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Astronomical Telescopes
and Our Place in the Universe
A brief history of telescopes, discussing their contribution to our understanding
of the universe and a brief description of plans for the next 20-30 years.
Jon Thaler
University of Illinois
Physics Department
Astronomy Department
Spring 2016
Outline
• In the beginning.
• How telescopes work.
• Science enabled by telescopes.
• The evolution of telescope technology.
° Mirrors vs lenses
° Clock drives
° Photography
° Computers and electronics
° Telescopes in space
° Adaptive optics
• The near future (i.e., 20-30 years).
I will not talk about
gravitational waves, but
° Dark energy and dark matter
can answer questions
° Exoplanets (exolife?)
at the end, if you want.
I will only discuss optical telescopes.
No radio, X-ray, gamma ray, cosmic ray, or neutrino instruments.
In the Beginning
An optics industry developed in the late 16th century, led by the Dutch.
Spectacles (one lens in front of each eye) were used as reading aids.
The invention of the two-lens combination is a bit obscure, but Hans
Lippershey’s binoculars were shown to the Dutch army in 1608. This was
initially treated as a military secret, but the knowledge spread quickly.
Galileo heard about it in May 1609 and immediately made a telescope with
a magnification of x3.
The compound microscope was invented at about the same time by Hans
and Zacharias Jansen (also Dutch). (You should ask for a biology talk.)
The quality of telescopes was limited by the ability to make surfaces with a
known curvature, and by the nonuniform optical properties of the glass.
How Do Lenses Work?
Single lens (spectacles or hand-held magnifier)
image
object
f
f the lens’ focal length.
f
What does magnification mean?
For a telescope the important thing
is the angular size of the image
(how big it appears to be),
compared to that of the object.
Here, magnification ~ 2:
Words
Words
I am stressing angles for a reason:
Telescopes are great at measuring angles,
but terrible at measuring distances.
This problem has plagued astronomy
up to the present day. Determining the
distance to an object requires other
information.
Is the orange word smaller than the
red one, or merely farther away?
Brightness Can Be as Misleading as Size
Which Planet Is Farther Away?
How do telescopes work (1)?
Two lenses (at least) are needed to manipulate images of distant objects.
This is Galileo’s first telescope (1609). The design was copied from Lippershey.
Objective
Eyepiece
object
image
Converging lens
Distances to
image and object
are very large.
f1 and f2 are the lenses’ focal lengths.
The object and image are both at “infinity”.
The magnification, m is the ratio f1/f2.
The image is upright. That’s important for
terrestrial viewing, but not for astronomy.
Galileo was not a professional optician.
He used existing designs.
About 2’ long.
Diverging lens
f1
Objective
f2
Galileo’s first telescope had
a magnification of about 3.
The ones he used to do his
science (1610) had 10-30.
I have two reproductions
of G’s 1610 telescopes.
They are very hard to use.
Eyepiece
How Did the Telescope Help?
The magnification improved his ability
to see fine detail, about 30 times
better than Tycho Brahe (the best
naked-eye astronomer).
The 5 cm aperture enabled observation of
Drawings in Sidereus Nuncius (Starry Messenger)
fainter objects (by about a factor of 300)
than he could see with unaided eyes (2-3 mm).
The moons
He was able to see Jupiter’s moons.
of Jupiter
One undesirable feature:
Galilean telescopes have a very small field of view.
Galileo could see about 1/3 the diameter of the Moon.
This is not good if you want to survey the sky, or
don’t know exactly where to look.
Imagine how tedious it was to make these drawings.
What Galileo Saw
Moons of Jupiter
Ptolemaic Cosmology:
The Earth is not the only center of motion.
Phases of Venus
Venus does not shine by its own light.
It goes around the Sun, not the Earth.
Craters & mountains on the Moon
The Moon is similar to the Earth.
The heavenly realm is not perfect.
Sunspots
The Sun is also imperfect, and is spinning.
This (multiple centers or motion) was
the final proof that Copernicus was right.
The Earth is not the center of the universe.
Most astronomers had already accepted
the Copernican theory.
