How Do Telescopes Work?

A telescope has two optical jobs. A large lens or mirror collects light from a distant object and brings it to a focus. A smaller eyepiece then spreads that focused image out so the eye can examine it.

The collecting part is what matters most. Your pupil is about seven millimetres across at night. A modest amateur telescope with a 200 millimetre mirror collects roughly eight hundred times more light, which is the difference between seeing nothing and seeing a galaxy.

Refracting telescopes use a lens at the front. They give crisp images but suffer from chromatic aberration, where different colours focus at slightly different points, and large lenses are heavy and sag under their own weight.

Reflecting telescopes use a curved mirror at the back instead. Mirrors do not split colours, can be supported from behind, and can be made far larger. Every major research telescope built in the last century uses mirrors for this reason.

Telescopes advertised by magnification are usually the wrong ones to buy. Magnification is just the ratio of two focal lengths and can be changed by swapping the eyepiece, so any telescope can claim a huge number.

What cannot be changed is the aperture, the diameter of the main lens or mirror. It determines two things:

• •How much light is gathered, and so how faint an object can be seen
• •How much fine detail can be resolved before physics sets a limit

Modern astronomy uses the whole electromagnetic spectrum. Radio telescopes use large dishes to collect long wavelengths, and several can be linked so they behave like one instrument the size of the distance between them.

X-ray and infrared telescopes usually go into orbit, because the atmosphere absorbs those wavelengths before they reach the ground. The James Webb Space Telescope observes in infrared with a 6.5 metre segmented mirror, which is why it can see dust shrouded regions and extremely distant galaxies.

Air turbulence blurs images, which is what makes stars twinkle. Ground based observatories are built on high dry mountains to sit above as much atmosphere as possible.

Many now use adaptive optics, where a flexible mirror is adjusted hundreds of times per second to cancel out the distortion, guided by a real star or an artificial one created with a laser. This lets the best ground telescopes approach the sharpness of space based ones.

The most common mistake is buying by magnification. A more useful approach is to consider aperture, mount quality and how easy the instrument is to set up, because a telescope that is awkward to use gets used rarely.

A few practical points are worth knowing:

• •A stable mount matters as much as the optics, since a wobbling image is unusable
• •Binoculars are an excellent and cheap starting point for the Moon and star clusters
• •Light pollution limits what is visible far more than telescope size does
• •A modest telescope used often beats an ambitious one left in a cupboard

Light behaves as a wave, and when it passes through an opening it spreads out slightly, a process called diffraction. Even a perfect telescope therefore turns a point of starlight into a tiny blurred disc surrounded by faint rings. The size of that disc depends on the wavelength of the light divided by the diameter of the aperture, so a mirror twice as wide produces a disc half as large and can separate two stars twice as close together. This is also why radio telescopes, working with wavelengths far longer than visible light, need to be enormous.

The move from lenses to mirrors has a long history. Galileo's early refractors from 1609 magnified only modestly, yet they revealed moons orbiting Jupiter and the phases of Venus. Isaac Newton, frustrated by colour fringes in lenses, built a small reflecting telescope around 1668, and the Newtonian design is still one of the most popular among amateurs. Later, glass mirrors coated with a thin layer of silver or aluminium replaced heavy metal ones, making large reflectors practical. Each step came from a physical limitation that someone understood and then designed around.

New telescope owners are often disappointed because they expect to see what space photographs show. Stars remain tiny points of light even in the largest telescopes, because they are so far away. Most galaxies and nebulae appear as faint grey smudges rather than vivid colour. That is partly because the eye's most sensitive cells in dim light, the rods, cannot detect colour, and partly because famous images are long exposures gathering light for minutes or hours, often through special filters. The Moon, planets, double stars and star clusters, by contrast, look superb even in a small instrument.

A few habits make a large difference. Your eyes need around twenty to thirty minutes in darkness to adapt fully, and a single glance at a bright phone screen resets much of that, which is why observers use dim red torches. For faint objects, try averted vision: look slightly to one side of the target rather than straight at it. This places the light on the edge of the retina, where rods are more plentiful, and a smudge that was invisible can suddenly appear. Start with low magnification, which gives a wider, brighter view.

Magnification is easy to calculate once you know two numbers printed on the equipment. Divide the focal length of the telescope by the focal length of the eyepiece. A telescope with a 1,200 millimetre focal length used with a 25 millimetre eyepiece gives 1,200 divided by 25, or 48 times. Swap in a 10 millimetre eyepiece and the same telescope magnifies 120 times. Nothing about the main mirror has changed. You have simply spread the same collected light over a larger image, which is why higher magnification also makes the view dimmer.

There is a practical ceiling. A common rule of thumb puts the highest useful magnification at about twice the aperture in millimetres, so roughly 400 times for a 200 millimetre telescope. Push further and the image grows larger but no sharper, which observers call empty magnification. On many nights, turbulent air limits useful magnification further, whatever the size of the telescope. Another handy figure is the exit pupil, the aperture divided by the magnification. At 48 times, a 200 millimetre telescope gives an exit pupil of about 4 millimetres, comfortably within a dark-adapted eye.

The Hubble Space Telescope shows how precise a telescope mirror has to be. Soon after its launch in 1990, astronomers found its images were blurred. The 2.4 metre main mirror had been ground to the wrong shape: towards its edge it was too flat by about 2.2 micrometres, roughly a fiftieth of the width of a human hair. That tiny error meant light from the edge of the mirror came to a focus at a different point from light near the centre, a fault called spherical aberration. The cause was traced to a testing instrument used during polishing, in which a lens had been set slightly out of position.

Because the error was known so exactly, it could be corrected rather than the mirror replaced. In December 1993, astronauts on the first servicing mission installed a device containing small corrective mirrors, along with a new camera that had the correction built into its own optics, much as glasses correct an eye that focuses imperfectly. The sharp images that followed made Hubble famous. Engineers took the lesson to heart. The mirrors of the James Webb Space Telescope were tested together as a complete system inside a giant cold vacuum chamber before launch, so that no single faulty measuring tool could quietly shape the result.

Telescopes work by collecting far more light than the eye can, using mirrors rather than lenses at large sizes, and aperture rather than magnification is what determines how much you can actually see.