CHESHIRE
STAR WATCH
With Binocularsby Jim Maynard
Star date: 02:27:97
The night sky is treasure trove of delightful objects, enticing those with a sense of overwhelming curiosity to set our sights on the stars. For thousands of years, the human race recorded images of the heavens with nothing more than the naked eye, a lot of patience, and diligent record keeping. Well, the record keeping was good at least some of the time. Galileo's early telescopes were no more powerful than today's binoculars, and they were of much lower quality. Yet, using low power instruments, Galileo discovered four moons of Jupiter, mountains on the Moon, and the phases of Venus. These remain lovely sights in binoculars today.
The four largest moons of Jupiter were the first objects found to be revolving around an object other than the Earth; this was part of a blow, when put together with the findings of Kepler and Issac Newton, from which the geocentric (earth centered) universe hypothesis never recovered. The Moon remains a lovely spectacle in the evening sky, which boasts majestic mountain ranges, sheer riffs and faults, and desolate plains. The Moon is always a beautiful sight in a pair of good binoculars; get yourself a moon map, and see how many objects you can find.
Try at the quarter moon phases (when you can see half
a circle); the craters are most visible then. Venus, the brightest planet
in the sky, displays phases just as our Moon does. We see this because Venus
is closer to the Sun than we are. Mercury also displays phases, but it is
so small, far away from us, and close to the Sun, that the effects are much
less prominent than they are on Venus.
Do binoculars have any advantages over telescopes? Well, it depends what you're intending to look at, and what your preferences are, but for many applications, binoculars can do just as good of a job, if not better than a telescope. But what are the advantages?
First is cost. $200 will buy a small decent beginner telescope
(if you shop carefully), or a very high quality pair of binoculars which
will last a lifetime. $100 cannot buy any more of a telescope then might
be recommended for a youngster, but that same $100 could buy a pair of binoculars
which will amaze and delight young and old alike for years to come.
The second advantage binoculars possess over telescopes is their large "field of view". This is a measurement of how wide an area which can be seen at a given distance, or another way of expressing the measurement is by denoting the number of degrees seen when looking through an instrument. There are 360 degrees in a full circle, and the Sun and Moon each fill about half a degree. While telescopes typically provide about half a degree to a degree of view at one time, binoculars yield around six to eight degrees. There are wonderful objects in the sky which fill far more area of the sky than a telescope will allow you to see at one time
While the moon is well within the area seen with a telescope, the Andromeda galaxy, M31 is not.
The spiral atoms of our nearest galactic neighbor span three degrees; six times the width of a full moon. This is well within the area seen through a pair of binoculars. Also lovely through binoculars are comets, such as Hale/Bopp, which should be seen in the evening sky starting around March l5th. Telescopes may be used to make detailed examinations of the heads of comets, but only binoculars will allow one to view significant areas of the comet at one time. Also large and dim, the Orion nebula is another treat for binoculars in the winter sky; find it as the middle "star" in the sword of Orion.
The third advantage binoculars possess over telescopes is their portability. While an amateur astronomer might not be inclined to set up a large telescope on a partly cloudy night, binoculars are almost immediately available for use, and are quickly torn down if a sudden rain starts to fall.
What are desirable qualities to look for in a pair of binoculars? The
question boils down to what you intend to use them for. For astronomy, one
of the most important factors to look for is the size of the objective (main)
lens. Just as you can run 
more water through
a larger pipe, you can get more light in through a larger lens. This measurement
can be found as the second number in the binoculars destination. For instance,
a pair of binoculars with a designation of 7X35 would have an objective
35 millimeters across (about 1.4 inches). For amateur astronomy, an objective
at least 50mm across is desirable. The first number designates the "power"
of the binoculars. In the case listed above, the 7X35 binoculars would make
the object appear seven times closer. When choosing binoculars for astronomy,
the power is not nearly as important as the diameter of the objective. In
fact, any power over lOX will be nearly useless without a tripod as the
shaking from your hands will cause the image to shake excessively. 10X50's
are a very good choice for amateur astronomy. Even here, a tripod is very
useful for holding the image steady as you view the object.
When shopping for a pair of binoculars, hold them at arms length and look through the eyepieces, aiming the binoculars at a distant, bright object. You will see two bright, round circles.
Move the binoculars so that the circles run around the edge of the eyepieces. The image should remain clear, bright and even. Another thing to look for is a brighter square inside the circles.
Nearly all binoculars will have this, but check to see if the image of
the square is much brighter than the circle. This is a sign of a possible
bad prism. The square should be at most noticeable, but not obvious. Also
try moving the focusing knob forward and back. The action should be smooth
and steady.
Speaking of prisms, there are two major kinds available, roof and porro. Roof prisms are lightweight and compact, meant for portability. Porros come in two varieties, BK-7 and BAK-4.
The BAK-4's are the best all around prism for use in binoculars.
Also important is the coating on the lenses. Since light traveling through glass has it's different colors of light bent to different degrees, lenses are coated to correct this effect. In order of quality, the choices are coated, fully coated, multi coated, and fully multi coated, with the fully multi coated being the best choice.
Good viewing to all, and clear skies.
James Maynard is head of the astronomy department at Earth Treasures in Keene, N.H.
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