women women pm pm
Page 19
Mount Palomar near San Diego: Mount Palomar near San Diego:
Prime focus cage and an inhabitant Prime focus cage and an inhabitant
• •
NOTE: NOTE:
Smoking and Smoking and
drinking are not drinking are not
permitted in the prime permitted in the prime
focus cage On web focus cage On web
page of Anglo page of Anglo
Australian Telescope Australian Telescope
• •
Until the 1970 Until the 1970
’ ’
s, women s, women
weren weren
’ ’
t permitted t permitted
either either
Page 20
Today Today
’ ’
s reflecting telescopes s reflecting telescopes
• •
Cassegrain Cassegrain
focus: focus:
– –
Light enters from top Light enters from top
– –
Bounces off primary Bounces off primary
mirror mirror
– –
Bounces off Bounces off
secondary mirror secondary mirror
– –
Goes through hole in Goes through hole in
primary mirror to primary mirror to
focus focus
Page 21
Examples of real telescopes Examples of real telescopes
• •
Backyard telescope: Backyard telescope:
– –
3.8 3.8
” ”
diameter refracting lens diameter refracting lens
– –
Costs ~ 300 at Amazon.com Costs ~ 300 at Amazon.com
– –
Completely computerized: it will Completely computerized: it will
find the planets and galaxies for find the planets and galaxies for
you you
Page 22
Largest optical telescopes in world Largest optical telescopes in world
• •
Twin Keck Telescopes on top of Mauna Kea Twin Keck Telescopes on top of Mauna Kea
volcano in Hawaii volcano in Hawaii
Page 23
36 hexagonal segments make up 36 hexagonal segments make up
the full Keck mirror the full Keck mirror
Page 24
Keck Keck
’ ’
s 10-meter diameter mirror is s 10-meter diameter mirror is
made of 36 segments made of 36 segments
Page 25
One Keck segment in storage One Keck segment in storage
Page 26
Future plans are even more Future plans are even more
ambitious ambitious
Thirty Meter Telescope Keck Telescope
Page 27
Future plans are even more Future plans are even more
ambitious ambitious
People
Page 28
Concept of angular resolution Concept of angular resolution
Car Lights Car Lights
Angular resolution Angular resolution
• •
The ability to separate two objects. The ability to separate two objects.
• •
The angle between two objects decreases as your The angle between two objects decreases as your
distance to them increases. distance to them increases.
• •
The smallest angle at which you can distinguish two The smallest angle at which you can distinguish two
objects is your objects is your
angular resolution angular resolution
. .
Page 29
How big is one arc second of How big is one arc second of
angular separation? angular separation?
• •
A full circle on the sky contains 360 degrees A full circle on the sky contains 360 degrees
or 2 or 2
π π
radians radians
– –
Each degree is 60 arc minutes Each degree is 60 arc minutes
– –
Each arc minute is 60 arc seconds Each arc minute is 60 arc seconds
1 arc sec 1 arc min
60 arc sec 1 degree
60 arc min 2 radians
360 degrees
❂
❂
2 60 60 360
radians = 4.8 10
-6
radian = 4.8 µrad 5 µrad
or 1 µrad 0.2 arc sec
Page 30
What does it mean for an object to What does it mean for an object to
“ “
subtend an angle subtend an angle
θ
” ”
? ?
θ θ
is the apparent angular size of the object is the apparent angular size of the object
Your eye A distant
object angle θ
Page 31
“ “
Small angle formula Small angle formula
” ”
• •
sin sin
θ θ
~ ~
θ θ
if if
θ θ
is is
1 1
radian radian
• •
s = d sin s = d sin
θ θ
~ d ~ d
θ θ
• •
Example: how many ar Example: how many ar
c sec does a nickel subtend if it c sec does a nickel subtend if it
is located 2 km away? is located 2 km away?
d s
θ
A dime is about 1 cm across, so s
d 1 cm
2 km 1 km
1000 m 1 m
100 cm =
1 2
10
5
radians 1
➭
rad 10
-6
rad = 5
➭
rad = 1 arc sec
Page 32
Concept Question Concept Question
From Earth, planet A subtends an angle of 5 arc sec, and From Earth, planet A subtends an angle of 5 arc sec, and
planet B subtends an angle of 10 arc sec. If the radius planet B subtends an angle of 10 arc sec. If the radius
of planet A equals the radius of planet B, then of planet A equals the radius of planet B, then
a planet A is twice as big as planet B. a planet A is twice as big as planet B.
b planet A is twice as far as planet B. b planet A is twice as far as planet B.
c planet A is half as far as planet B. c planet A is half as far as planet B.
d planet A and planet B are the same distance. d planet A and planet B are the same distance.
e planet A is five times as far as planet B. e planet A is five times as far as planet B.
Page 33
What do astronomers do with What do astronomers do with
telescopes? telescopes?
