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:
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 earths atmosphere @
500km therefore no atmospheric distortion.
Your head is in the way in front of the image.
Two arrangements have been devised to combat this:
fig 4.15 b and c
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.
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!
Cross section of glass mirror