Chapter 5
Light and Telescopes
Traditional
Telescopes
The 4-m
Mayall
Telescope at
Kitt Peak
National
Observatory
(Arizona)
Advances in Modern
Telescope Design (I)
Modern computer technology has made possible
significant advances in telescope design:
1. Simpler, stronger mountings (“Alt-azimuth
mountings”) to be controlled by computers
Advances in Modern Telescope
Design (II)
2. Lighter mirrors with lighter support structures, to
be controlled dynamically by computers.
Floppy mirror
Segmented mirror
The Keck Telescopes
The two Keck Telescopes on Mauna Kea, Hawaii.
Each telescope has a mirror diameter of 10
meters.
Examples of Modern
Telescope Design
The Large Binocular
Telescope (LBT)
The Very Lage
Telescope (VLT) in
Chile
The Future of Optical
Telescopes
The Giant Magellan
Telescope (2016)
The European Extremely Large
Telescope (E-ELT): 906 segments in a
42-m mirror!
Adaptive Optics
Computer-controlled mirror support adjusts the mirror
surface (many times per second) to compensate for
distortions by atmospheric turbulence.
Distortions by the atmospheric
turbulence are measured using a
laser beam.
Interferometry
Recall: Resolving power of a telescope depends on
diameter D:
amin = 1.22 l/D.
This holds true even if
the entire surface is
not filled out.
→ Combine the
signals from several
smaller telescopes to
simulate one big
mirror →
Interferometry
What a telescope does
• Typical resolution of eye is about 0.5 minutes
of arc or 30 arc seconds.
• HST has a resolution of 11.6/240 = .048 arc
seconds. With HST we can resolve objects
that are almost 1000 times smaller.
• NOTE the equation used above was given on
page 77 of the textbook.
CCD Imaging
CCD = Charge-coupled device
• More sensitive than
photographic plates
• Data can be read
directly into computer
memory, allowing
easy electronic
manipulations
False-color image to visualize
brightness contours
Negative Images
The galaxy NGC 891 as it
would look to our eyes
(i.e., in real colors and
brightness)
Negative images (sky =
white; stars = black) are
used to enhance
contrasts.
The Spectrograph
Using a prism (or a grating), light can be
split up into different wavelengths
(colors!) to produce a spectrum.
Spectral lines in a spectrum
tell us about the chemical
composition and other
properties of the observed
object.
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.
Radio Maps
In radio maps, the intensity of the
radiation is color-coded:
Red = high intensity;
Violet = low intensity
Just like optical telescopes, radio telescopes
should be built in regions with low average
rainfall and cloud cover, and low radio noise.
Radio Interferometry
Just as for optical
telescopes, the
resolving power of
a radio telescope is
amin = 1.22 l/D.
For radio
telescopes, this is a
big problem: Radio
waves are much
longer than visible
light.
→ Use
interferometry to
improve resolution!
The Very Large Array (VLA): 27 dishes are
combined to simulate a large dish of 36
km in diameter.
The Largest Radio Telescopes
The 100-m Green Bank Telescope in
Green Bank, WVa.
The 300-m telescope in Arecibo,
Puerto Rico.