Refracting & Reflecting Telescopes


telescopes-testing-lowres-3712.jpg

Right next to light, the telescope is an astronomer’s most valuable tool. There are so many different varieties of telescopes, it can be hard to keep them all straight. But they can all be sorted into a few basic types, and that makes choosing one a lot simpler.

Two very common types are reflectors and refractors, and each one in the image above is one of these. You can tell a reflector by its cylindrical design. They all look like cylinders, you say? Well…refractors are a little bit different.

Take the two telescopes on either end of this lineup, for instance. These two—the far left and the far right—are the only telescopes here that are refractors. And you may notice that, unlike all the rest, they’re not perfect cylinders.

Look closely. You’ll see that, not only is the end pointing up a bit wider than the rest of the telescope, but there’s a little tiny piece tacked onto the end. That same little tiny piece is tacked onto the side for the reflectors.

Every reflector and every refractor can be recognized by these basic qualities. But what they do with light is more important.

In order to better understand refractors, let’s take a look at one basic property of light.

Rainbow-Stretching-Hilly-Forest-Mountains.jpg.638x0_q80_crop-smart

Light forms rainbows.

You already know this. And you probably guessed that the light that forms rainbows is the exact same light that casts the bright, white beam of a flashlight. But how? How does white light turn into these colors?

The answer lies in refraction. And hey, guess what—that’s what refracting telescopes are named after.

Light_dispersion_conceptual_waves350px.gif

Refraction is a fancy word for bending. In this animation, a white beam of light is aimed at a prism. It passes through the prism, and along the way, it bends. But light is made up of many different wavelengths, and each one bends differently.

The red light doesn’t get bent much at all. The purple light, however, bends a ton. Because each color of light bends a different amount, it comes out the other end of the prism separated into distinct colors. All these colors blended together creates white light.

A refracting telescope takes advantage of the fact that light can bend. Unfortunately, it also splits out the colors of the rainbow in the process, which is part of why refractors aren’t common in major research observatories.

Here’s what basically happens inside a refracting telescope.

RefractorDiagram.jpg

Here, the blue arrows represent rays of light reaching the telescope from some distant object in the sky. That wider part at the up-pointing end of the telescope isn’t shown here, but all refractors have them. You can see that the little piece at the end is the eyepiece.

Any telescope’s job is to bring light to a focus.

Wait…but I thought a telescope’s job was to magnify objects?

No, no, that’s the eyepiece’s job. And you can change the magnification on any telescope just by swapping out the eyepiece. No, a telescope’s main job is to bring light to a focus. A refractor does that by bending light.

To do that, the telescope uses lenses. These lenses are little pieces of glass that have to be ground (shaped) absolutely perfectly. After all, light passes through them, and perfection is critical for aiming the light in the right direction.

Refractors use convex lenses, lenses that are rounded outward on either side.

Each lens acts as a prism, and bends the light so the once-parallel rays are all aimed at each other. They come to a focus—the focal point “f”—somewhere in the tube of the telescope.

After that, it’s the eyepiece’s job to make those rays of light parallel again and to magnify them the way you want. Most telescopes can accommodate just about any eyepiece; after all, that’s the power of the eyepiece. Each one magnifies a different amount.

Then, there are reflectors. And in order to understand them better, let’s take a look at another basic property of light.

cat in mirror.jpg

What’s happening here?

Well, obviously, a cat is being reflected in a mirror.

Um…wait. What is being reflected in the mirror?

It’s not the cat. Because in order for a cat to reflect off a mirror, it would have to do this:

cat reflecting off mirror.gif

Guys, reflecting means the act of bouncing off something. And things sure as heck don’t bounce off mirrors all the time when they’re “reflected” in them. What you’re really seeing is light being reflected.

And reflecting telescopes take advantage of the fact that light can reflect.

reflector.jpg

A reflector has two basic parts, and they’re both mirrors. Not lenses. Lenses bend (refract) light; mirrors reflect light.

Again, the blue arrows represent parallel rays of light hitting the primary mirror and reflecting off to hit the secondary mirror. The difference between these two mirrors is that one is concave and the other is flat.

Because light has to reflect at the same angle that it hits a mirror, the primary (objective) mirror has to be concave to aim the light in the right direction. The secondary mirror has to be flat because the angle of the light has already been changed.

After leaving the secondary mirror, light in a reflecting telescope crosses at its focal point, where it comes into focus. It then reaches the eyepiece, where the rays of light are straightened so they’re parallel again before hitting your eye.

Reflectors are much more common than refractors these days…because refractors are plagued by something called chromatic aberration.

chromatic_aberration_lens_diagram.png

Chromatic aberration is a problem for refractors because each color of light bends differently. That means that each color of light will focus in a different place (that’s the place where the light rays cross).

Here’s what can happen if you look at a large object, like the moon, with a refracting telescope…

moon chromatic aberration

This isn’t really a very good image period. It’s not in focus at all. But the blue and yellow glow on either side isn’t the photographer’s fault. That’s because of chromatic aberration.

In the case of this image, I can only guess that the photographer chose to bring either the blue or the yellow light into focus rather than the lunar surface. It looks to me like they chose the yellow light.

It’s possible to correct for chromatic aberration using an achromatic lens, but this is expensive—too expensive for astronomers to bother with. It’s already expensive to do their research. Why tack corrective optics onto the price?

For that reason, most major observatories—and, indeed, most large (and more expensive) telescopes—are reflectors. Reflectors just don’t have to deal with chromatic aberration. Or anything else of the sort.

If you’re out to buy a cheap telescope, a reflector is the type for you. It’s the most diameter per dollar, always a plus. And you won’t have to worry about rainbows on the moon.

About Emma

I'm a college student, astronomer, writer, Democrat, global warming activist, hardcore trekkie, daring adventuress, and dreamer. I have four blogs to date. The primary one is Science at Your Doorstep, but I also post to The Sojourns of Star Trek, Common Sense Politics (US), and Toleventures. I guest post to A Momma's View. I write for different reasons. Sometimes because I want to share my process of discovery with others. Sometimes because I think a story is too incredible to keep it to myself. Sometimes just because I want to be heard. Regardless, I hope you enjoy it!
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4 Responses to Refracting & Reflecting Telescopes

  1. jg says:

    Very nice. There are two typos, but I’m on my phone and can’t see them while commenting.

    Liked by 1 person

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