• •
Imaging: Imaging:
Taking digital pictures of the sky Taking digital pictures of the sky
• •
Spectroscopy: Spectroscopy:
Breaking light into spectra Breaking light into spectra
• •
Timing: Timing:
Measuring how light output varies with Measuring how light output varies with
time time
Page 34
Imaging Imaging
• •
Filters Filters
are placed in are placed in
front of a camera to front of a camera to
allow only certain allow only certain
colors to be imaged colors to be imaged
• •
Single color images Single color images
are then are then
superimposed to superimposed to
form true color form true color
images. images.
Page 35
How can we see images of How can we see images of
nonvisible nonvisible
light? light?
• •
Electronic detectors such as Electronic detectors such as
CCDs CCDs
can can
record light our eyes cant see record light our eyes cant see
• •
We can then represent the recorded light We can then represent the recorded light
with some kind of color coding, to reveal with some kind of color coding, to reveal
details that would otherwise be invisible to details that would otherwise be invisible to
our eyes our eyes
Page 36
Crab Nebula - supernova remnant Crab Nebula - supernova remnant
where a star blew up 1000 yrs ago where a star blew up 1000 yrs ago
Infra-red light Infra-red light
Visible light Visible light
X-rays X-rays
From above the
atmosphere
Page 37
In principle, larger telescopes In principle, larger telescopes
should give should give
sharper sharper
images images
• •
Concept of Concept of
“ “
diffraction limit diffraction limit
” ”
– –
Smallest angle on sky that a telescope can resolve Smallest angle on sky that a telescope can resolve
– –
Numerically: Numerically:
d
= D
radians
where = wavelength of light, D = telescope diameter in the same units as
diffraction limit = 2.5 10
5
wavelength of light diam of telescope
arc seconds
In same units In same units
Page 38
Image of a point source seen Image of a point source seen
through a circular telescope mirror through a circular telescope mirror
• •
Size of central spot ~ Size of central spot ~
λ λ
D D
Diffraction limit animation Diffraction limit animation
Page 39
Example of diffraction limit Example of diffraction limit
• •
Keck Telescope, visible light Keck Telescope, visible light
• •
BUT BUT
: Turbulence in the Earth : Turbulence in the Earth
’ ’
s atmosphere blurs s atmosphere blurs
images, so even the largest telescopes can images, so even the largest telescopes can
’ ’
t t
“ “
see see
” ”
better than about 1 arc second better than about 1 arc second
– –
A decrease of a factor of 1 0.0125 = 80 in resolution A decrease of a factor of 1 0.0125 = 80 in resolution
telescope diameter D = 10 meters
wavelength of light = 5000 Angstroms = 5
10
-7
meter
diffraction limit = 2.5 10
5
✭ ✮
5 10
-7
10 arc seconds = 0.0125 arc second
Page 40
Images of a bright star, Images of a bright star,
Arcturus Arcturus
Lick Observatory, 1 m telescope
Long exposure image
Short exposure image
Diffraction limit of telescope
Page 41
Snapshots of turbulence, Snapshots of turbulence,
Lick Observatory Lick Observatory
These are all images of a star, taken with very short exposure times 100 milliseconds
Page 42
How to correct for atmospheric blurring How to correct for atmospheric blurring
Measure details Measure details
of blurring from of blurring from
“ “
guide star guide star
” ”
near the object near the object
you want to you want to
observe observe
Calculate on a Calculate on a
computer the computer the
shape to apply shape to apply
to deformable to deformable
mirror to correct mirror to correct
blurring blurring
Light from both guide Light from both guide
star and astronomical star and astronomical
object is reflected object is reflected
from deformable from deformable
mirror; distortions mirror; distortions
are removed are removed
Page 43
Infra-red images of a star, from Lick Infra-red images of a star, from Lick
Observatory adaptive optics system Observatory adaptive optics system
With adaptive optics No adaptive optics
Page 44
Adaptive optics increases peak intensity Adaptive optics increases peak intensity
of a point source of a point source
Lick Observatory,
Near infrared images of a
star
No AO With AO
No AO With AO
Intensity
Page 45
Deformable mirror is small mirror Deformable mirror is small mirror
behind main mirror of telescope behind main mirror of telescope
Page 46
Mirror changes its shape because Mirror changes its shape because
actuators push and pull on it actuators push and pull on it
• •
Actuators are glued to back of thin Actuators are glued to back of thin
glass mirror glass mirror
• •
When you apply a voltage to an When you apply a voltage to an
actuator, it expands or contracts in actuator, it expands or contracts in
length, pushing or pulling on the length, pushing or pulling on the
mirror mirror
Page 47
Neptune in infra-red light, Neptune in infra-red light,
Keck Telescope adaptive optics Keck Telescope adaptive optics
Without adaptive optics With adaptive
optics