Astrophysics - telescopes

Question 1

What is an axial ray?

Answer 1

Ray parallel to the principle axis

Question 2

What does a converging lens do to a beam of axial rays?

Answer 2

Focuses an incident beam onto the principal focus

Question 3

What does a converging lens do to a beam of non-axial rays?

Answer 3

Focuses an incident beam at the focal length (some point along it)

Question 4

What happens to rays that pass through the centre of a lens?

Answer 4

They pass through undeviated

Question 5

What is one astronomical unit?

Answer 5

The earth too sun distance

Question 6

What is the equation for the angle subtended by an object?

Answer 6

α (in radians) = diameter of object / distance between object and observer

(due too the arc length equation r*α = arc length)

Question 7

How many arcseconds are in 1 arcminute, arcminutes in 1 degree and arcseconds in 1 degree?

Answer 7

60’’ = 1’
60’ = 1 degree
3600’’ = 1 degree

Question 8

What is the equation for magnification (angular size)?

Answer 8

M = α / B = f0 / fe
α = angular size viewed through telescope
B = angular size viewed by the naked eye
f0 = focal length of objective lens
fe = focal length of eyepiece

Question 9

What is chromatic aberration?

Answer 9

For non monochromatic light, the different wavelengths all have different refractive indexes and refract a different amount. The longer wavelengths refract more and locus closer too the lens.

Question 10

What is spherical aberration?

Answer 10

The curvature of a lens can cause rays of light at the edge (furthest from principle axis) to be focused closer to the lens than those near the centre, leading to image blurring and distortion.

Question 11

What are the disadvantages of refracting telescopes?

Answer 11

Large magnifications require large objective lenses and very long focal lengths.
Large-diameter lenses are heavy and tend to distort under their own weight as well as hard to manufacture with sufficient clarity and free from defects.
Heavy and difficult to manoeuvre quickly.
Very large.
Suffer from chromatic aberration and spherical aberration.

Question 12

What are the advantages of reflecting telescopes?

Answer 12

Large single mirrors can be made light
Mirror surfaces can be made a few nanometres thick, giving excellent image properties
Mirrors use only the front surface for reflection, so removing many of the problems associated with lenses
No chromatic aberration, and no spherical aberration when using parabolic mirrors
Relatively light mirrors allow rapid response to astronomical events
Smaller segmented mirrors can be used to form a large composite objective mirror