Astro-Physics

Question 1

Give two equations for magnification of a refracting telescope.

Answer 1

1) M = α / β
2) M = Fo / Fe (Focal Objective / Focal Eyepiece)

Question 2

Define 1 arc minute.

Answer 2

1/60 of a degree

Question 3

What is 1 degree in arc minutes

Answer 3

60 arc minutes

Question 4

What’s the equation for angular size of an object

Answer 4

Diameter of object / Distance to eye or lense

Question 5

List two “aberrations” of refracting telescopes

Answer 5

1) Chromatic Aberration
2) Spherical Aberration

Question 6

What is meant by Chromatic Aberration

Answer 6

The refractive index of a lens is different for different wavelengths so different colours are brought to focus at different points. SHORTER WAVELENGTH => SMALLER FOCAL LENGTH

Question 7

What is meant by Spherical Aberration

Answer 7

Due to manufacturing difficulties, imperfections in a lens cause rays further from the principle axis to be brought to focus closer to the lens.

Question 8

What criteria must be met for a a refracting telescope to be in “normal adjustment”

Answer 8

• Objective and eyepiece lenses share a principle focus (separation = Fo + Fe)
• Must be focused at infinity (meaning everything in frame is in focus)

Question 9

What is meant by the term Collecting power of a telescope

Answer 9

How much light enters the telescope hence determining the brightness of the observed image

Question 10

What is the relationship between collecting power and telescope aperture

Answer 10

Collecting power is directly proportional to (Diameter of Aperture)^2

Question 11

What is meant by the term Resolving Power of a telescope

Answer 11

How well the telescope can distinguish between two nearby objects

Question 12

What is an airy disc

Answer 12

It’s a circular diffraction patter around stars caused by diffraction at the telescopes aperture.

Question 13

What is the Rayleigh Criterion

Answer 13

The minimum angular separation for two object to be resolved

Question 14

When will the minimum angular separation for the Rayleigh Criterion occour

Answer 14

When the central maximum of one airy disc coincides with the first minimum of the second airy disc.

Question 15

List 4 disadvantages of refracting telescopes

Answer 15

• Very difficult to create large lenses
• Large lenses can deform under their own weight
• Very difficult to manoeuvre quickly
• Suffer from chromatic and spherical aberration

Question 16

List 4 advantages of reflecting telescopes

Answer 16

• Can be made of smaller composite mirrors
• Large single mirrors can be made fairly easily
• No chromatic or spherical aberration when using parabolic mirrors
• Light weight allows quick response to astronomical events