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Giving the Moon & Stars: Telescopes Differ According to Their Purpose

Posted on: Tuesday, 10 April 2007, 18:00 CDT

By Thomas R. Webber

A lot of people consider a telescope as a present, especially for their own families. But many find it can be an overwhelming decision.

If I had a nickel for every time someone asked me exactly what telescope they should go out and buy, I would be writing this from my yacht anchored off the coast of Honolulu.

Unfortunately, there is no simple answer - there are just too many variables, such as budget, options and intended use. What I might want out of a telescope could be entirely different from what someone else would want.

But you can start learning about telescopes to help you make an informed decision before buying, and this column can be a first step toward that goal.

As with all things, we need to start with some basic definitions.

Telescope terms

* Aperture - The width of the objective lens or mirror. The greater the aperture, the more light the instrument can collect, thus improving image quality.

* Resolution - The instrument's ability to render detail.

* Focal length - The distance between the reflecting surface (mirror) or refracting medium (lens) to the point where the instrument is in focus. The longer this distance, the greater the magnification.

* Magnification - How much larger (or smaller) a projected image is than the real one. It is calculated by dividing the focal length of the telescope by the focal length of the eyepiece.

For example, if a telescope with a focal length of 500 millimeters is used in conjunction with an eyepiece that has a focal length of 4 millimeters, the magnification would be 500/4 = 125, expressed as 125 power or 125x.

A good rule of thumb: Telescopes can only deliver about 50x magnification per inch of aperture. That is, a telescope with a 2.5- inch lens can provide 125x at best. Aperture is always a more important consideration than any claims of magnification.

* Focal ratio - the ratio of the focal length of the telescope to its diameter. A telescope with a focal length of 20 inches and an aperture of 5 inches would have a focal ratio of 20/5 = 4, written as f/4. Most telescopes fall between f/3 and f/20.

Telescopes with small focal ratios are shorter with lower magnification and a wider field of view. Conversely, telescopes with large focal ratios are longer, have higher magnification, and possess a narrow field of view.

Types of telescopes

Telescopes are divided into two basic categories, refractors and reflectors.

* Refracting telescopes utilize lenses with curved surfaces to bring light into focus. The very first telescope was a refractor, manufactured by Hans Lippershey in 1608, one year before Galileo started to make his historic observations.

First invented by Isaac Newton in 1668, reflectors use mirrors to focus light. There are many different types of reflectors, defined by how the mirrors are set up in relation with each other and the eyepiece.

The two most common types are:

* Newtonian Reflector - Contains two mirrors, a large primary at the bottom of the optical tube and a small secondary near the top, where light is reflected to the eyepiece on the side of the telescope.

* Cassegrain Reflector - Also contains two mirrors, but the secondary mirror reflects light back toward the primary mirror at the bottom of the optical tube, where it passes through a hole and to the eyepiece. Therefore, the eyepiece is at the bottom center of the telescope.

A Schmidt-Cassegrain telescope also has a corrective plate on the front of the optical tube to focus incoming light (also called catadioptric telescopes).

So which one is the best?

Sorry, it is not that straightforward:

Advantages of refractors: Totally clear aperture, low maintenance and durable.

Disadvantages of refractors: Expensive, difficulty in supporting heavier lenses and chromatic aberration (different wavelengths of light refract through a lens at different angles, meaning light can't be brought to a perfect focus).

Advantages of reflectors: Lower cost, mirrors can be supported from the rear, and reflection is not dependent on wavelength.

Disadvantages of reflectors: Obstruction caused by the secondary mirror in the light path and spherical aberration (light rays do not converge to a single point).

What Else?

The telescope is simply the optical system. The mount is used to support and move it. Most amateur telescopes use one of three basic mounts:

* Altitude-azimuth mount - allows the telescope to move left and right (azimuth), and up and down (altitude) in relation with the horizon.

* Dobsonian mount - an altitude-azimuth mount that has a dual pivot. Most often used in conjunction with a Newtonian reflector, it is very sturdy, stable and easy to use.

* Equatorial mount - designed to be aligned with earth's polar axis and equator. Once properly aligned and used in conjunction with a tracking motor, it is able to compensate for Earth's rotation, thus keeping a celestial object within its field of view.

While not technically a mount, "Go-to" drives and electronics are becoming popular accessories. Attached to the tracking motor, they contain a database and can locate celestial objects once the telescope and mount are polar-aligned.

Thomas R. Webber is director of the Heritage Planetarium in Blount County. If you have a question about an astronomical happening, email him at skyguy@blountk12.org.

Telescope buying tips

There are going to be people who are now upset with me. After all, I have not come right out and told you exactly what to buy and where to buy it.

It is just not that easy. The purpose of this piece was to act as an introduction, giving you a foundation to pursue an interest in amateur astronomy and eventually purchase the right telescope for your needs.

However, your ol' Uncle Tom is not going to leave you hanging, so I am offering my list of "Scopes and Whatnot, Facts and Recommendations" - or SAW FAR. Clip and save the following do's and don'ts:

* DO: Learn before buying.

Read issues of "Sky and Telescope" and "Astronomy" magazines and find amateur astronomy books appropriate for the level of the end user (child or adult).

* DO: Attend star parties.

Such soirees are offered by local astronomy clubs; you can get first-hand experience with a wide variety of telescopes.

* DO: Know how much you want to spend before you buy.

* DO NOT: Pay attention to claims about a telescope's power or magnification.

Remember, magnification changes with the focal length of the eyepiece, and your collection of eyepieces and filters will grow with your hobby. Your initial investment should be to purchase as much aperture as possible.

* DO: Be patient, be realistic and consider starting small.

If you are hesitant about investing in a telescope, or the purchase is for a child under 5, start with a good pair of binoculars and a book on backyard observing.Be patient. It takes time to learn how to use a telescope and the many different accessories that are available.

Be realistic. Nothing you see through a telescope will compare to computer-enhanced images from the Hubble Space Telescope or fancy special effects from science fiction.

The most efficient telescope, in terms of optical performance for a given aperture, would be a high-quality refractor, although they can be expensive.

For transportability and ease-of use, a Schmidt-Cassegrain reflector with an equatorial mount would be best.

The best performance on a tight budget would be a Newtonian reflector with a Dobsonian mount.

Most important of all: the best telescope for you is one you are going to use! A simple small telescope that is used regularly will spark imagination and interest, while a sophisticated large telescope gathering dust in the closet will only cause frustration.

Attending a star party

To experience observing first hand, take advantage of one of the following local opportunities.

Ask questions and mingle; most telescope enthusiasts enjoy sharing their passion with others.

Visit the Web site of the Smoky Mountain Astronomical Society - www.smokymtnastro.com .

The University of Tennessee has free observing on the first and third Fridays of every month (weather permitting). Check out www.phys.utk.edu/astronomy.html  or call 865-974-7815 for more information.

Or get info on the Tamke-Allan Observatory at Roane State Community College - visit www.roanestate.edu/obs

Source: News Sentinel

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