Two Kinds of Telescope
describe the features of reflecting and reflecting telescopes (detailed
ray diagrams not needed)
demonstrate an understanding of why the world’s largest telescopes are reflectors rather than refractors
Since the invention of telescopes astronomers have been able to see a huge amount more of what is out there including the detailed surface of the Moon and the planets. We can also see many more stars than we can with the unaided eye.
There are two main types of optical telescope. Refracting and reflecting.
When a wave such as light passes from one medium to another at an angle it changes direction. This is called refraction. A lens is a piece of glass designed to bend the light that passes through it in such a way that an image may be produced. A refracting telescope uses a combination of lenses to produce an image of a distant object, e.g. a star or planet.
A simple refracting telescope consists of two lenses, the Objective and the eyepiece. Basically the objective lens produces an image of a distant object at its focus and the eyepiece lens magnifies this image. We can represent this using a ray diagram which shows the path of imaginary rays of light passing through the telescope.
From the diagram above we can see that the distance between the lenses is the sum of the focal lengths of the two lenses.
The magnification of a telescope or any optical instrument is how much bigger the image is compared with the object. The magnification of a refracting telescope is equal to the focal length of the objective divided by the focal length of the eyepiece.
e.g. a refracting telescope has an objective of focal length 70cm and eyepiece 5cm. Its magnification will be 70 / 5 = 14.
Star gazers usually have a selection of eyepieces at hand. You don't always want an incredibly high magnification, e.g. when studying the Moon. When you want to observe a star you will often switch eyepieces. Less powerful to locate the star and more powerful to observe it closely. (eyepieces with smaller focal lengths are more powerful as they need to bend the light more).
This is a problem when using lenses. Different wavelengths of light refract by different amounts. Smaller wavelengths refract more than longer wavelengths. This is how a prism splits white light into the visible spectrum.
The result of this is that the edges of the image are blurred and coloured like fuzzy rainbows. One can get special lenses that have a special coating to reduce the amount of chromatic aberration but they are expensive.
Often called Newtonian telescopes as they were invented by Isaac Newton. They use a large concave mirror to create an image which an eyepiece lens then magnifies.
There are a number of possible designs, e.g. instead of a secondary lens one could use a photographic plate. The most powerful telescopes are reflectors.
So which is better then?
Why are reflecting telescopes better?
It is much easier and therefore cheaper to make a large parabolic mirror than it is to make a large lens. Reflectors are therefore a lot cheaper for the same magnifying power.
Chromatic aberration is not a problem.
They have large apertures which we will see makes a big difference to the quality of the final image.
They tend to be quite bulky. They are certainly less portable than refractors.
They require periodic maintenance to keep the optics properly aligned.
The secondary mirror and its supports can effect the image.
Find a website that sells both and compare the prices for the same magnifying power.