3-1 Telescopes

Types of Telescopes

A telescope is a device that lets you look at faraway objects as if they were up close. If you're interested in space, you can buy a small one to use yourself, or you can take a tour of a local observatory to see a really big one. (Most universities have observatories and they have public tours every so often, usually monthly.)

Telescopes do this by doing two main things:

1. Gathering a lot of light, using a large lens or curved mirror

2. Focusing that light to a point so it can be viewed or photographed

As suggested above, either a large lens or a large curved mirror is used. This determines the type of telescope that you have.

Refracting telescopes

A refracting telescope (or "refractor") uses a large lens to gather light, called the objective lens. It uses a another smaller lens, the eyepiece, to focus the light so you can see a clear image.

One of the first inventors of the modern telescope, Galileo Galilei, hand-made the two telescopes seen below in about 1610. They only magnified the image by about 20 times, but it was a vast improvement on the human eye and allowed Galileo to discover the four large moons of Jupiter.

Reflecting telescopes

Refracting telescopes were a good way to get started, but astronomers soon found that the large objective lenses needed to gather light would sag under their own weight. Plus, different colours of light would focus in different places, causing a blurry image.

A reflecting telescope (or "reflector") uses a curved mirror to gather the light, and focus it towards a small flat mirror that reflects the light towards the side of the tube. An eyepiece is used here again to focus the light so you can view a clear image.

This telescope seem a little strange: it's a big, open, mostly-empty tube. The light comes out the side of it, near the top. Very weird! The first reflecting telescope was probably built by Isaac Newton in the 1660s. Think about it: two of the biggest names in early science, Galileo and Newton, both built their own telescopes.

If you're interested in buying a small telescope for yourself, you can buy either a refractor or a reflector. Take it out to a park on a dark, clear, moonless night and see what you can see!

The Problem of the Atmosphere

Our atmosphere does a lot for us: it keeps us warm (possibly a little too warm these days), it burns up meteors that would otherwise it the ground, and blocks harmful ultraviolet rays with its ozone layer. It also provides oxygen for us to breathe, which is pretty nice too.

However, there's a problem with having an atmosphere when you're trying to see small points of light which are far away. The air, and the differences in temperature of the air, distort the picture you want to see.

One of the ways we can see this is when, on a hot sunny summer day, we can see heat distortion caused by a hot air rising from a dark surface such as a parking lot.

In the picture above, the hot air rising from the red-hot lava distorts the picture of the apartment buildings behind it. The uneven temperature of the air acts like a strangely-shaped lens, bending light out of focus and changing shapes.

This might be an interesting and pretty effect in photography, but if you want to take a clear picture of a tiny point of light, this will not help you at all.

Telescopes on Mountains

The air in the atmosphere distorts pictures, even with small temperature differences. So, you can try to put your telescope in a place where there'll be as little air between you and outer space as possible. One solution: build your telescope on the top of a mountain.

Above are two of the telescopes on Mauna Kea, an extinct volcano in Hawaii which has several telescopes on top of it. Its peak is over 4 km above sea level, and when you're on top of it, most of the atmosphere's gas, and clouds, are below you. Plus, the atmosphere above Mauna Kea is extremely stable most of the time, which makes for very clear pictures.

Telescopes in Space

While Mauna Kea is great, what's even better is to have a telescope above the entire atmosphere. This is a space-based telescope, and humans have put a few of them into orbit.

The most famous one is the Hubble Space Telescope, which was first launched in 1990.

See the open "lid" at the top end? That means the Hubble is a reflecting telescope. Here are some NASA scientists examining Hubble's mirror before the telescope was launched into orbit:

What these scientists didn't know was that a design mistake meant the mirror was curved just a little too much. How little? About two micrometres on the edges, about 5% of the width of a human hair! But it made all the difference. This article tells the fascinating story about the problem, and how NASA fixed it.

More recent than the Hubble is the James Webb Space Telescope (JWST), launched in 2021. Its mirror is much larger, which means it can gather much more light and take pictures of fainter objects.

