Cumberland Times-News

Bob Doyle - Astronomy

December 15, 2012

Here’s how to buy earth, sky telescopes

This column will cover locally available telescopes (monocular, binoculars and spotting scopes) that you can use both during the day and night (on sky objects such as the moon and planets).

Regular astronomical telescopes invert the image, causing both up and down and right and left to be reversed. This makes no difference when you are viewing the moon but a great deal of difference while looking at distant mountains or across a lake or bay.

Monoculars have the most value for their cost. Monoculars have straight tubes. A monocular is half of a binocular; you view with just one eye. But when you look with one eye though a monocular or a regular microscope, the other eye “shuts down.”

A monocular uses either extra lenses or a roof prism (to invert the image so it is right side up.). I have an 8-power monocular that I brought from a local hunting goods store (about $10). It came with a protective case.

This small monocular would easily fit in a knapsack or a women’s purse. This monocular would be fine for bird watching, spotting air planes or sporting events. The larger monoculars may have more magnification so the image may shake if you hand hold them.

Some of these monoculars have a opening for a quarter inch screw so they can be mounted on a camera tripod. A small; tripod could be placed on a standard table so you could use the monocular for sky objects and avoid neck strain while seated.

If you buy a monocular at a local store, you might be able to take it out of the box and see how well it works inside the store.

Binoculars employ twin telescopes, one for each eye. The bend on each side of the binoculars is to accommodate the two prisms that act to invert the image; this allows the binoculars to give you a normal image.

There are two crucial numbers for each binocular. The first is the magnification and the second is the width of the front lens in millimeters. Most binoculars have magnifications from 3 to 20 power.

A 7-power binocular will bring objects seven times closer. So if you looking at an object a block away (about 500 feet), the binocular will show the object as if it was 500/7 = about 71 feet away.

The larger number indicates how much more light the binoculars will collect than your eyes. During the day, the pupil of your eye (dark opening) may be 3 millimeters across. Now the area of a circle is Pi *Radius squared. Alternatively, a circle’s area is Pi*Diameter squared/4.

So in comparing the light collected by the front lens to the light collected by your pupil , we can just square the diameters. A 50-mm lens has an area in proportion to 50 squared = 50 x 50 = 2500. A 3-mm pupil has an area proportional to 3 x 3 or 9. You can see that a binocular with a 50 mm lens will collect 2,500/9 = about 278 times as much light.

At night, your pupils may grow to 6 mm if you are in a dark area for a few minutes. Then the 50 mm binocular lens will still have the same area but your pupil area will be proportional to 6 x 6 = 36. The light gain in using the binoculars will be 2500/36 = about 69.

When you go over 10 power with binoculars, the tremors in your arms become noticeable in the images you view. To lessen the shakiness of your view, lean against a wall, roof of car, etc. Tripod mounted binoculars are best.

I want to issue two warnings about purchasing binoculars. First, Inexpensive binoculars with prices around $10 don’t have high quality images, especially in viewing stars. They may do just fine for daytime viewing.

Second, zoom binoculars have images inferior to single power binoculars. Also, zoom binoculars tend to have smaller fields of view than single power binoculars at the same power.

A spotting scope is a good quality telescope that produces an erect (right side up) image. Spotting scopes come with their own tripod . You can expect to pay $40 or more for a spotting scope. Be sure that there is a bend near where you look, to avoid neck strain.

Spotting scope prices tend to be in proportion to the front lens diameter. The bigger the diameter of the front lens, the brighter will be the image. Just as zoom binoculars, zoom spotting scopes are subject to smaller field of view than a single power spotting scope.

SKY SIGHTS AHEAD: On Dec. 19, the moon will appear half full. For a few evenings before and after, the moon’s craters and mountain ranges will be at their best as seen through binoculars or telescopes.

Early Friday morning will be the official start of winter. At this time, the sun’s vertical rays will be farthest South at the Tropic of Capricorn (latitude 23.5 degrees south). southern Africa, central Australia, southern Brazil and northern Argentina are along this line.

Locally, the sun’s sky track will be lowest (27.5 degrees high in midday). Sunlight duration will be nine hours and 20 minutes.

Bob Doyle invites any readers comments and questions. E-mail him at rdoyle@frostburg.edu . He is available as a speaker on his column topics.

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Bob Doyle - Astronomy
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