Telescopes 101

Astronomy is the study of light. Whether you’re doing research, exploring the night sky, or even just looking up, everything that you learn comes in the form of light. In a dark sky away from the city, on a clear and moonless night, it definitely looks as if there's a lot to see! In fact, astronomers used to make painstaking efforts to document it all with the naked eye. But the human eye isn't so great in picking out the details of dim and distant objects, and even a clear and dark night reveals few secrets to the unaided observer. Enter the telescope.

A telescope is a tool that enhances the apparent brightness and size of any given target. That's a fancy way of saying it makes far away things look up-close and bright. The invention of the first telescope is a little sketchy. Most people attribute the first telescope to Galileo Galilei in 1609, but the knowledge required to build such an optical device existed before then. We just can't find any evidence that they had been built. The earliest telescopes we know of were made in 1608, in the Netherlands, by various optical craftsmen. One of them, Hans Lippershey, publicized his design well enough that news reached Galileo in Italy in 1609. When Galileo heard of the existence of a telescope, he constructed his own and within a year had drastically improved on Lippershey’s design. Galileo’s subsequent publicizing and improvement of his own telescopes, as well as his observations of Jupiter’s largest moons, have made his name synonymous with the invention of the telescope. History isn't always fair.

Reflecting vs. RefractingSimplified diagrams of reflecting and refracting telescopes. The refractor has to have an uninterrupted path for light to bend, while a reflector can be short and bounce light off many surfaces. (Richard Bloch)

So how exactly do telescopes work? Let's consider a basic telescope, like something you could find in a store. If you were looking for a telescope, an astute friend might tell you that there are two basic designs, refractors and reflectors. A refracting telescope uses lenses to bend incoming light through a tube to a focus at the far end - it's the kind of long telescope at which you imagine old-time astronomers sitting. The other kind, reflectors, use mirrors instead of lenses to reflect light to a focus. It's not really fair to say which one is 'better,’ because they both have their advantages and disadvantages, which are determined by what you want to look at, whether you want to take pictures, portability, and all kinds of other stuff. Most modern observatories use reflectors, but that's because their telescopes are so huge that on that scale refractors have no advantages. Issues like how heavy the lenses are, and how long the telescope tube is, are things you might think about when considering how you'd transport your personal telescope - but for a big observatory, these factors make construction of a refracting telescope almost impossible, and definitely less efficient than a reflector.

The world's largest refracting telescope.The world's largest refracting telescope. Its aperture is less than one-tenth as large as that of the world's largest reflecting telescope(By user Kb9vrg (from en: wp: user Kb9vrg transferred into commons) [Public domain], via Wikimedia Commons)

So let's say you've decided on your reflector/refractor. There are two other important aspects of telescopes you need to consider: aperture and focal length. The aperture of the telescope is the diameter of the opening on the front. The bigger the aperture, the more light the telescope can 'see.' Your pupil, or your body's aperture, is only ever as large as a few millimetres - optical telescopes rarely have apertures smaller than 8 centimetres, with big observatories having apertures that can exceed 10 metres! Light is hitting us from everywhere in the universe, but the dimmest objects don't give us a lot of light to work with - that's why we need big apertures. We collect as much light as we can.

Focal length is exactly that - a length. It's the length from the aperture to the focus point in the telescope. The longer the focal length, the smaller the patch of sky you're observing - but it also means there's a higher possible magnification. This is another area where reflectors have an advantage. Refractors bend light down their tube, so the tube has to be at least as long as the focal length. Since reflectors use mirrors, they can be shorter than their focal length - by reflecting light between multiple mirrors, the light still travels the full focal length, but the tube itself is not so long.

It may sound like everyone should have the biggest aperture and longest focal length possible, but aside from limitations like money and storage space, there are other reasons why this isn't a good idea. First, for anyone studying the solar system, aperture really isn't that important. Most of the planets are visible from the smallest telescopes, and objects like the Moon (and the Sun, properly filtered) are so bright that too much aperture may be an issue. Secondly, even smaller-sized telescopes (like 20cm apertures) are capable of seeing hundreds of galaxies and nebulae, some of which are almost a hundred million light-years away! It also helps to have low magnification for large objects. These objects are simply too large to 'zoom in' on - star clusters, for example, or even the Andromeda Galaxy (which is larger than the full Moon in the night sky)!

So now we have some basics. It's good to know the basic terminology for all this stuff. Luckily, astronomers keep their words simple:

The main components of reflecting and refracting telescopes.The main components of reflecting and refracting telescopes. (Richard Bloch)
  • Primary mirror: In reflectors, this is the big mirror at the back that starts to focus the light.
  • Secondary mirror: In reflectors, this is the smaller mirror at the front that redirects the light.
  • Objective: The element of the telescope that collects incoming light. In refractors this is the front lens; in reflectors this is the primary mirror.
  • Eyepiece: Exactly what it sounds like - a piece you put in the telescope to look through with your eye, to see the final image.
  • Angular resolution: The limit at which a telescope can detect a distinct object, measured as an angle of sky (usually in thousandths of a degree).
Demonstrating angular resolution
LEFT: An image of two stars through a telescope with low angular resolution. RIGHT: An image of two stars through a telescope with high angular resolution. (Richard Bloch)
Camera & resulting imagesLEFT: An astronomical camera. TOP: A raw image taken with the camera. BOTTOM: A processed version of the same image (Richard Bloch)

Whew! That sounds like a lot of stuff! Luckily, like anything else, the more time you spend around telescopes, the simpler they seem. So do astronomers learn the basics and then sit at telescopes looking through them all night? It turns out the answer is 'no.'