It might seem a little strange that the mirror and instruments are just sitting out there in the open, but so long as no stray asteroids or space junk hits the telescope, it should work just fine.

JWST's mirror is gigantic in comparison to Hubble:

The JWST's mirror exists in segments, which folded up so it could be launched into space more easily. The mirror then unfolded in space, the pieces can be turned to change the overall curvature of the mirror. The light takes a very strange path with several bounces before it gets to the big digital camera that captures the picture, in this video from NASA:

No wonder this took so long to build! But the pictures are worth it.

This is a part of a picture of the "Pillars of Creation" nebula. When the Hubble (HST) first took pictures of it in the 1990s, it was an amazing accomplishment and it let us see details we'd never seen before. You could buy posters with this picture on it.

But with the JWST a few decades later, just look at the extra details it can bring out. With a telescope as powerful as the James Webb, just imagine all the things we can see that we couldn't have seen before!

Astronomy at Different Wavelengths

As we saw before, electromagnetic radiation comes in different varieties. From lowest- to highest energy:

• radio waves

• microwaves

• infrared

• visible light

• ultraviolet

• X-rays

• gamma rays

When you look up at the sky with your eyes, you are only able to see the visible light coming from stars, planets and other galaxies. But, as it turns out, you're only seeing a tiny part of the whole picture!

The picture below shows the stars in the constellation Orion, an easily-observable constellation in the winter in the Northern Hemisphere. On the left is what it would look like to our eyes. But on the right, that is what it would look like if we were able to see infrared light.

We can still see the stars, but there's something very unusual going on at the end of the sword hanging off Orion's belt! As it turns out, that's the Orion Nebula, a fuzzy cloud which contains a lot of gas that will eventually turn into stars. Below is an excellent picture of the nebula.

Keep in mind that, with colours of light that are not within our visible light range, false colour has been used. The Orion infrared picture has been coloured red, orange and yellow, but it's not actually those colours. Different wavelengths of infrared (IR) are "seen" by an IR camera on a telescope, and a computer assigns a colour to these different wavelengths.

The Crab Nebula is the left-over parts of a supernova that we observed about a thousand years ago. Using different wavelengths of light, we can see many new details about its structure.

(Again, for everything except visible light, the above is false colour.) We can get different types of information using different wavelengths; as you can see here, there's a lot going on in the radio wave part of the spectrum, but not a lot with high-energy X-rays. However, in the low-energy X-ray part of the spectrum, something is definitely going on.

The general idea is this:

Or, put more simply:

High-energy objects give off high-energy electromagnetic radiation.

The picture below is an X-ray observation of a pulsar, a rapidly spinning neutron star that gives off regularly-timed blasts of radiation that cross our path. This is a high-temperature object, so it will give off a lot of X-rays; in visible light, it doesn't really look like much.

All the detail we see in this false-colour image can give us a better idea about how these objects work. This can help us better test out theories and models to see if what we think might be going on is actually happening.

However, there's a problem with some types of radiation: the atmosphere blocks a lot of it.

So, to see X-rays, gamma rays, most ultraviolet and infrared, and most microwaves, you have to make those observations above the atmosphere. For this you need a space-based telescope such as the Hubble or the James Webb. There are other space telescopes which are much more specialized, like the Chandra X-Ray Observatory, in orbit around the Earth.

Practice

The Basics

1. Describe the difference between a reflecting and a refracting telescope.

2. What is a space telescope?

3. What type of wavelength of electromagnetic radiation would you use to view (a.) a hot supernova explosion, or (b.) a cold interstellar gas cloud?

Extensions

1. Describe how a refracting telescope is actually a lot like a microscope.

2. A black hole gives off no light. However, black holes cause certain types of radiation to usually be emitted by something else close by, which gives away the black hole's (probable) location. Do some research to find out what type of radiation is often seen near (but not directly coming from) a black hole.