Spark image


The main purposes of a telescope used for astronomy are:
(a) to gather as much light as possible this is done by using a large aperture lens or mirror. The amount of light gathered depends on the AREA of the lens so a lens with an aperture of 300 mm diameter gathers four times a much light as one with an aperture of 150 mm diameter.
(b) to resolve fine detail this is also done by using a large aperture lens or mirror. The larger the aperture the finer the detail that can be seen. (Usually called the resolving power of the telescope.)
(c) to magnify the image of a distant object this is done by using a lens or mirror with a long focal length. The actual magnification of a telescope can be worked out from the formula:

Magnification = Focal length of the objective (F)/Focal length of eyepiece lens (f)

Example problem
Calculate the magnification of a telescope with an objective of focal length 1200 mm using two different eyepieces:
(a) focal length 25 mm
(b) 10 mm

(a) Magnification = 1200/25 = 48x         (b) Magnification = 1200/10 = 120x

Refracting telescopes telescope using large lenses for their objectives

Ray diagram for a refracting telescope

Normal adjustment and magnification

The telescope is adjusted so that the final image is at infinity so that the eye is completely relaxed when viewing it. This is called normal adjustment.

The magnification, strictly the angular magnification is:

Magnification (M) = Angle subtended by image at the eye (β)/Angle subtended by object at the unaided eye (α)

Magnification = Focal length of the objective (fo)/Focal length of eyepiece lens (fe)

Disadvantages of a refracting telescope

(a) the lenses are made of glass and because the light has to go through them the glass must be perfect there must be no bubbles of air in the glass
(b) the lenses can only be supported around their edges and this is where they are thinnest and weakest. The largest telescope lens in the world is on of a metre in diameter at Yerkes Observatory in the USA
(c) lenses suffer from colour distortion this means that when white light passes through the lens it is split into the colours of the spectrum. Because violet light refracts more than red light it is brought to a focus closer to the lens than the red light this makes the image coloured and blurred. This effect is called chromatic aberration.

However for small telescopes refractors are usually better than refractors because they can be made of high quality glass with no defects and the colour effects can be eliminated by using special lenses called chromatic lenses.

Sites for an observatory

Telescopes are best placed in observatories on the tops of mountains for the following reasons:
(a) they are above dust and other types of atmospheric pollution
(b) they are above low cloud, mist and fog
(c) they are far from light pollution of large centres of population
(d) the air is thinner and so there is less atmospheric absorption
(e) there are fewer convection currents in the air so that the image does not suffer so much from image shake
Of course the Hubble Space telescope is in an even better position there is no atmospheric absorption at all in space.


These are usually of one of two types:
(a) equatorial where one axis of the mounting is lined up with the axis of the earth. This type of mounting has the enormous advantage that when you have sighted on a star you only have to move the telescope about one axis in order to follow it across the sky.

(b) altazimuth here the axes are horizontal and vertical. These types of mountings are used by many very large telescopes as they can be computer controlled to move to follow the star, but the telescope must continually move on both axes up or down and across.

Reflecting telescopes ones that use a large mirror as their objective

Advantages of a reflecting telescope:

(a) the glass of the mirror does not have to be perfect throughout only to have a perfect surface
(b) the mirror can be supported across the whole of its back
(c) they do not suffer from the colour defects of chromatic aberration
For these reasons all the really large telescopes in the world today are reflectors. The largest ones have mirrors up to 10m in diameter.

Types of reflecting telescope

The Newtonian reflector has an eyepiece at the side of the tube which makes observing comfortable.
The Cassegrain has an eyepiece below the main mirror which means that much heavy detection equipment can be fitted here.

The secondary mirrors in the telescope tubes do not affect the quality of the image the small amount of light that they interrupt is negligible compared with the total amount received by the main mirror.

All mirrors used in astronomical telescopes are silvered on the front surface, otherwise the light would pass through the glass and both colour distortion and multiple images would result. The metal used is actually aluminium, vaporised onto the surface of the glass in a vacuum. It does not reflect quite so well as silver but is better over the complete range of wavelengths of visible light.

The curvature of the mirrors is usually accurate to within one eighth of a wavelength of green light!
Radio telescopes
Radio astronomy began in 1930 when Karl Jansky detected radio waves coming from a source in the Milky Way in the region of the constellation Sagittarius. In the 1940's Grote Reber made detailed radio maps of the sky and also detected radio emissions from the Sun. Radio telescopes operate at radio wavelengths which are considerably longer than optical wavelengths. For example neutral hydrogen within our galaxy and others emits at a wavelength of 21 cm and radio telescopes are designed to detect this.

This longer operating wavelength has a number of implications for the radio telescope:
(a) at 21 cm the Earth's atmosphere absorbs a different proportion of the radiation
(b) radio telescopes can be used in the daytime
(c) to get good resolving power a radio telescope must be made much bigger than its optical counterpart. In fact to achieve the same resolving power a radio telescope would have to be 50 000 times larger than its optical counterpart.
(d) the surface of a radio telescope does not need to be so 'smooth' as that of an optical mirror. Accuracies of 0.005 m are quite acceptable as against 0.000 000 1 m for the optical mirror surface.

See also: Schmidt telescopes

© Keith Gibbs