1. Types of Reflecting Telescopes
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Types of Reflecting Telescopes
Each design incorporates a small mirror just in front of the prime focus to reflect the light to a convenient location for viewing.

2. - most common form of astronomical telescope
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Cassegrain reflector
- most common form of astronomical telescope
- allows room for large instruments

3. Schmidt-Cassegrain focus most common in small telescopes
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Schmidt-Cassegrain focus most common in small telescopes
- uses catadioptrics - combines optical advantages of both lenses and mirrors
- light enters through a thin aspheric Schmidt correcting lens
- focal length increased by the magnification of the correcting lens
- lens carefully matched to the primary concave mirror to correct for spherical aberration
- too slightly curved to introduce serious chromatic aberration
- shorter physical length
- lighter and more compact
- easy to use
Aspheric
lens

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The Keck Telescopes
On Mauna Kea in Hawaii. 36 hexagonal mirrors function as single 10-meter mirror.
- segmented mirrors
- more economical - segments can be made separately
- weighs less
- cools rapidly
- less distortion from uneven expansion and contraction
- optical shape maintained by computer-driven thrusters

5. Two Fundamental Properties of a Telescope
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Two Fundamental Properties of a Telescope
Resolution
smallest angle which can be seen
 = 1.22  / D
The angular resolution of a reflecting telescope is dependent on the diameter of the mirror (D) and the wavelength of the light being viewed (). Called Dawes’ Limit
Light-Collecting Area
think of the telescope as a “photon bucket”
The amount of light that can be collected is dependent on the mirror area A =  (D/2)2
These properties are much more important than magnification which is produced by placing another lens - the eyepiece - at the mirror focus. Astronomers do not look through telescopes with their eyes - a light gathering detector (for instance a camera) records the image which can later on be magnified to any desired size.

6. Angular Resolution The ability to separate two objects.
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Angular Resolution
The ability to separate two objects.
The angle between two objects decreases as your distance to them increases.
The smallest angle at which you can distinguish two objects is your angular resolution.

7. Angular Resolution of Car Lights Animation
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Angular Resolution of Car Lights Animation
The maximum angular resolution attainable by the human eye is about one arcminute - in other words two stars will appear distinct if they are separated by more than one arcminute - remember that Tycho Brahe produced the best naked eye star charts ever - had resolution of one arcminute.

8. Angular Resolution amin Resolving power: Wave nature of light
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Resolving power: Wave nature of light
=> The telescope aperture produces fringe rings that set a limit to the resolution of the telescope.
=> Two point sources are just resolved when the principal diffraction maximum of one image coincides with the first minimum of the other. If the distance is greater, the two points are well resolved and if it is smaller, they are regarded as not resolved.
Resolving power = minimum angular distance amin between two objects that can be separated.
amin
amin = 1.22 (/D)
For optical wavelengths, this gives
amin = 11.6 arcsec / D[cm]

9. Mirror Angular Resolution Animation
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So: the angular resolution/resolving power of a reflecting telescope is dependent on the diameter of its mirror
Mirror Angular Resolution Animation
and the wavelength of the light
Wavelength Effect on Resolution

10. Light Gathering Ability: Size Does Matter
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Light Gathering Ability: Size Does Matter
1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to diameter squared: D
A = (D/2)2

11. Light Collecting Area Animation
PHYS-3380 Astronomy
So: light collecting ability of a reflecting telescope is dependent on the area of the mirror
Light Collecting Area Animation

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Magnifying Power
Magnifying Power = ability of the telescope to make the image appear bigger.
The magnification depends on the ratio of focal lengths of the primary mirror/lens (Fo) and the eyepiece (Fe):
M = Fo/Fe
A larger magnification does not improve the resolving power of the telescope!

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Interferometry
Recall: Resolving power of a telescope depends on diameter D:
amin = 1.22 /D.
This holds true even if not the entire surface is filled out.
Combine the signals from several smaller telescopes to simulate one big mirror  Interferometry

14. Uses of Telescopes Imaging use a camera to take pictures (images)
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Uses of Telescopes
Imaging
use a camera to take pictures (images)
2. Photometry
measure total amount of light from an object
3. Spectroscopy
use a spectrograph to separate the light into its different
wavelengths
4. Timing
measure how the amount of light changes with time (sometimes in a fraction of a second)

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Imaging
In astronomy, filters are usually placed in front of a camera to allow only certain colors to be imaged
Single color images are superimposed to form true color images.

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Spectroscopy
The spectrograph reflects light off a grating: a finely ruled, smooth surface.
Light interferes with itself and disperses into colors.
This spectrum is recorded by a digital detector called a CCD.

