## 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.

#### Background:

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