now, you tend to undo that advantage with more magnification, so expect to be able to see dimmer objects with 7×50 binoculars than with 10×50 binoculars. Also, the larger the objective diameter, the more detail that you will generally see with the same magnification. But don’t go wild and buy the biggest diameter binoculars that you can afford. As with magnification, you get into problems if the binoculars are too big to hold steady. Most people have trouble with holding steady binoculars over about 60mm aperture. Among the most commonly available binoculars at sporting goods stores, 10×50 binoculars tend to be about the highest combination of magnification and aperture for astronomy. Again, this depends upon the person. I used to go out with someone who had a high quality pair of 7×35 binoculars which she liked much better for skywatching because she was somewhat petite and found holding 10×50’s steady a bit of a challenge.
Another issue is exit pupil. This is essentially the size of the light cone that emerges from the eyepiece. Too small of an exit pupil is difficult to look at, and it allows floaters in the eye to become a major interference to viewing. Too big of an exit pupil may be larger than the eye’s own pupil, in which case some of the light is truncated, and you are not seeing all of it (thus canceling out some of the advantage of a larger aperture). The exit pupil can be found by dividing D/M. Note that 10×50 and 7×35 same the same exit pupil: 5mm. That is a good size. Younger people can tolerate bigger, since a 20 year old’s dark adapted eyes have a pupil diameter of a bit over 7mm. However, the older you get, the less your eyes dialate at night. By the time that you are 50, your pupils don’t get much bigger than 6mm. You normally want the exit pupil of the binoculars to be 1 or 2mm smaller than than your eye’s pupil diameter. Again, that makes 10×50 and 7×35 binoculars excellent choices.
So, what do I recommend? I generally suggest that you try holding 10×50 binoculars and look at the Moon or something. It is harder to hold them steady looking up than looking at the horizon, so try that. If you can hold them steady, then go for them. If not, then try 7×35’s. But, whatever you pick, you should select some good quality binoculars.
-Astroprof
(Clip Art from Connections)
Gopal Ray on February 15, 2007 at 6:27 am: 1
Good Article. I wish you could have added a few sketches to elucidate the main attributes.
Did you write anything on protection of binos from Fungus and metods of removing fungus.
Astroprof on February 16, 2007 at 1:31 pm: 2
Storing your binoculars with desiccant helps keep the fungus at bay. My understanding is that the fungus grows on the lens coatings, so removing the fungus often damages the coatings. It is best left to people who specialize in such things.