Lecture 12: Tools of Astronomy - II
Astronomy 101/103
Terry Herter, Cornell University
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Lecture
Topics
  • Interferometers
  • Where to put your telescope
  • Telescopes around and above the world

Interferometers
  • Interferometry synthesizes a larger diameter telescope with a set of smaller telescopes spaced a large distance apart.
  • Achieves high angular resolution roughly equal to the largest telescope spacing.

Schematic Interferometer

Simple Interferometer Picture


Schematic Radio Telescope

A schematic view of a radio telescope is shown below. Key component are the primary mirror (or dish), the secondary mirror, and the receiver.

Simple Picture of a Radio Telescope

An operating radio telescope is shown below.

This is an 25-m, 240 ton radio telescope located in Fort Davis, Texas. It is part of the VLBA network (see below). You can find pictures of all the VLBA antennas which are located across the United States at the VLBA website.

Features of Interferometers
  • Advantage: Cheap to build compared to a single large telescope.
  • Disadvantage: Most photons hit the ground between the dishes.
  • Thus interferometers give excellent angular resolution but are much less sensitive than a single "filled aperture" telescope would be.

Radio Interferometers

Most interferometry is done in the radio.

The Very Large Array (VLA)
  • 27 radio dishes, each 25 m in diameter spaced in a Y pattern which is 20 km along each leg.
  • Simulates a 40 km diameter telescope
  • At 2 cm the angular resolution is about 0.1"
    • Could see a dime in Elmira!

Above is a picture of the VLA. This picture and others can be found at the VLA web site. Click on the "What is the VLA?" link and go to photographs of the VLA.

The Very Long Baseline Array (VLBA)

  • 10 radio dishes, 25 m in diameter located across the world from Hawaii to the Virgin Islands.
  • The baseline is about 8,000 km.
  • The resolution at 2 cm is ~0.0003" !!
    • Could "split a hair" in Elmira!

Telescope Summary
  • Reflecting telescopes are the best.
  • Larger telescopes collect more photons => larger is better.
  • Angular resolution: theta ~ lambda / D
    => larger telescopes are better
  • Interferometry allows a large aperture to be simulated.

Below is a list of some telescopes from around the word. You can click on the names to get to an observatory web page. Most have public informtion areas.

Name
Size (m)
Wavelength Measured
Location
Palomar*
5
Visible/IR
S. Calif.
MMT
6.5
Visible/IR
Arizona
Gemini
2 x 8.1
Visible/IR
Arizona/Chile
VLT
4 x 8.2
Visible/IR
Paranal, Chile
Keck
10
Visibl/IR
Hawaii
AST/RO
1.7
Submm Radio
Antarctica
JCMT
15
Submm Radio
Hawaii
IRAM
30
mm Radio
Spain
Nobeyama
45
mm Radio
Japan
Effelsberg
100
cm Radio
Germany
GBT
100
cm Radio
Greenbank, WV
Arecibo*
300
cm Radio
Puerto Rico
VLA
27, 25
cm Radio
N. Mex.
VLBA
10, 25
cm Radio
N. Hem.
*Cornell affiliation

Telescope
location

Where To Put Your Telescope

  • Everyone (astronomers anyway) wants one, but not all locations are equal.
  • Put on barren mountain tops
    • For best "seeing"
  • Keep away from big cities
    • Avoid light pollution
  • But not always enough!
    • -- Ain't no mountain high enough!


Transparency
of the
Earth's
Atmosphere
Simple Interferometer Picture


Wavelengths
of
Light

Why Do We Care to Observe at all These Wavelengths?

At different wavelengths, we see different objects:

Wavelength
Characteristic Object
Gamma-Ray
Compact object which collapsed
X-Rays
Neutron stars
Ultraviolet
Hot stars, quasars
Visible
Stars
Infrared
Red giant stars, galactic nuclei
Far-IR
Protostars, dust, planets
Millimeter
Cold dust, molecular clouds
cm Radio
HI 21-cm line, pulsars

 

Getting Rid of the Atmosphere
  • On the surface of the Earth, only a few parts of the spectrum are transparent (mostly in the visible and mm-cm radio). The rest are opaque.
  • In the visible, we are plagued by the turbulence in the atmosphere -- seeing.
  • Put telescopes above the atmosphere!
    • Eliminates "seeing" as a problem.
    • Gets above the absorbing atmosphere.
  • Possibilities
    • Airborne Observatories
    • Balloons
    • Spacebased observatories

Space --
The New
Frontier

Some Observatories and Wavelength Bands

  • Past
    • Einstein: X-ray
    • IRAS (Infrared Astronomical Satellite)
    • GRO: Gamma Ray Observatory
    • ISO: Infrared Space Observatory
  • Present
    • Hubble Space Telescope: UV, Optical
    • Chandra (formerly called AXAF: Advanced X-ray Facility)
    • Spitzer (formerly called SIRTF: Space Infrared Telescope Facility)
  • Future:
    • JWST: Next Generation Space Telescope (2006?)
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