Chapter 1 Telescopes

Question 1

What are the two types of lenses?

Answer 1

Converging Lens & Diverging lens

Question 2

What does the Converging Lens do?

Answer 2

A converging lens makes parallel rays converge to a focus. The point where rays parallel to the principal axis are focused to is called the principal focus or the focal point of the lens

Question 3

What does the Diverging Lens do?

Answer 3

A diverging lens makes parallel rays diverge. The point where the rays appear to come from is the principal focus or focal point of this type of lens.

Question 4

what is the focal length?

Answer 4

The focal length is the distance from the center of the lens to the principal focus

Question 5

What is a Real Image?

Answer 5

A Real Image is an image which is formed from a converging lens when an object forms where the light rays from the lens meet.

Question 6

What is a virtual image?

Answer 6

A virtual Image is an image that is formed from a converging lens when an object forms where the light rays appear to come from.

Question 7

How do you draw a ray diagram

Answer 7

Ray 1 must go horizontal until it reaches the lens, then it must be refracted through the focal point.
Ray 2 must go through the center of the lens and continue going straight.
Ray 3 must go through F and continue until it reaches the lens, then it will refract and travel horizontally (opposite of Ray 1).
The image appears where the three lines cross.

Question 8

What makes a refracting telescope

Answer 8

Two converging lenses of different focal lengths, with the one with the longer focal length known as the objective lens, while the one with the shorter one being the eye piece

Question 9

How do refracting telescopes work

Answer 9

Refracting telescopes work by having two lenses work in tandem, the objective lens (the lens with the large focal length) first focuses the image which allows the light rays to travel through the telescope until they reach the second converging lens which enlarges the image.

Question 10

What is collecting power

Answer 10

Collecting power is the amount of ligt a telescope collects, and is proportional to the square of the objective diameter

Question 11

What is a Cassegrain reflecting Telescope

Answer 11

A concave mirror instead of a converging lens is used as the objective of a reflecting telescope, with the concave mirror having a shape such that parallel rays directed at it are reflected and focused to a point by the mirror, with this point being known as the principal focus. near this point will be a secondary convex mirror, which is used to focus the light onto a small hole at the center of the concave mirror, with the light passing through this hole reaching the eyepiece and being visible to the user

Question 12

What is the angular magnification of a cassegrain reflecting telescope

Answer 12

the angular magnification of a cassegrain telescope can be obtained by deviding the focal length of the objective by the focal length of the eyepiece

Question 13

Why shoud the primary mirror of a Cassegrain Reflecting Telescope be parabollic and not spherical

Answer 13

to reduce Spherical Abberation, if the primary mirror is spherical the light rays can reflect to different positions, making the image losing focus.

Question 14

What are the differences between Reflecting Telescopes and Refracting telescopes

Answer 14

Reflecting telescopes are shorter than refracting telescopes with the same angular magnification.
Reflecting telescopes also have greater angular magnification than refractors of the same length and therefore produce greater magnification of distant objects such as the Moon and the planets.

Question 15

What does a refracting telescope do which a reflecting telescope doesn’t

Answer 15

Refracting telescopes use lenses only and do not contain secondary mirrors and supporting frames which would otherwise block out some of the light of the object.
Refracting telescopes also have a wider field of view than reflectors of the same length, so they are better to locate distant objects with rather than the much more focused reflecting telescope

Question 16

what is the minimum angular resolution

Answer 16

The minimum angular resolution (sometimes called resolving power) is used to describe the quality of a telescope in terms of the minimum angular separation is can achieve.

Question 17

What is a CCD

Answer 17

A CCD (Charge-coupled device) is an array of light-sensitive pixels which become charged when exposed to light. after being exposed for a pre-set amount of time, the array is connected to an electronic circuit which transfers the data about the proportion of charges to a screen, allowing the image to be seen

Question 18

what is the Quantum Efficiency of a CCD when compared to an eye

Answer 18

The quantum efficiency of a CCD is around 70%, while the quantum efficiency of an eye is around 1-2%

Question 19

What is Quantum Efficiency

Answer 19

Quantum efficiency is the percentage of incident photons that can liberate an electron

Question 20

advantages of a CCD

Answer 20

Can be used to record changes of an image
It is much more sensitive than the human eye, being able to see light from wavelength between 100nm to 1100nm whereas the human eye can only see light from 350nm to 650nm
Much higher quantum efficiency than that of a Human Eye

Question 21

Where are Radio Telescopes stationed

Answer 21

Radio Telescopes can be stationed anywhere on Earth, and their dish can be rotated to make up for Earth’s rotation, meaning that they can track one section of space to get a prolonged reading.

Question 22

How is the Collecting area of a Radio Telescope determined

Answer 22

(piD^2)/4, where D is the diameter of the dish of the radio telescope

Question 23

How is the minimum angular resolution of a Radio Telescope calculated

Answer 23

λ/D - Where λ is the wavelength of the radio waves and D is the Diameter of the dish of the radio telescope

Question 24

What are Infrared Telescopes

Answer 24

Infrared telescopes are telescopes that have a large concave reflector which focuses infrared radiation onto an infrared detector at the focal point of the reflector, because of this, they can detect objects which cannot be seen using optical telescopes

Question 25

Where are Infrared Telescopes?

Answer 25

Infrared Telescopes can be Ground-based or on satellites, with Infrared Telescopes on satellites being more effective as they are not affected by water vapour like Ground based Infrared telescopes

Question 26

Advantages and Disadvantages of Ground-Based Infrared Telescopes and Infrared Telescopes on Satellites

Answer 26

Ground-Based:
Easy to build and maintain when compared to satellites containing Telescopes
Water vapour absorbs Infrared Radiation, so need to be stationed at places with a very dry atmosphere while also being high-up.

Satellite-Based:
No need to worry about Water vapour absorbing Infrared Radiation
Needs to be at a few degrees above zero to detect infrared radiation from weak infrared sources
Difficult to maintain due to the difficulties of getting someone up there

Question 27

Where are Ultraviolet Telescopes situated

Answer 27

Ultraviolet Telescopes are always carried on Satellites

Question 28

Why are Ultraviolet Telescopes on Satellites

Answer 28

Ultraviolet telescopes are on satellites due to the Earth’s Atmosphere absorbing Ultraviolet Radiation.

Question 29

What are Ultraviolet Telescopes used to detect

Answer 29

Ultraviolet Telescopes are used to map hot gas clouds near stars, as well as study hot objects like glowing comets, supernova and quasars

Question 30

Where are X-Ray and Gamma-Ray Telescopes

Answer 30

Both X-Ray telescopes and Gamma-Ray Telescopes are carried on satellites

Question 31

Why are X-Ray and Gamma-Ray telescopes on satellites

Answer 31

They are both on Satellites since the Earth’s Atmosphere also absorbs X-Rays and Gamma-Rays

Question 32

How do X-Ray and Gamma-ray Telescopes Work

Answer 32

X-Ray telescopes work by reflecting X-Rays off highly-polished metal plates at grazing incidence onto a suitable detector

Gamma-Ray Telescopes work by detecting gamma photons as they pass through a detector containing layers of pixels, triggering a signal in each pixel it passes through, which determines the direction of the photon.