Epicycle
Venus
Phases and distance
tell us about the orbit.
The Size of the Solar System
The Copernican theory tells us the relative sizes of orbits and objects in the
Solar System, but not the actual distances. Telescopes only measure angles,
so how to obtain a distance? The first methods (Cassini & Richer, 1672)
used parallax:
Cassini, in Paris
7070 km
Animals (even insects)
use motion parallax
to estimate distances
a
Richer, in Cayenne
Mars
(South America)
1 arcsecond =
1
degree
3600
Stellar parallax was not
observable at that time.
Stars are very far away.
The angle, a, is very small, about 25 arcseconds (1% the diameter of the Moon)
At its closest approach, Mars is 55 million km from Earth.
The Solar system is very large !!
The Greeks knew this in the
3rd century BC, but didn’t
have precise numbers.
Telescopic accuracy is required.
Parallax is useful to distances of a few thousand light-years (still within our galaxy).
The Speed of Light
Between 1671 and 1690, Cassini, Römer, & Huygens studied discrepancies in the orbit
of Io. They measured that when the Earth and Jupiter are on opposite sides of the Sun,
the orbit lags by about 22 minutes, compared to when they are on the same side.
Earth
Earth
opposite
side
same
side
Sun
Io
Jupiter
They attributed this to the time it takes light to travel across Earth’s orbit.
Their answer, 132,000 mi/sec was about 30% smaller than the currently accepted value.
(Their time measurements were incorrect.)
Astronomy and physics have a
continuing, close relationship.
Mirrors
A curved mirror can also make images:
A mirror can be thin.
A lens must be thick.
Advantages (compared with lenses):
• No chromatic aberration (rainbow effect).
• Lighter at larger diameters.
Mass of a lens depends on volume (R3).
Large lenses (over 1 meter) sag.
Mass of a mirror depends on area (R2).
Disadvantages (Early mirrors were made of metal):
• Reflectivity was poor (66%, at best)
• The surface corroded fairly quickly.
• The focus is in the path of the incoming light.
Like lenses, the spherical shape is not optimal.
One wants a parabolic mirror.
Because of the disadvantages of metal, lenses were preferred
over mirrors until silvered glass (>90% reflectivity) was invented
in 1856 (by Steinheil and Foucault).
Focus
Yerkes 40” Refractor (the largest).
It’s in Wisconsin; you can visit it.
The First Reflecting Telescopes
To solve the “light path” problem, another mirror was added:
Newton’s solution (1662):
Eyepiece
Cassegrain’s solution (1672):
Eyepiece
or camera
Most large modern telescopes have Cassegrain geometry.
It puts the eyepiece (or camera) at the bottom.
Eyepiece
The Rotation of the Earth
If you’ve ever used a telescope, you’ve noticed the annoying fact that the Earth rotates.
Objects move out of the field of view.
For naked-eye observation, this is merely an annoyance.
For long exposure astrophotography, this is a disaster.
About a 30-minute exposure.
North pole
The Equatorial Mount
How to compensate for the Earth’s rotation:
Mount the telescope on an axle aligned with the Earth’s axis.
Rotate the axle counter to the Earth’s rotation.
This will stabilize the object in the vield of view, enabling more precise viewing.
To the
north pole
91 cm Cassegrain, at Sapporo
Hale 200” Cassegrain reflector on Mt. Palomar
(largest equatorial mount)
west
Counterweight
No telescope “tube”
mirror
camera
Clock Drives
Throughout the 18th century, maintaining proper
telescope pointing was very labor intensive, with the
observer having to regularly instruct an assistant to
move the telescope. Fraunhofer solved the problem
problem in 1825 by the use of a clock drive.
Dorpat 9.5” refractor
Automation improves observing efficiency.
Friedrich von Struve used this
telescope to survey 120,000
stars, including 3,000 double
stars, about four times the
previously known number.
Beginning to map the
structure of the Milky Way.
Accurate telescope drives
are necessary for long
exposure photography
(20 years later).
To North pole
Clock drive
weights
The Altitude-Azimuth Mount
Mount the telescope on two perpendicular axes, one vertical and one horizontal.
