Lecture 8: Telescopes & Space Observatories

Question 1

The Universe looks very different depending on the 1? used to observe it
—> we need to cover as much 2? (EM) spectrum as possible to get the most complete view possible

Answer 1

1) wavelength
2) electromagnetic spectrum

Question 2

Refracting telescope

• A lens is used to 1? the incoming light and focus it at one point (2? point)
• The lens must be 3? so the 4? light rays 5? to a focus
• A 6? telescope consists of a large- 7? objective lens with a long 8? length and a small 9? lens of short focal length.
• The eyepiece lens 10? the image formed by the 11? lens in its focal plane (dashed line)

Answer 2

1)bend
2)focal
3)convex
4)parallel
5)converge
6)refracting
7)diameter
8)focal
9)eyepiece
10)magnifies
11)objective

Question 3

Refracting telescopes: main problems

Chromatic 1? : different colours
do not 2? at the same 3? (same
effect as a 4?)
* Can be solved by using two 5?

Answer 3

1)abbertion
2)focus
3)point
4)prism
5)lenses

Question 4

Refracting telescopes: main problems

Other problems are:
1. The 1? from which the lens is made must be totally free of 2? very challenging from a 3? point of view
2. 3? blocks certain kinds of light. In particular, 4? light is 6? almost completely and visible light is 7? substantially
3. Manufacturing large lenses is impossible, limiting the 8? of the telescopes
4. Because the lens is only supported around its 9? , a large lens tends to 10? and distort under its own weight as it tracks 11? through the night —> bad image 12?

Answer 4

1)glass
2)defects
3)manufacturing
4)glass
5)ultraviolet
6)absorbed
7)dimmed
8)size
9)edges
10)sag
11)distort
12)objects
13)quality

Question 5

Reflecting telescopes

A concave, 1? mirror is used to 2? the incoming light and 3? it in one point
A reflecting telescope consists of one 4? mirror (concave) at the 5? of the telescope and a secondary mirror (6?) that deflects the light rays to 7? side.
* There are several types of 8? telescopes based on the 9? of these two mirrors

Answer 5

1)parabolic
2)reflect
3)focus
4)primary
5)bottom
6)flat
7)one
8)reflecting
9)configuration

Question 6

Reflecting telescope

1? aberration: different
parts of a spherical mirror focus
light onto slightly 2? spots
—> 3? images
* Can be solved by using
4? mirrors or correcting
lenses (not that common

Answer 6

1)spherical
2)different
3)fuzzy
4)parabolic

Question 7

Issues with parabolic mirror

Coma: 1? images far from the 2? of the field of view are 3? looking like 4?
* Also, parabolic mirrors reduce the 5? view (we see less 6? of the sky)

Answer 7

1)star
2)centre
3)elongated
4)teardrops
5)wide - angle
6)portion

Question 8

Properties of telescopes

There are three basic properties that describe the main capabilities of a telescope: what are they?

Answer 8

1.Light gathering power
2. Magnifying power
3. Angular resolution

Question 9

Light gathering power

directly 1? to the 2? of the mirror, A large 3? captures more light, produces 4? images and allows astronomers to detect 5? objects

Answer 9

1)proportional
2)area
3)diameter
4)brighter
5)fainter

Question 10

Magnification

m = fobj/feyepiece
the 1? of an object’s 2) diameter seen through the telescope to its 3) angular diameter

Answer 10

1)ratio
2)angular
3)naked - eye

Question 11

Light waves spread out when
they are confined to a small
area like lenses/mirrors due to
1?
* Two objects 2? to each other
might be seen as one big 3?
object

Answer 11

1)diffraction
2)close
3)fuzzy

Question 12

Angular Resolution:

The angular resolution (or spatial resolution) of a telescope describes its ability to distinguish between 1??????? or 2??????????? (e.g. Sun’s spots)
* θ is measured in arcsec and represents the 3?????????? for which a telescope is capable to 4??????????.
* λ is the wavelength (in meters) at which the telescope works and D the 5???????????? (meters)

Answer 12

1) objects close to each other
2) small features in an object
3) minimum angular separation
4) differntiate between two objects
5) diameter of objective

Question 13

In reality, the atmosphere 1? starlight in 2? directions, making it harder for 3? to distinguish between two close objects.
* To solve this issue, 4? telescopes are built at high 5? where the atmosphere is more 6? or in space (e.g. Hubble).
* Another solution is to use 7? optics on the primary mirror to
correct in real time the 8? fluctuations

Answer 13

1)refracts
2)random
3)telescopes
4)optical
5)altitudes
6)stable
7)adaptive
8)atmosphere

Question 14

Radio telescopes must be really 1?
to have decent spatial 2?
(~300 km to achieve Hubble’s
resolution)

Answer 14

1)big
2)resolution

Question 15

Two 1? 2? 3? can be made to have the same resolution
as a large single dish that has a 4? equal to the 5?
between the two small ones

Answer 15

1)small
2)radio
3)dishes
4)diameter
5)distance

Question 16

The main advantages of radio telescopes over optical telescopes are:
* They can be used 1? 2? 3?
* They can be used on 4? 5?
* They are not affected by 6? 7?
* They can peer into vast regions of space 8? from the
“visible” world
* They are less affected by 9? in the galaxy => we can see
through dusty regions that are 10? from view

Answer 16

1)during
2)the
3)day
4)cloudy
5)nights
6)atmospheric
7)distortion
8)hidden
9)dust
10)obscured