Telescopes - There are two kinds of telescopes

Document Sample
Telescopes - There are two kinds of telescopes Powered By Docstoc
					Telescopes (cont.)

Review of Last Time


There are two main types of telescopes
 

Refractors use lenses to bring the light to a focus Reflectors use mirrors to do this

Anatomy of a Telescope

1. 2. 3. 4. 5.

Several important parts of a telescope are:
The aperture The primary The secondary The eyepiece The optical tube The finder The detector

6. 7.

Refracting and Reflecting Telescopes

Telescope Properties


Astronomers are most concerned with a telescope’s collecting area




A large collecting area collects more photons from faint sources Just like a large net can catch more fish

Telescope Properties


Resolution is another important property for astronomers
 



Measure resolution in terms of angular size Resolution of a telescope defines the smallest angle that we can see clearly Trees on a mountain

Telescope Properties


There is a natural limit to the resolution of a telescope

1.22   R D



We can’t do better than this, and many telescopes do worse

Telescope Properties


The atmosphere also distorts images, decreasing resolution



Solution is to go UP

Telescope Properties


Another good reason to go above the atmosphere is because many wavelengths of light are blocked before they reach Earth’s surface

Telescope Properties

This Time


Continue talking about telescopes
  

More properties Some famous telescopes Detectors



Then it is on to stars…

Magnification


Astronomer’s do like magnification, too




But note that it does not matter how much you magnify something…if you cannot resolve it, magnification does you no good Think of a pixelated image

Pixelation

Magnification


With that in mind, here is the jist on magnification




Magnification is defined as the increase in angular size The formula to calculate magnification is

fT M  fE

Magnification
 

fT is the focal length of the telescope fE is the focal length of the eyepiece

What will give you a larger magnification?
1.

2.

A short eyepiece focal length A long eyepiece focal length

50%

50%

a. ..

fo c

ec e

ye pi

or te

sh

0 of 5

A

A

lo

ng

ey e

pi ec

e

fo

ca .

..

Magnification


A longer focal length for the telescope, or shorter focal length for the eyepiece, means a larger magnification

Cost






Cost is of course a practical concern, but it needs to be taken into account Telescopes can cost as much as hundreds of millions of dollars, and guess who pays for most of it… Space telescopes are especially expensive, so we need to make compromises

Nickels and Dimes…


Since it costs lots of money to put a telescope in space, space telescopes are usually smaller


No atmosphere, but less collecting area, lower limiting resolution



There are also lots of practical concerns

Precision Instruments




If astronomers want the best possible data, their telescopes have to be very, very good Consider that in order to get a clear image, your mirrors or lenses need to be perfect on scales of about the wavelength of light you are looking at


This can be nanometers

Some Famous Telescopes

Some Famous Telescopes

Some Famous Telescopes

Some Famous Telescopes

Some Famous Telescopes

Some Famous Telescopes

Some Famous Telescopes

Some Famous Telescopes

Some Famous Telescopes

Some Famous Telescopes

Local Telescopes

Local Telescopes

Detectors
 



So, we collected all our light…now what? It doesn’t do us any good if we can’t see the light Of course, we always have our eyes, but…

Do you think that you would ever see an image like this one if you looked through a telescope using only your eyes?
1. 2.

Yes No

50%

50%

s

Ye

0 of 5

N

o

Sorry




I hate to tell you, but the answer is definitely no Why not?




Well, remember, most things we look at through a telescope are really faint Even though we collect more photons with telescopes, we need to collect them for long periods of time to make images like those

More Reasons Your Eyes Aren’t Good




 

Our eyes are also only sensitive to visible light They are subjective tools They cannot store images long term They are horribly inefficient

Photographic Plates




When the photograph was invented, it revolutionized astronomy You could expose of long periods of time and have a permanent record

Photographic Plates




  

But photographic plates have lots of shortcomings They over expose easily They have a non-linear response You cannot actually count photons They are not very efficient

CCDs
  

CCDs revolutionized astronomy again CCD stands for charge coupled device This is the same technology at use in digital cameras

CCDs

CCDs

CCDs


CCDs are great because
 

 

They are very efficient They allow you to take digital data…analyze on computer They have a linear response They have a wide dynamic range

CCDs




CCDs are by far the most common detector in astronomy Although some others exist, it is not worth talking about them here

Spectrographs
 



We don’t always want to make an image Sometimes, we want to split the light into its spectrum We use spectrographs for this

Spectrographs


There are two basic types of spectrographs
 

Prisms Gratings



Combining the two, we get Grisms

Prisms

Prisms




Prisms work because light of different wavelengths takes a slightly different path Comes out at a different place, and is thus spread out

Gratings

Gratings




Gratings are made up of hundreds or thousands of tiny grooves They use a phenomenon of light known as diffraction to split the light

Grisms
 

Grisms use both effects Gratings and grisms are the most commonly used spectrographs in astronomy

Spectrographs




Once we split the light by wavelength, we usually use a specially designed CCD to actually measure the photons Since we splitting the light up and smearing it, spectroscopy needs lots of photons, i.e. a bright source

The Stars Above

What They Are and How They Live

A Seemingly Simple Question




We are now going to switch gears and start to actually talk about objects in space My first question is, “What is a star?”

