Telescopes can be used for loads of different things, there are the great big ones that orbit the earth and send us photos back about what’s happening on Mars, there are the long ones that people have in their attics or on their rooftops to peer at the night sky and try to catch a glimpse of a shooting star, there are those little portable ones which you can carry around for a bit of bird spotting (of the feathered variety!) – yep, they do come in all shapes and sizes.
Telescopes basically though, can be one of two different types – they can be refractors, or they can be reflectors.
How They Work – Refractor Telescopes
Refractor Telescopes have a glass lens at the front of the telescope, which bends (or refracts) the light as it passes through it. This is how the first telescopes were designed and built, and are still used to this day in the small, everyday telescopes (remember the bird watching!) or for sights on rifles and that sort of thing.
The advantages of refractor telescopes are:
- Don’t need cleaning too much – the glass surface is actually sealed so doesn’t get grimey from the atmosphere. No cleaning isn’t just good for lazy people, it means that you also get a good clear view of whatever you’re looking at.
- Refractor telescopes aren’t too sensitive – once they’ve been aligned they more or less stay aligned even if you move them about a bit – hence they’re good for putting in your pocket or your rucksack.
- Refractor telescopes offer steady and sharp images. The tubes are completely sealed so aren’t affected by changing temperatures or air currents, so size for size, the pictures are much clearer than those in a reflector telescope.
Unfortunately, you have to take the rough with the smooth and there are some problems with refractor telescopes which is why nobody has bothered to try and make a really huge one to be used for serious astronomical research.
The disadvantages of refractor telescopes are:
- Somewhere over the rainbow – no, I haven’t been put under a spell by the Wicked Witch of the West, but all refractor telescopes have a certain amount of chromatic aberration – you know that pretty rainbow of colors. Right, well it might look pretty if that’s what you’re after, but it can actually be a nuisance if your trying to identify a star which is 3 zillion light years away! Hey, that star is beautiful, it’s all rainbow colored man . . .
- It’s really difficult to make a lens which doesn’t have any imperfections and has perfect curvature not just on one side, but on both.
- You can only support the lens at the ends, which means that if the lens was too big it would very possibly sag from its own weight.
- As the lens gets thicker, the efficiency of the light passing through it decreases.
OK, so there are plenty of reasons why refractor telescopes are perfect, just as long as they don’t get too big.
How They Work – Reflector Telescopes
Reflector Telescopes however, use a mirror with which to capture and focus light. All big celestial telescopes are reflectors, after all, the stuff that you’re looking at is so far in the distance that their light rays are parallel once they get to Earth. Because of this, the mirror on a reflector telescope is a parabolic shape.
The advantages of reflector telescopes are:
- they don’t suffer from rainbow colors because each of the wavelengths is reflected in the same way off the mirror.
- the mirror can be supported right across it’s back surface, so you can make them BIG.
- Size for size, reflector telescopes are actually cheaper than the refractor type.
- Reflector telescopes only need to have one perfect side because the light bounces off it instead of passing through.
The disadvantages of reflector telescopes are:
This is a bit like the opposite of the above isn’t it?
- The optics can easily be moved out of alignment.
- The optics need to be cleaned a lot, as they are open to the outside elements
- If another (or a secondary) mirror is used to send the image to somewhere where it’s a bit easier to see it, you can sometimes get a diffraction which gives a lovely “Christmas Star” effect. Very nice in season but not ideal when you’re trying to discover some other celestial beings.