This is mechanically much simpler and, for large telescopes, more robust.
However, unlike the equatorial mount, tracking objects requires a computer,
because the motor speeds are constantly changing. (It’s a trigonometry problem.)
Every large telescope built since 1970 uses this mount.
A simple altazimuth mount.
A Newtonian reflector!
The LSST: 8.4 m (under construction)
A different optical design. The camera is up here.
Navigation
A practical spin-off from basic science.
If we can measure the time when a known star passes through
due North (or due South), we can calculate our longitude.
What time does each star
One minute time accuracy leads to ¼° longitude accuracy
(about 15 miles, depending on lattitude).
The first sufficiently accurate (to 5 seconds during a
transatlantic voyage) marine chronometer was tested
by John Harrison in 1761.
Of course, the positions of the stars on the sky must be
accurately measured. This was one of the important tasks
of the Greenwich Observatory (and others).
?
cross this line?
Photography
The advent of photography dramatically increased the capability of telescopes.
Putting a camera at the telescope’s focus produces several benefits:
• Fainter objects can be seen. The human eye integrates light for about 1/20 second,
so, looking at a faint object longer does not significantly improve one’s ability to see it.
• A camera is usually smaller that a person. The secondary mirror isn’t required.
• Photography can be used in the ultraviolet and infrared.
• One can study atomic spectra (which require long exposures).
Modern cosmology would be impossible without photographic techniques.
John Draper took a daguerreotype of the Moon in 1840 (a 20 minute exposure).
This image of the Orion Nebula in 1883 (by Andrew Common,
an amateur!) was the first to show stars too faint to be seen
with the human eye.
This allowed more detailed structure of the Milky Way
(our galaxy) to be mapped.
The Longest Photographic Exposure
In 2003, the Hubble Space Telescope’s ACS camera took a one million second
(about 11 days) exposure. There are very few stars in this photo, and
about 10,000 galaxies, the faintest of which are about a billionth as bright
as can be seen by eye.
We are seeing the light that they
emitted about 13 billion years ago.
(because they are so far away)
The Hubble
Ultra Deep Field
Spectroscopy
One can determine the chemical composition of
astronomical objects by measuring the spectra of
the light they emit.
One can also measure the speed toward or away
from the telescope by observing the spectral
(Doppler) shift.
The shift (to the red) shown here corresponds to
9% the speed of light away from the observer:
Lab
Incandescent
Mercury
Lithium
Cadmium
Strontium
Barium
Calcium
Sodium
Star
Helium
Hydrogen
Spectroscopy requires a lot of light: Long exposures and large telescopes.
In cosmology, one talks about redshift, z, the fractional change of wavelength.
Two Important Discoveries
In 1800, William and Caroline Herschel discovered that sunlight has considerable energy
beyond the red end of the spectrum. This is infrared radiation.
Thermometer becomes hot.
In 1868, Janssen & Lockyear saw an unknown line in the solar spectrum, which Janssen
attributed to a then unknown chemical element. Helium is rare on Earth (not found
until 1895), but is 27% of the Sun.
Spectral analysis (requires photography) has become a key tool in astronomy.
Because different atoms emit different colors, we can determine the chemical
composition of even the most distant objects.
Spectral analysis enables tests of the constancy of the physical laws as the universe evolves.
No evidence of change so far.
Extragalactic Cosmology
The simultaneous measurements of speed (using spectroscopy) and distance (using stars
of known brightness) enabled the beginning of modern cosmology.
• Many nebulas turned out to be “island universes” – other galaxies (i.e., outside the
Milky Way). This was a contentious issue until the 1920’s (partly as a result of some
incorrect measurements).
• Distant galaxies are moving away from us. The speed is proportional to the
distance. This implies that the universe is expanding (a prediction of general relativity).
This led to the concept of a “big bang” (very controversial until the 1960’s).
Hubble’s original data (1929).
2007 data (a compendium)
His distance calibration was off by x 1/7.
