Week 4 Telescopes, Spectroscopy, Detectors, Stars, HR Diagram

Question 1

what is the telescope magnification equation

Answer 1

M = θ' / θ = fo / fe
θ' is the exit angle
θ is the angle of entry
fo is the focal length of the objective
fe is the focal length of the eye

Question 2

define image scale

Answer 2

size of the image of the sky in the focal plane

Question 3

what is the image scale equation

Answer 3

p = θ / s = 1 / f
θ is the angle separation
s is the distance to the object

Question 4

why is the image scale important

Answer 4

it is important for matching the size of the detector to the area of the sky being measured

Question 5

define the collecting area of a telescope and explain why its important

Answer 5

the collecting area is the area of the mirror or the objective. It is important because it determines how much light is collected

Question 6

what is the focal ratio equation and how does it relate to the image

Answer 6

F = f / D
f is the focal length
D is the aperture
a smaller focal ratio value means a brighter image

Question 7

define the resolution of a telescope

Answer 7

it is the ability of the telescope to separate two closely spaced sources

Question 8

what determines the resolution and how do they differ wrt each other

Answer 8

diffraction limit of the telescope

a larger diffraction limit value means a poorer resolution

Question 9

what is the diffraction limit equation

Answer 9

θ = 1.2λ / D

Question 10

how does the atmosphere cause problems for ideal conditions for telescopes and what are the solutions to mitigate the effect

Answer 10

the atmosphere has pockets of air of different temps that are moved by the wind. These pockets deflect the light in different direcitons causing problems
solutions:
1 Build telescope high up
2 Deform the mirror to compensate

Question 11

define interferometry

Answer 11

combining telescopes together to improve resolution

Question 12

what is the interferometry collecting area equation

Answer 12

A = NπD^2 / 4
N is the number of telescopes
D is the diameter of the telescopes

Question 13

what is the interferometry resolution equation

Answer 13

θ = λ / d

d is the distance between the telescopes

Question 14

what are the two methods to obtain an optical light spectrum

Answer 14

Prism

Grating

Question 15

how does the prism spectrum method work

Answer 15

light enters at some angle to the first surface refracting and exits the second surface where it is refracted again splitting up the different colour wavelengths

Question 16

what is the Snell’s law equation

Answer 16

n = c / v

n is the refraction index

Question 17

how does the grating spectrum method work

Answer 17

shine light at an opaque screen with two holes. These two holes now effectively act as light sources and interfere with each other to create the spectrum

Question 18

what is the wave front interference equation

Answer 18

dsinθ = mλ
d is the separation
m is an integer

Question 19

what is the general name given to modern optical wavelength detectors

Answer 19

charge couple device CCD

Question 20

explain briefly how CCDs work

Answer 20

They use the photoelectric effect

Photons are used to release electrons from a material in order to induce a voltage that is readout

Question 21

what is used to represent the wide range of stars as their different categories easily

Answer 21

Herzsprung-Russel Diagram

HR Diagram

Question 22

describe what the HR diagram looks like

Answer 22

it is split into four sections
white dwarfs, main sequence stars, giants, supergiants
white dwarfs occupy the bottom left of the diagram
main sequence form a downwards diagonal in the middle
giant occupy the area just to the right and slightly above the main sequence
supergiant occupy the top area above all other star types