Question

1.The Celestar 8 telescope available in the Physics Lab is described on Blackboard. The primary mirror of a Celestar 8 has an aperture of 203 mm, and the secondary mirror that blocks the primary mirror has a diameter of 70 mm. The focal length of the primary mirror is 2032 mm. Using this information calculate the following:

a) [2 marks] The fraction of the incoming light blocked by the secondary mirror.

b) [3 marks] The light gathering power of the Celestar 8 telescope, relative to the human eye with a pupil size of 7 mm, in each of these cases: i) Taking into account that the secondary mirror blocks some of the incoming light. ii) Ignoring the blockage of light by the secondary mirror.

c) [2 marks] The resolution of the telescope under ideal conditions (no air turbulence) measured in seconds of arc. Assume that you are observing with the visible wavelength of 550 nm.

d) [2 marks] The diameter, in metres, of the smallest crater that you can observe on the Moon’s surface with the Celestar 8 telescope, assuming ideal seeing conditions (no atmospheric turbulence), visible light with wavelength 550 nm, and the distance to the Moon found on the course formula sheet.

e) [1 mark] The magnification of the Celestar 8 telescope if a 15 mm focal length eyepiece is used.

Answer 1

a) The fraction of incoming light blocked by the secondary mirror can be calculated as ration of their respective diameters, hence,

  

Secondary mirror will block the 0.34 fraction of incoming light.

b) Light gathering power

(ii) Light gathering power can be calculated as follows (without blockage of light)

Light gathering power of Celestar 8 telescope is 841 times that of human eye.

C) Resolution of the telescope can be calculated as follows,

d) The diameter of smallest crater that you can observe can be calculated as follows,

First you need to calculate the (theta)

By using Rayleigh Criterion

rad

Now, the size of the smallest crater that can be resolved can be calculated as follows,

r = distance to the moon

The moon is approximately 400,000 km = 400,000,000 meters

Hence the diameter of the smallest crater that can be resolved is approximately 1201m.

Hope this helps. All the best!