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Frequently Asked Questions 

 

Choosing a New Telescope

Telescopes are designed with differing purposes in mind, but with one thing in common: they all help us to see far off or faint objects more clearly.

If you are new to astronomy, and are still learning about the wide variety and price ranges of telescopes, you probably do not yet have a good feel for what is available  on the market today. A good way to get personally acquainted with telescopes is to check them out during a star party. 

Telescope designs vary as widely as the design of people. Diverse sizes, shapes, and colors, as well as many other aspects of telescope design, can be seen as you walk through the observing field at a star party. You will not usually see two sets of exactly the same equipment set up on any one observing field.

Telescopes fall into basically two categories: reflecting telescopes and refracting telescopes. A refractor has a lens on the front end called an objective, which forms the images viewed through the eyepiece. In contrast, the objective in a reflector telescope is a curved mirror mounted at the bottom, which reflects images back up into the eyepiece. The most obvious external differences are due to the various ways telescopes are mounted.

Selecting a telescope should not be decided on a whim. Before you buy, think about what you want to do with your telescope, what size you can physically handle, and the price range you can afford. Impulse purchasing is often regretted and observers are easily disappointed when they  find out that the telescope they just bought is not the one they really want. Think things out before you buy.

Everyone who has been an active amateur astronomer for very long usually has their own favorite type of telescope and accessories. At a star party, you will get a good feel of what is available and at what price, and learn what its owner thinks about his own equipment. Ask questions and look through as many telescopes as you can. Many of the major organized star parties will also have an area set aside for vendors, where you can conveniently compare and shop various types of equipment.

When you finally decide what telescope you want, and how much you are willing to spend, browse through the ads in the astronomy magazines, the internet, and catalogs. You will be surprised to find out how much prices can vary from one distributor to another for the same equipment. Sometimes you can find what you want from an individual. Check around.

Types of Telescopes Used by Amateur Astronomers


A Newtonian Reflector 
Mounted on a German Equatorial Mount


A Newtonian Reflector 
Mounted on a Dobsonian Alt-Az Mount


A Typical Department Store 60-mm Refractor Mounted on a Tripod with Alt-Az Controls


A Schmidt-Cassegrain Catadioptric 
Reflecting Telescope with Equatorial Wedge Mounted on a Field Tripod

Telescope Images Drawn by George Proctor. Copyright 1987

Telescopes fall into basically two categories: reflecting telescopes and refracting telescopes. A refractor has a lens on the front end called an objective, which forms the images viewed through the eyepiece. In contract, the objective in a reflector telescope is a curved mirror mounted at the bottom, which reflects images back up into the eyepiece. The most obvious external differences are due to the various ways telescopes are mounted.

Optical Systems Commonly Used in Amateur Telescopes

 

Refractor 
Optical System

Newtonian Reflector 
Optical System

Catadioptric  
Optical System

Light Path optics-rfr.jpg (21787 bytes) optics-rfl.jpg (19472 bytes)

optics-sct.jpg (22669 bytes)

Aperture Range in Common Use 2 to 4 inches
The 60-mm (2.4-inch) is the most popular
3 to 12 inches
The 6-inch is the most common. Dobsonians up to 32-inches
3-inches and up. The Meade ETX Telescope
Focal Ratios f/8 to f/17 f/5 to f/10
f/4 Wide Field often found
f/10 to f/15

Telescopes and Magnification

Many people purchase a telescope because they want to look at celestial objects at high power. While magnification is certainly a function of a telescope, equally important is the light gathering power of the optical system. The human eye, due to the small size of the opening of the pupil, can gather only so much light. One thing a telescope does very effectively is gather more light and concentrate it is such a way that it is easily seen by the eye. Put simply, the bigger the telescope's objective, the brighter the image will be in the eyepiece. Furthermore, a large objective also means higher effective magnification. That is to say, telescopes with a larger objective can be used at higher powers.

According to AAAA member Doug Kniffen, diffraction theory states that 50X per inch of aperture is the limit. Even with very good optics, this is rarely achievable. Further, while this limit may prove true with an "artificial eye," it ignores the difference between detection in the eye and perception in the brain. "Among my telescopes," Doug says, " I've used as much as 75X per inch, and as little as 25X per inch."

While some of the difference can be attributed to the instruments, most is due to the air at the time of observation.  The atmosphere, from inside the telescope to the edge of space, will moderate your practical magnification limit. More important than the total magnificatio, is matching the eyepiece exit pupil to your eye. Higher magnification does not always show more, since small angular scale image distortions will become visible.

Occasionally, increasing the magnification beyond the apparent limit  imposed by the atmosphere will actually improve the image if the distortion temporal rate is much less than the response rate of the eye, and the angular size of the distortions are relatively large compared to the airy disk size.


The Meade ETX

Q: I am interested in purchasing a telescope.  I have been looking at the Meade ETX125.  Is there a resource at your organization that could tell me that this would be a good purchase for the money or is there something else that would be better for that same amount of $s?  I know you probably want to know how much star gazing I do and if this is my first purchase.  I have always been interested in astronomy.  I do not own a telescope.  This would be my first, but I want to get a good one that I would not have to upgrade anytime soon.  I would like to be able to observe the planets and moons, our moon and also possibly some nebulas.  I would appreciate any advice that you could give me.

A: The Meade ETX line seems to be a very popular scope, and Meade would not be able to sell so many of them if they were not a good product.

The main thing to keep in mind is aperture, that is to say, the size of the objective or light gathering part of the telescope. I understand that the optics in the Meade ETX series are good, but they are still small telescopes. This is fine for viewing the planets and the moon. However, if your interest is really looking at fainter galaxies, nebulas and clusters, you might consider getting a telescope with a much bigger aperture. In any case, even if you do end up buying an ETX, try to buy the one with the biggest aperture you can afford.

One of the popular features of the ETX series is the computer controlled GOTO capability of these scopes. If you are a beginning astronomer, you will probably find this feature a real advantage, especially for such a relatively inexpensive piece of equipment.

There is a website created by Mike Weasner devoted solely to the ETX telescopes. The URL is http://www.weasner.com/etx/menu.html

You might also spend some time exploring the AAAA site, as it contains a lot of information of interest to beginning astronomers.

Sincerely,

Ed Flaspoehler, President
American Association of Amateur Astronomers
http://www.astromax.com



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