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Nonvisible Light
Special detectors/receivers record light invisible to the human eye - gamma rays, x-rays, ultraviolet, infrared, radio waves.
- each type of light can provide information not available from other types.
Digital images are reconstructed using false-color coding so that we can see this light.
Chandra X-ray image of the Center of the Milky Way Galaxy

18. The Crab Nebula Visible Infrared Radio Waves X-rays
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The Crab Nebula
Visible
Infrared
Radio Waves
X-rays

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Atmospheric Effects
Earth’s atmosphere causes problems for astronomers on the ground:
Bad weather makes it impossible to observe the night sky.
Man-made light is reflected by the atmosphere, thus making the night sky brighter.
light pollution
The atmosphere absorbs light - dependent on wavelength
Air turbulence in the atmosphere distorts light.
That is why the stars appear to “twinkle”.
Angular resolution is degraded.

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Light Pollution

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Radio Astronomy
Recall: Radio waves of  ~ 1 cm – 1 m also penetrate the Earth’s atmosphere and can be observed from the ground.

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Radio Telescopes
Large dish focuses the energy of radio waves onto a small receiver (antenna)
Amplified signals are stored in computers and converted into images, spectra, etc.

25. Radio Telescopes The wavelengths of radio waves are long.
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Radio Telescopes
The wavelengths of radio waves are long.
So the dishes which reflect them must be very large to achieve any reasonable angular resolution.
305-meter radio telescope at Arecibo, Puerto Rico

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Radio Interferometry
The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter.
Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very Long Baseline Interferometry)!

27. Most sensitive VLBI array in the world - European VLBI Network (EVN).
PHYS Astronomy
Most sensitive VLBI array in the world - European VLBI Network (EVN).
brings together the largest European radiotelescopes for typically week-long sessions
Very Long Baseline Array (VLBA)
uses ten dedicated, 25-meter telescopes spanning 5351 miles across the United States
the largest VLBI array that operates all year round as both an astronomical and geodesy instrument.
Global VLBI
Combination of the EVN and VLBA
Space Very Long Baseline Interferometry (SVLBI)
dedicated VLBI placed in Earth orbit to provide greatly extended baselines.
HALCA, an 8 meter radio telescope - launched in February made observations until October 2003,
small size of the dish - only very strong radio sources could be observed with
Spektr-R (or RadioAstron) - launched in July 2011.
When Global VLBI combined with one or more space-based VLBI antennas gives resolution of microarcseconds.

28. Science of Radio Astronomy
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Science of Radio Astronomy
Radio astronomy reveals several features, not visible at other wavelengths:
neutral hydrogen clouds (which don’t emit any visible light), containing ~ 90 % of all the atoms in the Universe.
molecules (often located in dense clouds, where visible light is completely absorbed).
- Radio waves penetrate gas and dust clouds, so we can observe regions from which visible light is heavily absorbed.

29. Atmospheric Distortion
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Atmospheric Distortion
The turbulence (ever-changing motion) of the atmosphere causes distortion - twinkling of starlight. Bends light in constantly shifting patterns. Like looking down the road on a hot day and seeing distant cars rippling and distorting. Why best viewing is when it is cold and calm.
Atmospheric Distortion Animation

30. Adaptive Optics (AO) AO mirror off AO mirror on
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Adaptive Optics (AO)
It is possible to “de-twinkle” a star.
The wavefronts of a star’s light rays are deformed by the atmosphere.
By monitoring the distortions of the light from a nearby bright star (or a laser):
a computer can deform the secondary mirror in the opposite way.
the wavefronts, when reflected, are restored to their original state.
Angular resolution improves.
These two stars are separated by 0.38
Without AO, we see only one star.
AO mirror off
AO mirror on

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The Sun 33

34. The Sun’s Energy Source
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The Sun’s Energy Source
The first scientific theories involved chemical reactions or gravitational collapse.
- chemical burning ruled out…it can not account for the Sun’s luminosity
- conversion of gravitational potential energy into heat as the Sun contracts would only keep the Sun shining for 25 million years
late 19th-century geological research indicated the Earth was older than that
Development of nuclear physics led to the correct answer
- the Sun generates energy via nuclear fusion reactions
- Hydrogen is converted into Helium in the Sun’s core
- the mass lost in this conversion is transformed into energy
- the amount of energy is given by Einstein’s equation: E = mc2
- given the Sun’s mass, this will provide enough energy for the Sun to shine for 10 billion years 34

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Striking a Balance
The Sun began as a cloud of gas undergoing gravitational collapse.
- the same heating process, once proposed to power the Sun, did cause the core of the Sun to get hot and dense enough to start nuclear fusion reactions
Once begun, the fusion reactions generated energy which provided an outward pressure.
This pressure perfectly balances the inward force of gravity.
- deep inside the Sun, the pressure is strongest where gravity is strongest
- near the surface, the pressure is weakest where gravity is weakest
This balance is called gravitational equilibrium.
- it causes the Sun’s size to remain stable 35

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One second of output from the Sun (luminosity) would provide power for the human race for the next 500,000 years 36

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Layers of the Sun 37

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Core
T = 1.5 x 107 K; depth = 0 – 0.25 R
Density - up to 150,000 kg/m³ (154 times the density of water on Earth)
Pressure 200 billion times that on the surface of Earth
This is where the Sun’s energy is generated. 38