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Wednesday, February 27, 2008

Reflectors versus Refractors


I'd like to take a few minutes to discuss one of the most fervently debated topics in amateur astronomy. Of course, one's personal preference and budget comes into play here, but hopefully this article will help to dispel some of the inevitable rumors and misconceptions out there.Which is right for you? The answer to this question, for you, may be one or the other, both, or neither! (Don't forget compound scopes!) I'm going to keep this article from being biased in one direction or the other; there'll be no scope bashing here!

We'll start by discussing both of the major telescope types' configuration and workings. Then we'll discuss each one's particular advantages and disadvantages. Each individual scope will have its own inherent advantages and disadvantages. There are far too many types of telescopes to cover in depth in one article, so we'll be speaking mostly in generalities.

Reflectors


To start off, we'll take a look at the reflecting telescope. The most common type of reflecting telescope is the Newtonian telescope. The Newtonian telescope uses two mirrors to focus the light into a cone. The primary mirror establishes this cone, and the secondary mirror bends this cone so that it can be viewed from the side of the telescope. An eyepiece is then used to view this image. The image to the left is a representation of the typical Newtonian telescope. The common Dobsonian telescope is also a Newtonian, albeit with a different mount.

The Newtonian design is a very popular telescope, and many of the entry level telescopes are Newtonians or Dobsonians. This is usually because the Newtonian design is very simple. The two mirrors are relatively cheap to manufacture, and are mechanically very simple. Therefore, one can make a Newtonian with a large aperture for very little money. The aperture of the telescope is a measure of how large the objective lens or mirror is. Basically, the bigger the better, and some Dobsonians are huge! I have a 10" model (which weighs almost 70 pounds with base), and that is considered medium sized! Some companies are producing Dobsonian telescopes as large as 30+", for a cost less than a used car.

The Newtonian design is not without flaws, however. The primary mirror of a Newtonian uses a parabolic shape. One drawback of this mirror shape is the aberration known as coma. Coma is an optical aberration that makes stars near the edge of the field of view appear as a tiny check mark. Coma can degrade sharpness of extended nebula and galaxies, and impact views of star clusters. Coma is more readily apparent in fast, large telescopes, although it can be corrected and/or reduced using a Parracor eyepiece. Another drawback of this design is the central obstruction. The secondary mirror sits inside the light cone, so it does slightly reduce contrast. A Newtonian telescope must also be collimated, or have its mirrors aligned, quite frequently. This sounds like a daunting task, but it is really quite easy with a few days experience.

Refractors


Next, we can discuss the refracting telescope. The refracting telescope works on an entirely different principle than the reflecting telescope. The image to the right shows the basic workings of a refracting telescope. Light enters the objective lens and is focused through several more lenses, then passes into an eyepiece. The refracting telescope is the oldest design for a telescope: it is the type of telescope that Galileo used to view the moons of Jupiter. Because of this, some people feel a nostalgic connection to the refracting design, a true telescope in a sense.

An advantage of the refracting design is that it has no central obstruction. Its entire aperture is clear, and hence, tends to give higher contrast than a reflecting telescope. Another nice thing about the refractor is the position in which you observe. Typically, a refractor will be outfitted with a 90* diagonal mirror before the eyepiece. This makes it so you can observe by looking down into the eyepiece, which is much more comfortable than some of the contortions that an equatorially mounted reflector can get you into. Also, a refractor is nearly permanently aligned. The objective lenses and such don't need frequent collimation as do reflectors. Refractors are pretty much ready to go after a cool-down period.

Refracting telescopes, however, also suffer from an optical aberration. While they don't display a reflector's coma, they do have what is known as chromatic aberration. Chromatic aberration is due to the fact that lenses bend light of different colors with different indices of refraction. This means that the different colors of an object will come to focus at different points. Chromatic aberration can become especially apparent when viewing bright objects such as the moon or planets. High-end refractors, called apochromats, use special lenses to reduce or nearly eliminate chromatic aberration. These telescopes can be quite expensive, however. Refractors also cost much more per inch of aperture, due to the precision demanded of their multiple lenses. You won't see a 12" refractor for a working man's budget!

Compound Scopes


While reflectors and refractors may not be the only types of telescopes, they are the two main categories. One other common type, however, is the compound telescope. The compound telescope is a "best of both worlds" telescope. It combines pieces of the refracting and reflecting design, using both mirrors and lenses. Some advantages of this design are its long focal length, for high magnification and its small central obstruction, for better contrast. This design is not as expensive as some high-end refractors, but it is very large, and can be quite heavy. They also take a long time to reach thermal equilibrium due to their large, enclosed design.

Think about it this way: refractors are the shiny Ferraris and Lamborghinis of the telescope world, while reflectors are the powerful muscle cars of the telescope world. They both go fast and are fun to use, but go about doing so in a completely different way. For a beginner, I would recommend a Dobsonian, not just because they're my favorite, but because they're cheap, big, and easy to use. If you're budget supports a large refractor, this is also a great option.

The different types of telescopes, however, should not be looked at as competitors, but rather as complements to each other. It is not uncommon to see a refractor piggy-backed onto a reflector. This is because where one falls short, the other excels, and vice versa. The two should be used to complement each others' capabilities. Many people have several of each kind! I hope that this article has helped you in understanding the differences between the different types of telescopes. Whether you're a new astronomer looking to see which type to buy, or a veteran of one camp curious about the other, its good information. Next time you get a chance, look through another type of telescope, you might just come to like that design, too!

Clear skies!

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Images courtesy of Celestron.

4 comments:

Mang (433rd) said...

Great article!

I had done an article for beginners on their first telescope at http://mangsbatpage.433rd.com/2007/11/your-first-telescope.html
It was really a suggestion to get binoculars first and then figure out what you need or want.


This provides the perspective on the strengths, weaknesses and compromises in choosing a telescope.

Thanks
Mang

Sean Welton said...

I checked out that page and it has some good information on it! I don't seem to have covered binoculars here, but they are, as you state, refractors.

Thanks!

February 28, 2008 at 2:01 PM

Ndellie said...

Great article!! Thanks for the information. I am now ready to buy my first telescope...reflector of course!

Anonymous said...

GREAT ARTICLE

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