Far, Far Away…

Forget your troubles for a moment and look at the sky.

The night sky, I mean.

Sadly in an urban metropolis such as the one I live in there isn’t much to see at night besides the faint glow of a near Full Moon. A telescope, however, paints on the black canvas of the night sky and unlocks an entirely new world.

For centuries we’ve used telescopes to discover more about this world. A simple optical telescope (i.e. one that detects visible light) consists of two pieces of glass. Sometimes three. That’s literally it. Seems a little anticlimactic, right? You would expect there to be many delicate mechanisms at work.

There are currently two major types of optical telescopes. The first type is the refracting telescope, or refractor. The earliest refractors were developed in the Netherlands in 1608, and its invention is generally credited to spectacle-makers Hans Lippershey and Zacharias Janssen, and to instrument-maker Jacob Metius. The famous Italian polymath Galileo Galilei, after having heard of this news, constructed his own the following year and pioneered the use of telescopes in astronomy, hence why he has been called the father of observational astronomy (and even the father of modern science).

The two pieces of glass that a refractor uses are both lenses. At the front of a refractor is an objective lens, which gathers as much light as possible from a faraway object (say, a planet) and brings it to a focus (think burning ants with a magnifying glass). Behind the objective lens is the eyepiece lens, which takes light from the focus and spreads it out so that your eye can produce an image from it. The key to this configuration is that the distance between the two lenses is equal to the sum of their focal lengths, and everything else falls into place.

refracting-telescope
Image: Las Cumbres Observatory.

These telescopes are often tweaked slightly, because you wouldn’t want a telescope designed to look at nearby galaxies to spy on your neighbour’s garden, of course. The magnification (how zoomed in the image is) is maximised by increasing the ratio of the powers of the eyepiece lens to the objective lens, and the resolution (how detailed the image is) is maximised by increasing the size of the aperture (in this case the diameter of the objective lens). In some telescope configurations, a mirror is also added at the end so that you look through the side of a telescope instead of along the axis of the telescope.

The second type of optical telescope is the reflecting telescope, or reflector. The reflector uses two mirrors instead of lenses (hence the name), and Isaac Newton is generally credited for constructing the first reflector in 1668.

In a reflector, the light from the faraway object first hits a primary mirror, which bounces the light backwards to a secondary mirror. The secondary mirror then directs it towards your eye.

11_physics
Image: BBC.

And then there are other more complex designs that use a mixture of lenses and mirrors.

Our place in the universe can be discovered by a few fragments of glass. Amazing, is it not?

Yanhao

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