How They Work
A telescope is an amazing device that has the ability to make faraway objects appear much closer! Telescopes come in all shapes and sizes, from a little plastic tube you buy at a toy store for $2, to the Hubble Space Telescope weighing several tons. Amateur telescopes fit somewhere in between, and even though they are not nearly as powerful as the Hubble, they can do some incredible things. For example, a small 6-inch (15 centimeter) scope lets you read the writing on a dime from 150 feet (55 meters) away!
Most of the telescopes you see today come in one of two flavors:
- The refractor telescope, which uses glass lenses.
- The reflector telescope, which uses mirrors instead of the lenses.
Both types accomplish exactly the same thing, but in completely different ways!
To understand how telescopes work, let's ask the following question. Why can't you see an object that is far away? For example, why can't you read the writing on a dime when it is 150 feet (55 meters) away with your naked eyes? The answer to this question is simple: the object does not take up much space on your eye's screen (retina). If you want to think about it in digital camera terms, at 150 feet the writing on the dime does not cover enough pixels on your retinal sensor for you to read the writing.
If you had a "bigger eye," you could collect more light from the object and create a brighter image, and then you could magnify part of that image so it stretches out over more pixels on your retina. Two pieces in a telescope make this possible:
The objective lens (in refractors) or primary mirror (in reflectors) collects lots of light from a distant object and brings that light, or image, to a point or focus.
An eyepiece lens takes the bright light from the focus of the objective lens or primary mirror and "spreads it out" (magnifies it) to take up a large portion of the retina. This is the same principle that a magnifying glass (lens) uses; it takes a small image on the paper and spreads it out over the retina of your eye so that it looks big.
When you combine the objective lens or primary mirror with the eyepiece, you have a telescope. Again, the basic idea is to collect lots of light to form a bright image inside the telescope, and then use something like a magnifying glass to magnify (enlarge) that bright image so that it takes up a lot of space on your retina.
 This is the simplest telescope design you could have. A big lens gathers the light and directs it to a focal point and a small lens brings the image to your eye.
A telescope has two general properties:
- how well it can collect the light
- how much it can magnify the image
A telescope's ability to collect light is directly related to the diameter of the lens or mirror -- the aperture -- that is used to gather light. Generally, the larger the aperture, the more light the telescope collects and brings to focus, and the brighter the final image.
The telescope's magnification, its ability to enlarge an image, depends on the combination of lenses used. The eyepiece performs the magnification. Since any magnification can be achieved by almost any telescope by using different eyepieces, aperture is a more important feature than magnification.
Refracting telescopes work by using two lenses to focus the light and make it look like the object is closer to you than it really is. Both lenses are in a shape that's called 'convex'. Convex lenses work by bending light inwards (like in the diagram). This is what makes the image look smaller. The biggest refracting telescope in the world is located at the Yerkes Observatory of the University of Chicago at Williams Bay, Wisconsin. It has an objective lens that's 1.02 meters (40 inches) across, and it's total power is 195x.
Reflecting telescopes, on the other hand, don't use lenses at all. Instead, they use mirrors to focus the light together. In this case, the type of mirror that they use is a concave mirror. Mirrors of this shape also accomplish the goal of bending light together, except that they do it by reflecting the light instead of bending it as it passes through (like lenses do).
The problem with these telescopes is that in order for a person to see the image at the focal point, the telescope has to be so big that a person can actually sit in the middle. (Or they have to put a small mirror in the middle to reflect the image out the side of the telescope, instead.) This is why the reflecting telescope built in the 1780's by the famous astronomer William Hershel was so big:
Some more complicated telescopes work by combining both
lenses and mirrors, but there are so many different
ways of doing this that I'm not going to try to get
into them. The other way you may have heard of telescopes
being categorized is by what kind of light they pick
up, rather than how they work. This is what they're
talking about when they say things like Radio Telescopes
and X Ray Telescopes, which are still made by some combination
of refracting and reflecting.
About
helioseismology |
