April 4, 2013, jeb,

Outline

• Astronomical Applications of Spectroscopy
• Spectral Analysis - Chemical ID
• The Message of Starlight
• Telescopes
• Refracting Telescopes
• Reflecting Telescopes
• Types of Reflectors
• Why Do We Want BIG Telescopes?
• Atmospheric Blurring
• Recording Images
• New Telescope Designs
• Radio Astronomy
• Other Astronomies


• Summary

Astronomical Applications of Spectroscopy



By the late 19th century, spectroscopists were making many detailed measurements of the radiation from space, identifying the elements of the stars

Some unfamiliar lines appeared in the solar spectrum

It was assumed this must be due to an unknown element, which was called helium after the Greek word helios meaning Sun.

Nearly 30 years after it was discovered in the Sun's spectrum, helium was discovered on Earth in 1895.

Spectral Analysis -> Chemical ID 



Each chemical element produces its own unique set of spectral lines.

These spectral lines serve as a "fingerprint"
- revealing the elements from which they are emitted, as well as the properties of the emitting object, such as temperature, speed, and density.


A spectrograph is an optical device that records a spectrum. 

Old fashioned spectrograph
• Light dispersed by prism 
• Deficiencies:
  • uneven spread of frequencies
  • uneven absorption of frequencies

Modern Device
• based on diffraction grating
• thousands of closely spaced parallel lines cut into glass
• as light passes through a grating, diffraction causes the different colors to be separated

Chemical ID

The emission spectra of some well known elements.

The visible spectrum of the Sun.

The emission and absorption lines of Sodium.



From the observed spectrum (analyzed with a spectrograph) we can determine the chemical elements involved.


Spectrum of the hot gases in a nearby star-forming region, the Omega Nebula (M17)

Spectroscopic Thermometer
Many properties of the spectra reveal temperature
• peak frequency or wavelength of continuous spectra
• lines
• line intensities
• line widths

Measurement of Radial Velocity
• Doppler effect will shift the spectral lines
  • Example of Doppler shift

Line Broadening
• Line broadening is caused by the environment in which the emission or absorption occurs
  • Example of the broadened line
  • Thermal broadening
  • Rotational broadening
  • Collisional broadening
  • Broadening by magnetic fields (Zeeman effect)

The Message of Starlight

Telescopes

There are two basic categories of optical telescopes
• reflectors
• refractors

Refracting Telescopes

lens concentrates light

Problems:
• Chromatic aberration
  • (focal length depends on wavelength, but can be corrected)
• Hard to Make large and free of defects like bubbles
• Very hard to support :
  • Heavy
  • cannot block light
• Light is absorbed and scattered in glass

Largest refracting telescope was completed in 1897 
Yerkes Observatory, near Chicago 
102 cm (=1.02 meters) diameter  

Reflecting Telescopes




mirror concentrates light 

1) avoids chromatic aberration 
2) easier to mount (lighter and can support from back) 
3) Limited by turbulence and disturbances in air that twinkle stars 

Largest telescopes:


• 5 meter - Mt. Palomar, California
  • built in 1948
    • largest in the world for nearly 30 years
  • illustration
  • photograph
  • photo of Edwin Hubble with telescope
  • photo of Einstein with Edwin Hubble and telescope


8.3 meter - Subaru, Mauna Kea, Hawaii
• 1999 - scientific observation begins


8.4 meters (two) - Large Binocular Telescope (LBT), Mt. Graham, Arizona
• 2005 - scientific observation begins

Multi-mirror telescopes:

10 meter - Keck at Mauna Kea, Hawaii 

• 36, 1.8 m hexagonal mirrors working together
• two such 10 meter telescopes
• operated separately or together

Table of largest telescopes

Hubble Space Telescope
• Placed in orbit by NASA's Space Shuttle Discovery in 1990
• Cassegrain design
• capable of measurements in optical, infrared, and ultraviolet parts of spectrum
• extremely successful scientific program


Best observing conditions in the world: 

Mauna Kea Volcano, Hawaii - 14,000 feet 

Types of Reflectors 

• Newtonian
  • Light directed to eyepiece on side of instrument
  • Particularly popular for smaller telescopes, typically used by amateurs.
• Cassegrain
  • Light is reflected to rear of instrument
  • Useful for bringing light to heavy equipment, such as spectrometer
  • More likely choice for large telescopes, typically used by professionals.

Why Do We Want BIG Telescopes? 

