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Reflecting telescopes

A parabolic mirror reflects and focuses parallel rays to a point.

Copy fig.4.15a P. 51 – Ingham

Reflectors have these advantages over refracting telescopes:

  1. The mirrors can be supported from behind – so can be made larger – improving the resolving power and light gathering power
  2. Since light does not have to pass through the glass of the mirror it can be made slightly less than perfect – therefore cheaper.
  3. No refraction at mirror surface – therefore no chromatic aberration.


 Eg. William Hershel telescope with a 4.2m mirror has light gathering power over 300 000 times that of human eye.  Could detect a candle flame 15 000 miles away.

Hubble space telescope – although main mirror is only 2.4m diameter – above earth’s atmosphere @ 500km – therefore no atmospheric distortion.

Problem:  Your head is in the way in front of the image.  Two arrangements have been devised to combat this: 

Newton and Cassegrain arrangement

Sketch fig 4.15 b and c

Cassegrain system

Question 4.8 P.52 Ingham

a) Silvering of mirror is on top surface – reduces multiple images caused by spurious reflections:  (1) reflections off the surface of the glass and (2) total internal reflection within the glass.

b)  Newtonian focus: for a large telescope the length of the telescope would be possibly much taller than a person – this person would have to find a way of climbing halfway up the telescope to use it!

c) Both Cassegrain and Newtonian focus have a small area that is blocked by the second reflector – for large telescopes this will be proportionally small and therefore not significant


Cross section of glass mirror