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Skygazing: Astronomy through the seasons, 2018-12-11
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Binoculars for Christmas
Tapping, Ken
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Binoculars for Christmas
Ken Tapping, 11thDecember, 2018Every astronomer, young or old, beginner or experienced, should have a good pair of binoculars, maybe more than one. They can be used for a short bit of observing, or a session of serious astronomical study. These are the best tools for searching for comets, exploring the Milky Way, or just touring the sky.
A pair of binoculars consists of two telescopes fixed together. Magnifications are generally fairly low, typically between 7 and 10, although some offer a higher power. From an astronomical point of view, their main function is to collect lots of light. For example, binoculars with objective lenses with a diameter of 50mm will collect about 100 times as much light as our dark-adapted eyes. They will reveal the structure of the Milky Way, the bright clouds where new stars are being born, the four brightest moons of Jupiter and how they change positions from night to night, and many other things that would otherwise be too faint to see. Binoculars are described by two numbers, separated by an “x”. For example 8x30, 7x50, 10x50, 12x75 and so on. When discussing them, we pronounce the “x” as “times”, as in “10 times 50”. The first number is the magnification, and the second is the diameter of the objective lens in millimetres. The objectives are the big lenses at the front of the binoculars. They fulfil two functions; they collect the light and form an image out of it. We then view that image with the smaller eyepiece lenses at the back end of the binoculars.
However, from a practical point of view, there is a limit to the size of the objectives and how much magnification we should have. Binoculars with objectives bigger than about 50mm are too heavy for the average observer. Those with objectives bigger than about 75mm will definitely need a tripod. Magnification is an issue too. In addition to making things look larger, the field of view is smaller, meaning it gets hard to find or follow things, and any shakiness of your hands gets magnified too. Some sort of support helps.
This problem has been solved with the invention of binoculars with “image stabilization”. Sensors, movable mirrors or prisms and some clever electronics sense the shaking of the binoculars and correct the image so that it does not move. These instruments are still expensive and most have rather small objective lenses, but they are improving rapidly and falling in price. At the moment, for less than the price of image-stabilized binoculars you can buy a really high-quality pair of the standard kind.
It pays to try binoculars before you buy them. A problem with less expensive ones is that the two telescopes are often not pointing in exactly the same direction. If this problem is bad, you will notice it immediately when you try to use them. If it is less serious your brain will correct it, producing a slight feeling of disorientation and eventually a bad headache. No matter what the salesperson says, it should be possible to adjust the binoculars to make them comfortable to use. If he or she does not know how to help you, shop elsewhere. If you cannot get utterly comfortable, don’t buy them. These issues might sound scary to a beginner or non-astronomer. However, the solution is simple; seek advice. If you have a science store nearby, go there. If there isn’t one, contact the local centre of the Royal Astronomical Society of Canada. The RASC has many amateur astronomers who can give really good advice on what to get and where to get it. You could buy the family astronomer a membership while you’re at it. You’ll locate your nearest centre by having a look at
https://www.rasc.ca/locations-across-Canada. Mars lies low in the south after sunset and Venus shines low in the southeast before dawn. Mercury is there too, lurking low in the dawn twilight. The Moon will reach First Quarter on the 15th.
Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.
Tel (250) 497-2300, Fax (250) 497-2355