Nowadays, most astronomical work is done with cameras and computers. It turns out that (big surprise) astronomical cameras are more sensitive than the human eye to light. Not only that, but where a human may tell you "that galaxy kind of looks bright in the centre, and maybe has a spiral structure," the camera will tell you exactly how bright it thinks every pixel of the image is, and how it changes over time. Then astronomers can work with that highly precise data.

Cameras can also have attachments called spectroscopes that work like a prism, and break up white light into its constituents. The camera can pick up small changes in these colours, and, since different elements affect colours in a unique way, astronomers can study these changes to better understand what is happening in the rest of the universe. Almost everything we know about the universe today has been discovered without ever having left the surface of the Earth.

Light comes in different wavelengths.Light comes in different wavelengths.

So why do colours matter, aside from taking pretty pictures of space? Well, different colours represent different wavelengths of light. If you think of a wave, you can imagine stretching that wave so that it makes very gradual crests, or compressing it so that it makes very sharp crests. The wavelength is the distance from crest to crest. We see rainbows go from red to violet, but that isn't the entire picture - that's just the picture we can see. Red has longer wavelengths than violet, but light can have wavelengths much, much longer than red, and much, much shorter than violet. We can only see a tiny fraction of all light, and we call the entire possible range of light the electromagnetic spectrum. We break it up into different ranges of light, and from longest wavelength to shortest wavelength they are: radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma ray.
The EM Spectrum
An overview of our classification of the electromagnetic spectrum. (NASA)

The visible spectrum is so small that it doesn't even deserve its own classification - we just have one to distinguish which part of the EM spectrum we can see. So what does this have to do with astronomy?

Everything. Just because we see visible light doesn't mean the rest of the universe has to operate in visible light. There are events and objects that can only be detected in different spectra, and even details on visible objects (like the Sun) that are only present when viewed at these other wavelengths. So what do astronomers do? They build telescopes that can see these wavelengths.

Radio waves have the longest wavelengths, so radio telescopes need to be huge in order to collect them. These are the giant dishes you may have seen in movies.

LEFT: The Very Large Array in New Mexico. These dishes are capable of detecting radio waves, but linked together they function as one giant dish as wide as the array itself. RIGHT: The 76-metre wide Lovell telescope at the Jodrell Bank Radio Observatory. Single radio telescopes like this are also in use around the globe. (Left, NASA. Right, Richard Bloch)
Recently, radio observatories have actually been able to network different dishes together so that they can all operate as if they were one giant dish. This process is called interferometry. Some of the most distant objects in the universe give us light in radio waves.

There are other telescopes that study phenomena in other wavelengths. Microwave observatories have given us insight into the cosmic microwave background, a radiation field that permeates the entire universe, created shortly after the Big Bang. Infrared telescopes are good for peering through the dust in our galaxy that blocks visible light, allowing us to see things that might otherwise have remained invisible. Ultraviolet telescopes have given us insight into the chemical composition of our galaxy, which helps us understand how it evolves with time. X-ray and gamma ray telescopes can detect highly energetic particles from some of the most exotic objects in the universe, such as neutron stars and black holes, giving us plenty of data with which to study these objects.

Telescopes have changed our perspective on the universe around us, allowing us to learn about the cosmos long before our technology will allow us to explore it. But not all telescopes do their work from the ground. In fact, Earth's atmosphere has an annoying habit of distorting images and even absorbing different wavelengths of light. This is great for life - harmful X-rays and gamma rays from the Sun stay out, while infrared is absorbed, keeping our planet warm. Good for life, bad for astronomy. As a result, many telescopes are actually launched into space to do their work in orbit. Most famously, the Hubble Space Telescope (launched in 1990) has been peering at the near-infrared and visible wavelengths for years. Other telescopes, such as the Chandra X-Ray Observatory (which aptly observes X-Rays) can only do its work outside of our absorbent atmosphere.
The airborne telescope SOFIAThe airborne telescope SOFIA (Stratospheric Observatory for Infrared Astronomy) in flight with its telescope door open. The plane is based on the design of a Boeing 747SP, but has been modified for astronomical purposes. It flies above 99.9% of all water vapour. (By Jim Ross [Public domain], via Wikimedia Commons)

Some telescopes aren't put into space, but are sent high into the air, above most of that absorbent atmosphere! Some, such as the Kuiper Airborne Observatory and SOFIA, are mounted on airplanes, which take measurements mid-flight at around 12 000m (40,000 feet), where water vapour is sparse. Not only does water vapour produce clouds, but it's also great at absorbing infrared radiation - which is exactly what these telescopes were designed to observe. Other airborne telescopes are actually mounted to balloons, like Stratoscope and BLAST, which allow these telescopes to fly up to three times higher than airplanes!

In the end, it takes a lot of different telescopes observing a lot of different wavelengths to get a complete picture of our universe. But regardless of what part of the spectrum you observe, there's always something new out there to look at and study. So whether it's online images from NASA or your friend's backyard telescope, new sights are only a peep away!

Richard Bloch

Richard Bloch is a student of political science and astrophysics at York University in Toronto. He spends his days with work, classes, video games, and slacking off, but spends every night either reading about astronomy or practicing it out of the city. He sleeps when he's lucky, and doesn't when he's luckier.

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