1.) Light-Gathering Power 

• to produce brightest images and to detect faintest stars
• Light-Gathering Power depends on AREA

2.) Resolving Power 

• angular resolution - the ability to distinguish between two adjacent objects in the sky
  • illustration of improving angular resolution
• to sharpen image and see details
  • these images show angular resolutions 10', 1', 5", 1"
• Resolving Power depends on DIAMETER



angular resolution (arc sec) = 0.25 wavelength(microns) / diameter(meters)


Examples: 

200 inch Palomar Observatory 

has twice the resolving power 
has four times the light gathering power 

of the 100 inch Mt. Wilson Observatory 

Atmospheric Blurring

The telescopes on the Earth's surface are limited by the blurring due to atmospheric turbulence
• Seeing - a term used by astronomers to characterize the ease of making observations on a given night
• Seeing disk - circle over which the stars light is spread
• without special techniques, the best achievable angular resolution is about one arc-second
• to minimize the effects of atmospheric blurring, telescopes are usually placed on mountain tops, but there one just achieves the one arc-second resolution on a good night
  • European Southern Observatory in the Andes, Chile


• Hubble Space Telescope (launched in 1990) avoids this problem by getting above the atmosphere
  • 2.4 meter diameter mirror
  • 0.05 arc-second angular resolution

Recording Images

Photography 

Invented during the 19th century and served astronomers well 

Advantage: Long-term storage of information 

However "Old Fashioned" 
e.g. only records 5% of light 

Charged-coupled device 

Silicon device: 

Device mounted at focus of the telescope

Send data directly from CCD to Computer 

"Electronic" image 
Uses (records) as much as 90% of light 


Image Processing
• View of Star Cluster R136
  (a) from the Earth's surface
  (b) from Hubble Space Telescope (HST) in original configuration without processing
  (c) original HST with processing
  (d) repared HST in 1994

Wide Angle Telescopes
• Invented and built in 1930s by Berhard Schmidt
• Schmidt telescope in Chile

New Telescope Designs

• active optics
  • Image is analyzed while light is being collected
    • telescope is adjusted
    • R136 star cluster without and with active optics
• adaptive optics
  • Laser beam probes the atmosphere to allow corrections in the mirror surface thousands of times a second
  • Example of rich star cluster NGC 6934, some 50,000 light years away (resolution = 0.1 arc-seconds)
  • Double star Castor with adaptive optics (resolution = 0.1 arc-seconds)

Radio Astronomy

Radio Astronomy was started with the work of Karl Jansky in 1931
• discovered a radio "hiss" from the center of the Milky Way Galaxy
• Jansky is regarded as the father of Radio Astronomy
Radio technology got big push during Radar work of World War II, and after the war Radio Astronomy developed rapidly

Radio Telescopes: Made from dishes which focus the radio signals on a receiver.

The RESOLVING POWER for radio signals is inherently worse than for visible light due to the much LONGER wavelength.



angular resolution (arc sec) = 0.25 wavelength(microns) / diameter(meters) Examples


To improve resolving power, ASTROPHYSICISTS link multiple dishes to achieve the effect of a larger receiver.

This is called INTERFEROMETRY.


• the Very Large Array (VLA) in New Mexico
  • mounted on railroad tracks for easy repositioning

the VLA, can achieve a resolution better than an optical telescope.
• image of the spiral galaxy M51 taken with the VLA and image taken with the 4 meter Kitt Peak optical telescope

Very-long-baseline interferometry (VLBI)
Link up radiotelescopes on different continents
• Thousands of kilometer separation
• Better resolution than optical (even to 0.001")

Other Astronomies


Optical (visible light) and Radio Astronomy are the oldest and best-established branches of astronomy. However, there is information at all wavelengths of the electromagnetic spectrum.

The other astronomies are:
• Infrared
  • Infrared can penetrate dust
    • images near San Jose, California and of the Orion Nebula
    • infrared image of the Orion region
      • Do you see Betelgeuse?
    • infrared images of the M81 galaxy at 4, 8, and 24 microns, and combined
  • Infrared light is absorbed by water vapor in the atmosphere,
    so infrared telescopes must be
    • raised with a balloon, launched into outer space, or located at very high altitude, such as Mauna Kea
• Ultraviolet (uv) Astronomy
  • UV does not penetrate the atmosphere, and therefore these types of telescopes must go into space
  • UV image of Tycho Supernova Remnant
• High-energy (X-ray or gamma-ray) Astronomy
  • design of X-ray telescope
  • Chandra X-ray Observatory shown in 1998, before launch in 1999
  • x-ray image supernova remnant Cassiopeia A by Chandra
  • the Gamma-Ray Observatory and obtained gamma ray image of the distant galaxy 3C279, known as a blazer

The Milky Way as it appears
• at radio wavelengths,
  infrared wavelengths,
  visible wavelengths,
  X-ray wavelengths,
  and gamma-ray wavelengths

Summary

• Message of Starlight
  • Temperature
  • Composition
  • Turbulence
  • Rotation speed
  • Density
  • Magnetic Field
  • Line-of-sight velocity
• Telescopes
  • Reflecting
  • Refracting
  • Newtonian
  • Cassegrain
• Charge-coupled Devices (CCDs)
• Light gathering power
  • collecting area
• Angular resolution
  • larger telescopes favored
  • limited by "seeing"
• Active and adaptive optics
• Radio telescopes
• Interferometry
• Infrared, ultraviolet, and high-energy telescopes