Stars & EA 7

Question 1

Why do we use telescopes?

Answer 1

• They have a bigger collecting area than the eye; so we can study fainter sources
• They increase the apparent angular diameters; we get better detail and obtain accurate positions.

Question 2

What are the two types of telescope?

Answer 2

• Reflector

- Refractor

Question 3

Describe refractor telescopes.

Answer 3

They have an objective lens and an eyepiece. The largest that is now obsolete is 1.0m Yerkes (1888).
A diagram describing how it functions would be useful here, treat yo self to some paper and a pen!! Remember to include:
objective; focal plane; eyepiece; eyepiece focal length; effective focal length.

Question 4

Describe 4 reflector designs.

Answer 4

Newtonian focus
Prime focus
Cassegrain focus
Nasmyth focus
Could you draw diagrams of them?

Question 5

Describe the telescope at St Andrews.

Answer 5

A Schmidt has a combination of lens and mirror.
The James Gregory telescope at St Andrews is a hybrid between the Schmidt and Cassegrain design.
Draw a schematic diagram of the telescope.

Question 6

What is aperture?

Answer 6

The aperture is the space through which the light passes in an optical instrument.
The light collecting area can be described as (pi)D^2
D = diameter of the hole/aperture.

Question 7

How is the magnitude limit of the aperture described? And what does this mean for a 1m telescope?

Answer 7

The magnitude limit is:
m = 6mag + 5log(D/0.006m)
For a 1m telescope this means it can see 11 magnitudes fainter than the eye.

Question 8

What is the focal ratio?

Answer 8

The focal ratio tells you how fast the beam converges on the focal plane.
f-ratio = fe/D
fe = (effective focal length)

A diagram is the most effective way to explain this Jen, and I’m pretty sure you’ve been ignoring all the cards that say draw a diagram so far; so you should go back and draw some diagrams.

Question 9

Pros and Cons of a small focal ratio.

Answer 9

AKA : ‘fast’
Pros : Good for surveys because it gives wide-field images
Cons : Small image scale; arcsec per pixel

Question 10

Pros and Cons of a large focal ratio

Answer 10

AKA : ‘slow’
Pros : Good for more details for extended sources as it gives small images
Cons : large image scale

Question 11

What is resolving power?

Answer 11

This tells you how much detail can be seen

Question 12

Explain resolution.

Answer 12

Resolution is the minimum angular separation of two sources on a sky so that the sources can be seen as two separate sources.

Question 13

What are the limits of resolution?

Answer 13

• Diffraction limit : caused by the diffraction of light around the edge of the optics; 0.01-1” for typical astronomical telescopes
• Atmospheric ‘seeing’ : caused by disturbances in the atmosphere of the Earth; 0.5-1” for typical sites.

Question 14

Describe the diffraction limit using maths!!!

Answer 14

I may be incorrect o this so maybe look it up as well.
Rayleigh’s criterion gives angular resolution.
(alpha) = 1.22 (lambda)/D radians which is around 2.5x10^5 (lambda)/D arcsec

D is the diamter of the aperture or main mirror of the telescope at optical wavelengths e.g. (lambda) = 500nm
(alpha) = 1 arcsec for D = 0.15m

This means (alpha) is around 0.03 arcsec for a 4m telescope.