Research into Telescopes

Jackson Heddy Page

Telescopes

The first record of a device that acted like a telescope was made in October 1608. It came from Holland where the state discussed the patent applications of and Jacob Metius. The records describe a device for "seeing faraway things as though nearby." It was the most simple form of telescope consisting of a tube and a concave lens at one end and a convex lens at the other and gave a magnifying power of three or four times.

The earliest known illlustration of a telescope.
Giovanpattista della Porta included this sketch in a letter written in August 1609

But the man who made telescopes really famous was Galileo Galilei. By October/November 1609 he had constructed a 24x telescope and made observations of the moon and discovered the four major moons of Jupiter.

Galileo’s telescopes were simple refracting telescopes that were of less quality than those you can buy cheaply in the high street. They were based on simple convex lens to focus the light and magnify that image.

Convex lens’s work on the principle of refraction. When a wave enters a material that is denser than the original substance, it ‘bends’ towards the normal (the normal being perpendicular to the plane the denser material). The opposite occurs when the wave leaves the denser material. Upon leaving the denser material the wave is ‘bent’ away from the normal. This is called refraction. Using this it is possible to focus images.

Once the image is focused it can be magnified using another convex lens as such.

When you view an image by focusing it on to your retina rather than a screen or something similar it is called a ‘virtual image’. When you focus the image on a screen and view that it is called a ‘real image’.

It was with these simple instruments that Galileo found evidence for Copernicus’s heliocentric solar system rather the Ptolemaic system, which is geocentric, and so angered the church and consigned himself to a life of house imprisonment.

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Once the image is focused it can be magnified using another convex lens as such.

The refracting telescope was a great leap forward in science, it allowed the study of the heavens which led on to the theory of gravity and many other things. But for many the most important part of its use from the proof that the church was wrong and the following divorce of the church and science. Galilean refractors have the problem that they are limited in their maximum size and also can produce false colour. The size limit is due to the fact that lenses to be effective must be held around the edge. When you increase the lens size (so as to get a greater light gathering power) past a certain point its mass grows greater than can be supported around its edge and so the lens can warp under it own weight. This warping can lead to false readings and poor images.

False colour is a bane for astronomers using refracting telescopes as it spoils observations and is only fixable using a complicated arrangement of lenses made of different glasses to make an achromatic objective. This extra system in the objective makes detailed study of the sky a lot harder as all the extra glass absorbs light and worsens the image. False colour comes about from the fact that the shorter the wavelength of a light beam the more it is refracted. This means the focal point of violet is different from that of red light. In astronomy this creates coloured rings which surround the object under study and though pretty are a nuisance in science.

I keep mentioning the term ‘focal point’ so its best that I explain it. The focal point is simply the location where beams being converged by convex lenses are focused. The focal length (which I will build upon later is related to this) is related to the focal point in being the distance from the lens to the focal point.

The next leap in telescope engineering came from Isaac Newton. He created a new type of telescope, now called a Newtonian Reflector. This worked on the principles of reflection rather refraction and only contained one convex lens and uses a curved mirror to focus the light for the eyepiece (the one convex lens). The benefits of this means there is little problem of false colour (due to less refraction) and have fewer limits on size as the objective piece (the mirror) can be supported from behind as it increases in size.

In Newtonian Reflectors the image is reflected for a second time off a flat mirror into the eye at right angles to the incoming light rays. The other type of reflector, the Cassegrain reflector instead has a secondary convex mirror to focus in back down the support tube in parallel to the incoming light. The advantages means that the focal length of the objective can be greater controlled as it is split in to parts.

The magnifying power of a telescope is related to the power of the lens. And the lens power is the inverse of its focal length. So we can say that the magnifying powers of a lens is proportional to the focal lengths of lens (or mirrors) involved. The equation linking focal lengths to the magnification is

Magnification = Focal Length of Objective

Focal Length of Eyepiece

M = F

One of the parts of our visit the Royal Observatory Greenwich was a talk on the 28-inch Photo-visual Refractor. We were told it was commissioned in 1885 and completed in 1893. It was later moved to Herstmonceaux in 1957. After retirement it was returned to Greenwich to become an exhibit.

In the visit to the Royal Observatory Greenwich we were told that the 28-inch Photo-visual Refractor had an objective lens of 28 feet and a focal length of 1-5cm depending on which interchangeable eyepiece was used. If we change the units of the focal length into metric we can then work out the range of magnifying powers. 28 feet converted into meters comes out to 8.534

So M = 8.534

0.01

= 853.4

and M = 8.534

0.05

= 170.68

So the magnifying power of 28-inch Photo-visual Refractor is 850x – 170x (we can only use 2 significant figures as that was what the focal lengths were given in).

The 28-inch Photo-visual Refractor at twilight.