1000
km/s
Hubble’s
data is here:
~6.5 Mly
~ 2.8 Gly
An Important Cosmological Concept
Because the speed of light is an invariant (everyone measures the same value),
when we look at distant objects we are seeing them as they were in the past:
1 light-year
= 1 year
1 billion light-years = 1 billion years
That is how we measure the evolution of the universe.
Extrapolating the Hubble expansion backwards in time, we learn that he universe is about
13.7 billion years old. Light can only have traveled 13.7 billion light-years since the
beginning of the expansion.
I will not go into the details of the cosmological time-line. That’s a whole other talk.
Computers (1)
Computers and electronics enable several big advances:
• Bigger telescopes, using altitude-azimuth mounts.
Following the circular path of a star with only horizontal and vertical controls
is like drawing a circle with an Etch-a-Sketch. Computers are good at that.
• Better optics. A mirror or lens surface can be made (at some cost) with an
arbitrary shape.
• Digital photography , using CCDs.
° CCDs are more sensitive than film, and easier to calibrate.
° Images can be transmitted to scientists over the internet.
Some telescopes in Chile send their data to NCSA for analysis.
° Digital images can be analyzed more quickly (by computer!).
• Large data sets
The LSST camera will have 3 gigapixels, and each image will be
6 gigabytes. In 10 years, it will take nearly a million pictures.
The final data set will be ~ 100 petabytes (100 million gigabytes).
About 300 times more than
your 10 Megapixel camera.
Giga = billion
Tera = trillion
Peta = quadrillion
Computers (2)
• Robotic telescopes (no humans involved!)
Example: The PROMPT project (led by UNC), is
used for optical follow-up of gamma-ray bursts
(huge explosions at the centers of galaxies).
These bursts are very short, and a few-second
response time is needed. The internet is required.
° Space-based astronomy.
PROMPT, at Cerro Tololo, Chile
The Hubble Space Telescope
Computers (3)
Adaptive optics:
Undistorted incoming light
Inhomogeneous atmosphere
Looking at stars through the atmosphere is like looking at objects
at the bottom of a swimming pool. If we can quickly measure the
distortion (before the distortion changes), we can correct it.
This requires:
• Powerful computers (1000 measurements per second).
• Flexible mirrors, supported by computer controlled fingers,
to change the shape of the mirror as needed.
One can achieve ideal (diffraction limited) resolution.
Image quality
is crucial for
exoplanet studies.
Distorted light
Computers (4)
• Multiple mirrors. The Thirty Meter Telescope (TMT, construction about to begin)
will have a 30 m mirror that consists of 492 1.5 m hexagonal mirrors. It is not
practical to consider a single 30 m (98 ft) piece of glass.
Alignment of the mirror segments so they function as a single “mirror” is similar to
adaptive optics.
More on TMT science in a minute ...
The Discovery of Dark Energy
Dark energy was discovered in 1998 by observing the apparent luminosity of
“type Ia” supernovas as a function of distance. Distance was measured using the
fact that these supernovas are “standard candles” (all about the same brightness).
The inverse-square law tells us the distance.
The data came from a combination of ground and space-based telescopes. Accurate
measurement of the cosmological parameters requires lots of data, accurate
calibration, and the ability to see in the infrared. The plot shows the accuracy of the
data we will achieve with 4000 supernovas using the Dark Energy Survey (data being
taken now).
Astronomical magnitude
(inverse
brightness)
m
Comparison of supernova brightness
in a universe with dark energy to a
universe with without it.
Measure
the curvature
0.1
Redshift
( a measure of distance)
Dark energy decreases
the brightness
by about 15%.
About
7 billion
years ago
z
A Supernova Video
Data from the Berkeley Supernova Cosmology Project.
The supernova reaches peak brightness in about two weeks,
and fades over several months.
Telescopes in Space
The advantages of putting a telescope in space:
Space
Ground
• Space is almost black, the terrestrial sky is not.
One can see fainter objects.
These two “stars” are the same brightness:
• The atmosphere fuzzes out images.
The best terrestrial “seeing” (without adaptive optics)
is about 5 times worse than Hubble’s image quality.
