Binoculars

In astronomy, it pays to go easy on equipment. Equipment makes it possible to see things that are impossible with the naked eyes, but it can also lead one away from the beauty of the sky. A good way to bridge toward fainter objects is to invest the time necessary to learn about the sky with a pair of binoculars.

I know the purists would like to say that the device I’m writing about is a binocular. Well, if it makes someone feel better, that is correct. It is just one thing, and it can be held with one hand. However, I’m not one to stand in the way of clear communication or to try to make someone feel insecure about their use of common language. I’ve always called ‘em binoculars, or a pair of binoculars. If you feel better about it that way, you have my official permission to keep calling them that too. (Yeah, I guess that kind of attitude is a little red neck of me – but you get what you get.)

What’s the big deal about binoculars? Why do they help a sky observer to see more? Well, it is a matter of physics. When I look up at the sky toward the Andromeda Galaxy from a dark sky site, what is entering my eye are photons that left that galaxy about 20 million years ago. Those photons cause chemical changes in my retina and my brain perceives it as light – a really dim light – that is spread out across an area larger than the size of the moon.

The cross section of the receiving area is the area of my cornea above my open iris in each eye. How big is that area? You may remember a little math from high school that says that the area of a circle is pi*r^2, or in words “pi r squared.” Pi is a constant that is about equal to 3.14. The radius of a pupil when it is dilated may be about 3mm. So the area of the pupil is 3.14*3*3 or about 28 square millimeters.
For two eyes, the area is about 56 square millimeters.

When I pick up a pair of astronomical binoculars, like the Orion Scenix 10x50 binoculars I bought on line for $100, those numbers mean that the binoculars magnify the image by 10 times and the lenses in the front are 50 mm across (almost two inches for the metrically challenged.) If the diameter is 50 mm, then the radius is 25mm and the area of the lens is 3.14*25*25, which is about 1963 square millimeters or 3925 mm^2 for the pair of lenses. That is about 70 times the area of my pupils.

The effect of this is to funnel 70 times as much light through my pupils from an area of the sky that has been magnified ten times. And the main effect of that is not so much seeing things bigger, as seeing more things! Stars that were not visible to my eyes are now easy to see. If the Pleiades are up, I can see six stars in the open cluster in good sky conditions. With a pair of binoculars, I can see more than fifty.

There only two real problems with binoculars for looking at sky objects.

First, some objects are not large enough to make out enough detail to make them interesting. That is especially true for objects in our solar system. Ten times magnification is not thrilling for the moon, Jupiter, or Saturn. It is about what Galileo had in 1609, and with it I can see the moons of Jupiter, the rings of Saturn, and craters on the Moon. This lack of magnification is not as much of a problem for many sky objects. In fact, many deep sky objects are beautiful at low magnification if you can get enough light into your eye to enjoy them. And that is what binoculars excel at doing.

The second problem is a real bother. Binoculars are really hard to hold still. If I try to see the moons of Jupiter with a pair of hand-held binoculars, all I see is a bright dot swimming in front of my eyes. To get the most out of all that optical power, I need to find a way to hold the binoculars still.

I’ve tried holding binoculars against a tree, against the back of a chair, and I’ve tried to hold my breath to hold them still. None of this works very well. When I was out at the Texas Star Party in April 2009, I came across a device for holding binoculars called the Couch Potato Telescope. Designed by SIM PICHELOUP, this is a rotating platform that holds a beach chair and an adjustable frame to which the binoculars clamp. While I was out at the TSP, I bought a kit from Sim and built it when I got back to San Antonio. It makes binocular observing a dream! (If you want your own Couch Potato Telescope, Google the term and write to Sim.)

There are a number of special observing lists devoted to binocular observing. They are all worth spending the time to find the objects on that list. So Far, I have gotten the TSP binocular list done, and the Binocular Messier list of the Astronomical League. I am hoping to do the AL Deep Space Binocular list this summer and fall.

Maybe I will invest in a larger pair of binoculars for the Messier Marathon this coming spring. A friend of mine was able to see over a hundred of the Messier objects with a pair of 80 mm binoculars across a wonderful night of observing. Of course those 80 mm lenses are bigger than mine and collect a lot more light. 3.14*40*40*2 is 10 square centimeters or two and a half times more light than my binoculars gather. Sounds like a nice Christmas present, Diane.