Download Telescopes

Telescopes
Collecting Light
The simplest means of observing the Universe is
the eye.
The human eye is sensitive to light with a
wavelength of about 400 and 700 nanometers.
In a dark-adapted eye, the iris
is fully open and the pupil has
a diameter of about 7 mm.
pupil
Collecting Light
Limitations of the human eye:
• No ability to build up exposure
(equal to about 1/15th of a second)
• Fixed field of view / magnification
• Small collecting area
• Small wavelength coverage
Collecting Light
Around 1600, it was discovered that glass lenses
could be used to correct human vision.
During the next decade, several lens makers
realized that combinations of lenses could be
used to augment normal vision.
The first telescopes used lenses, but later
telescopes were developed that used mirrors.
Galileo Galilei built a much improved telescope
and used it to survey the night sky.
Collecting Light
A reflecting telescope
A refracting telescope
Collecting Light
Modern, large telescopes are all reflectors:
1. Light traveling through glass is bent
differently depending on wavelength.
2. Glass is not perfectly transparent. Some
light traveling through a lens is absorbed.
3. Large lenses can be very heavy, and can be
supported only at their edges. Large mirrors
can be supported across their back surfaces.
4. Lenses need two optically polished surfaces,
mirrors need only one.
A small reflecting telescope
Newton’s telescope (3.5 inch diameter)
A big reflecting telescope
Mayall 4 meter telescope (Kitt Peak, AZ)
Prime focus
Cassegrain focus
An even bigger reflecting telescope
Large Binocular Telescope (Mt. Graham, AZ)
(Two 8.4 meter mirrors in tandem or solo operation)
A truly huge reflecting telescope
The Thirty Meter Telescope (Planned for 2016)
(492 segments form a 30 meter collecting area)
Collecting Light
The light-collecting power of a telescope equals
the area of its primary mirror or lens.
1-meter
diameter
mirror
10-meter
diameter
mirror
How much more area
does the larger mirror
have over the smaller
mirror?
Recording Light
The recording device may form an image from
the light (a camera) or break it into a spectrum
(a spectrometer):
Recording Light
The modern standard for collecting visible light is
the charge-couple device (CCD).
The CCD chip is made of a silicon wafer that
absorbs photons of light and records the electric
charge in microscopic “wells”.
CCDs can only measure brightness. They do not
measure color directly.
e-
“False color” or “Color composites”
The assignment of color values in an image may
not reflect the wavelengths of the spectral lines
captured in the image.
Image of Eagle Nebula
Red: red sulfur line,
green: red hydrogen line,
blue: green oxygen line
This representation gives
more information about
the nebula than a “true”
color image.
Image credit: T. Rector and B. Wolpa (NOAO)
Image Quality
Angular resolution is the ability of a telescope to
distinguish neighboring objects in the sky as
separate. Here is 4 images of a cluster of stars:
The image on the left has poor “resolution” while
the image on the right as high “resolution”.
Image Quality
Angular resolution is limited by the diameter of
the telescope and the wavelength of the light.
Telescopes with higher resolution:
• Larger diameter
• Observing shorter wavelengths
Going to Mountaintops: Most observatories are
placed at high altitude to minimize the blurring
of the atmosphere.
Large Binocular Telescope atop
Mt. Graham, AZ, 10700 ft. altitude
Cerro Tololo International Observatory,
Chile, 7200 ft. altitude
Kitt Peak National Observatory, AZ,
6900 ft. altitude
Mauna Kea Observatories, Big Island of
Hawaii, 13700 ft. altitude
Going to Space: The primary reason for placing
the Hubble Space Telescope in orbit was to get
above the blurring effect of the atmosphere.
Radio Telescopes
Radio telescopes are like
optical reflecting telescopes:
• wavelengths are longer, so
surface is not polished
• relaxed engineering means
larger telescopes
• larger diameter partially
offsets long wavelength
Design of 100-m Green Bank
Telescope, WV
Largest radio telescope: 300-m dish at Arecibo, PR
Radio Telescopes
Radio telescopes observe the range of frequencies
that pass through Earth’s atmosphere (the “radio
window”).
The longer wavelength of radio waves means that
radio telescopes must be much larger than other
telescopes to achieve good resolution.
But there are advantages in radio astronomy:
• Radio telescopes observe 24 hours a day
• Clouds and rain don’t interfere with observing
• Different frequencies, different spectral lines
Interferometry
• Combine signal collected from widely-spread
telescopes as if they came from a single antenna
• Resolution will be that of antenna whose diameter
equals the separation between dishes
Very Large Array (VLA) in western New Mexico
Interferometry
For example, the Very Long Baseline Array uses 10
radio telescopes spread out across N. America.
Images routinely
achieve resolution
of a few microarcseconds
or
0.00001 arcsecond
Allows parallaxes
of Milky Way and
other galaxies!
Atmospheric Absorption
Most of the electromagnetic spectrum is blocked
by Earth’s atmosphere, only visible light and radio
penetrate all the way to Earth’s surface.
Optical and infrared images of nebulas in Orion.
What are you seeing in each image?
Optical image from NOAO
Infrared image from 2MASS
Optical and x-ray images of the Crab supernova
remnant. What are the temperatures of the gas
emitting visible light and x-rays?
Optical image from ESO VLT
X-ray image from Chandra Observatory
Optical and ultraviolet images of spiral galaxy M81.
What kinds of objects are producing most of the
visible light? The ultraviolet light?
You want to observe the emission lines from cold
clouds of interstellar gas (at about 10 Kelvin).
What part of the spectrum would you observe?
A. gamma rays
B. ultraviolet
C. visible light
D. radio
Where would your telescope need to be?
You want to observe the thermal spectrum of a
newly-born white dwarf (about 100,000 Kelvin).
In which part of the spectrum would the white
dwarf be brightest?
A. gamma rays
B. ultraviolet
C. visible light
D. radio
Where would your telescope need to be?
You want to observe the population of cool G-type
and K-type stars across our Milky Way galaxy.
In which part of the spectrum would these types of
stars be brightest?
A. x-rays
B. ultraviolet
C. visible light
D. infrared
Where would your telescope need to be?
Suppose you wanted to observe the superheated
gas (1 million Kelvin) falling into black holes at the
centers of galaxies.
What part of the spectrum would you observe?
A. x-rays
B. ultraviolet
C. visible light
D. infrared
Where would your telescope need to be?