Here, the ground image (same total brightness) is 40% bigger.
• No gravity.
No distortion of the mirror shape when the telescope points a different direction.
• The atmosphere absorbs ultraviolet and infrared (and is weather dependent).
The ability to observe infrared is important to cosmology (due to the red shift).
The gain from better image quality in space is partially reduced by the fact that
terrestrial telescopes can have larger mirrors.
Adaptive optics also helps to mitigate atmospheric effects.
The Future
Very large terrestrial telescopes (TMT, E-ELT) .
30-40 meter diameter.
They must be multi-mirror. One cannot make a single piece of glass that large.
Adaptive optics yields exquisite resolution in a tiny field of view.
New space telescopes (James Webb Space Telescope)
Six meter diameter. Multi-mirror to fit in a rocket.
Operates in the infrared, where space has a big advantage.
See older objects (higher red shift). Look for the first stars.
Moderate size, wide-field telescopes (LSST)
Eight meter diameter. Field of view 10x Moon’s diameter.
Good, but not exquisite resolution.
Rapid image acquisition. Designed for all-sky surveys and searches for new phenomena,
as well as dark matter and dark energy studies.
Very Large Terrestrial Telescopes
The Thirty-Meter Telescope (TMT) will have a small field of view (1/3 the size of the
Moon, about the same as Galileo’s), but will have diffraction limited (best possible)
image quality and more than a million times the light gathering power.
Planetary imaging:
The angular resolution (with adaptive optics) will be about 10-3 arcsecond:
• About 1 cm when looking from NYC to LA.
• About 100,000 km when looking at nearby
human
stars.
That’s good enough to image large planets
(super-Jupiters) and study planetary
atmospheres.
Look for life!
492 computer controlled
mirror segments.
21 different surfaces.
30 meters
Exoplanet Images
These pictures were taken with 8-meter telescopes (one in Chile and one in Hawaii)
without adaptive optics.
The point is, we can see exoplanets now. We’ll study them with the next generation
of very large telescopes.
Both stars are sun-like and are a few hundred light-years away.
All four planets are super-Jupiters (8 to 70 times as massive). They are quite far from
the star – comparable to or farther than the distance to Pluto.
Very Large Terrestrial Telescopes (2)
Study the first stars and galaxies.
• The first stars appeared less than a billion years after the big bang, so an infrared
camera is needed (due to the large Doppler shift).
• They are also very faint, so a large telescope is needed.
Observations may start in 2024.
Space Telescopes
A new space telescope: James Webb (JWST), viewing in the infrared.
The mirror must be segmented to fold up into the rocket.
Many of the science goals are similar to the TMT. It will launch in 2018.
The End of the Dark Ages: The first stars and galaxies.
Stellar and Planetary Evolution the Origins of Life: The physical and chemical
properties of solar systems (including our own) and where the building blocks of
life may be present.
Mirror mass
is only 700 kg!
6.5 m
2.4 m
Sun shield.
To operate in the infrared, the
entire telescope must be cold.
Wide-Area Survey Telescopes
Rapid all-sky surveys (LSST). They require a large field of view, and automated
operation (an exposure every 17 seconds). Survey the sky every 2-3 days. This enables
the study of rare transient phenomena. Some objectives:
• Near Earth (“killer”) asteroids. A 90% complete survey of everything > 100 m.
LSST
• Milky Way & other galaxy structure.
G
• Transient phenomena (supernovas, gamma ray bursts, active galactic nuclei).
Moon
• Cosmology: Supernovas, galaxy clusters, gravitational lensing.
A large (100 petabyte!) publicly accessible data set, for everyone’s use.
Under construction now; it will start in 2021.
An f1.2 camera!
8.4 m
Extra slides
Problems with Lenses
The index of refraction varies with color (chromatic aberration):
The lens surface needs to be a hyperboloid,
not a sphere (spherical aberration).
A lens at each end
Both can be corrected,
but not with 17th – 18th century technology.
Both effects are smaller with long focal lengths,
leading to very long telescopes !!!
Hevelius’ 150’ telescope (~1670)