Lecture 15 - Interferometry (11/3/98)
Telescopes --- | ---
Earth
Reading:
Chapter 9 (ZG4)
The Very Large Array, 27 telescopes of 25m diameter that
can be moved out to spacings of 45km! (courtesy NRAO)
 |
Key Question: |
How do you combine two telescopes to
function as a single telescope?
|
|---|
 |
Key Principle: |
Interference and Diffraction
|
|---|
 |
Key Problem: |
What is the resolution of a VLBI
array the size of the Earth at 6cm wavelength?
|
|---|
 |
Key Quote: |
|
|---|
Investigations:
- Resolution and Diffraction contd.
- What is the significance of the aperture plane and
image plane in an optical system?
- For two points along the aperture plane at separatipn D/2, what is the
condition for destructive interference for wavelength lambda and
small inclination angle theta?
- What is the integral of the wavefronts along the one-dimensional
aperture of diameter D?
- What do we mean when we say the resolution of a telescope is
lambda/D?
- What limits the resolution of large telescopes from the ground
to 1 arcsecond or so in the optical?
- Observatories
- Why are optical observatories placed on mountaintops?
- How does the atmosphere limit the resolution of a telescope?
- What is the novel design of the 10-meter Keck telescopes?
- How does the Arecibo telescope manage a 300-meter diameter?
- Why must X-ray, Gamma-Ray, Infrared, and Ultraviolet telescopes
work from space or very high in the atmosphere?
- Radio Telescopes
- How do you make a radio telescope?
- What is the resolution of a 300m radio telescope at a wavelength
of 6 cm?
- How large a diameter is needed to obtain a resolution of 0.4 arcsec?
- What kinds of detectors are used at the focus of a radio telescope?
- Interferometry
- What is the relation between diffraction and image formation
in a telescope?
- What happens to the image if you block part of a telescope aperture?
- What is the resolution of a two-element interferometer of
baseline D?
- What does it mean to correlate the signals from two telescopes?
- How many baselines are there in an N-element interferometer array?
- What is the Very Large Array (VLA)?
- What is the resolution of the VLA with a maximum spacing of 36 km
at a wavelength of 6 cm?
- What is the equivalent diameter single dish with the same sensitivity
as the 27 VLA antennas of 25m diameter?
- How does Very Long Baseline Interferometry work?
- Why is optical interferometry so difficult, yet so desirable?
Interferometry:
Interference of wavefront at telescope causes blurring (resolution):
Interference between wavefronts received at separate telescopes makes
an interferometer work:
Some Tours of Observatories:
Just to remind you to look at these pages...
Take a guided
photographic tour of observatories I have visited.
For some images and links to observatories of the world, see this
Special Lecture.
Also, for more on interferometry with the VLA and VLBA, see the
National Radio Astronomy Observatory Home Page.
Prev Lecture ---
Next Lecture ---
Astr11 Index ---
Astr11 Home
smyers@nrao.edu
Steven T. Myers