A Star Is…


Here is my definition:


An object composed primarily of gas and plasma that supports itself against gravitational collapse through outward pressure supplied by selfsustaining fusion

A Star Is…


Let’s take that statement piece by piece


Composed primarily of gas and plasma – not made of solids or liquids


Plasma is gas that is completely ionized



Gravitational collapse – the star’s own selfgravity is squeezing it, trying to crunch it into a little ball

A Star Is…




Supports itself against collapse – something is stopping the collapse…the star is stable Self-sustaining fusion – the pressure that keeps the star stable comes from fusion
 

This fusion occurs naturally in stars We will see why shortly…

Stated Another Way


A star is a stable ball of gas that stops the crunch of gravity by generating fusion

Fusion
  



Fusion is what powers the stars Without it, there would be no life on Earth There would not even be an Earth So, yeah, this is pretty important stuff

What is Fusion?
1.

2.

3.

le i

an d

le us

nu c

at te r

nu c

tw o

a

he n

he n

he n

W

W

0 of 5

Th

W

e

po

w er

m

a

of

fiv

e

4.

When a nucleus splits apart to make two new nucleus When a two nuclei come together to make a new nucleus When matter and antimatter collide to create energy The power of five blades

25% 25% 25% 25%

lit ...

an t.. .

..

sp

bl ad

co

es

Fusion


Fusion is the process by which two nuclei come together to make a new nucleus

How Does Fusion Work?




We want to understand how fusion can generate energy It is related to the concept of binding energy

Binding Energy


It takes a certain amount of energy to hold anything together


We call this energy, binding energy



Think of a spring


To keep it compressed, you would have to exert some energy

Binding Energy
 

Atomic nuclei are no different Some nuclei are more tightly bound than others

Binding Energy


Consider a baseball held at 100 miles above the Earth
 

It is not very tightly bound to the Earth If we drop the baseball to the Earth’s surface, it speeds up, gaining kinetic energy

Binding Energy


When the baseball finally stops, it has released a good bit of kinetic energy Not as tightly bound as one Helium nucleus



Now consider 2 Deuterium nuclei




When the Deuterium fuses to form Helium, it releases energy just like our baseball

Mass Into Energy


But where did this energy come from?


Remember E=m*c2



Let’s do some atomic book keeping

Mass Into Energy


How much Hydrogen did we start with?


6.693 x 10-27 kg



How much Helium did we finish with?


6.645 x 10-27 kg



So we lost some mass
6.693 x 10-27 kg – 6.645 x 10-27 kg = 0.048 x 10-27 kg

Mass Into Energy


This missing mass went into energy


E = m*c2

E = 0.048 x 10-27 kg * (3 x 108 m/s)2 = 4.3 x 10-12 J


4.3 x 10-12 Joules is only 1.2 x 10-18 kilowatt hours


Enough to power a 60 Watt light bulb for 0.00007 nanoseconds

This seems pretty wimpy. Is fusion really all that powerful?
1.

2.

Yes…we got this energy from just one reaction. With more reactions we would get more energy No…fusion is not really that powerful at all

50%

50%

en er ..

th is

go t

we

s…

0 of 5

Ye

No

…

fu s

io n

is

no tr ea

l..

Boom Tomorrow…


How much energy would we get by fusing a person sized chunk of Hydrogen?
  

I am about 70 kg That is 5 x 1027 fusion reactions 600 kilotons, or about 40 Hiroshimas

More On Fusion
 

Fusion is extremely powerful As another example, fusing just 1 milligram of Hydrogen would release as much energy as 30 gallons of gas

More On Fusion


It would be a clean, efficient, and practically never-ending source of power


IF we could do it here on Earth in a controlled way

There has never been a fusion reaction on Earth during human existence.
1. 2.

True False

50%

50%

Tr ue

0 of 5

Fa

ls e

Fusion On Earth


We have had fusion reactions




Unfortunately, they were either uncontrolled (Hydrogen bombs) Or they were very short lived (less than a second)



But hopefully in the future

Why Can’t We Do It?
 

Fusion is hard Nuclei are all positively charged, and they repel


A lot


A lot, a lot


No, seriously, I mean A LOT

Why Can’t We Do It?


Two protons repel each other due to their like-charges 1036 more strongly than they are attracted by gravity!!


Gravity is puny!!

The Nuclear Strong Force


But if you can get the nuclei close together, an entirely new force takes over


This is called the nuclear strong force

How Close?


Within 10-15 meters


Strong force is only strong at short distances

So How Does It Work?


How can we get them that close if the electromagnetic force is repelling the nuclei?


If the nuclei have enough energy, they can overcome the repulsion

Like A Ton of Bricks


If you drive fast enough at a brick wall, you can smash through it


Not recomended

Hotttttt


The equivalent of driving fast for particles is being hot




Remember, temperature measures average kinetic energy If the temperature is high enough, particles will have the energy needed to fuse

How hot does it need to be for fusion to occur in the Sun?
1. 2. 3.

4.

6000 Kelvin 1 million Kelvin 15 million Kelvin 100 million Kelvin

25% 25% 25% 25%

vi n

n

n

Ke l

Ke lv i

el vi

K

on

00

on

m ill i

m ill i

15

1

0 of 5

10

0

m ill i

60

on

K

el vi

n

Hotttt




For the Sun, we need a temperature of about 15 million Kelvin Having high densities helps, too


Makes the chances of a collision more likely


				
DOCUMENT INFO
Shared By:
Stats:
views:82
posted:4/17/2008
language:simple
pages:83