Download Lecture 14: Non-Optical Telescopes )( )( 25.0)(" mD m a

Lecture 14: Non-Optical Telescopes
•When light enters a telescope, it is bent slightly:
Wave fronts
Light rays
D
•The angle of bending limits the resolution of the telescope
•This depends on the aperture of the telescope, D=2 x R
Resolving Power
•The angle of bending limits the resolution of the telescope
•This depends on the aperture of the telescope, D=2 x R
•The bending angle is given by
a(" ) = 0.25 ×
λ ( µm)
D ( m)
•here, the wavelength is measured in micrometers and the
aperture is measured in meters
•This angle is the theoretical limit of resolution for the
telescope
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Resolving Power
•Examples:
l= 4,000 Angstroms (blue light)
¾
D = 1 meter
a = 0.1” (less than the atmospheric limit of 1”)
¾
l= 4,000 Angstroms (blue light)
D = 5 meters (Hale telescope)
a = 0.02” (less than the atmospheric limit of 1”)
¾
l= 1 cm = 108 Angstroms (radio)
D = 43 meters
a = 1’
a (" ) = 2.5 × 10 −5 ×
λ (A )
D (m)
Resolving Power
•Examples:
¾
l= 4 mm = 4 x 107 Angstroms (radio)
D = 100 meters
a = 10” (best we can do using one radio telescope)
¾
l= 5,000 Angstroms (green light)
D = 2.4 meters (Hubble Space Telescope)
a = 0.05” (less than the atmospheric limit of 1”)
This resolution can be achieved
using the Hubble, since the telescope
a (" ) = 2.5 × 10 −5 ×
λ (A )
D (m)
is located above the atmosphere!
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Optical Telescopes
•We have learned that larger telescopes
¾Collect more light
¾Have better angular resolution
•Using high magnification in a small telescope results in dim,
fuzzy images because you are exceeding the theoretical limit
for the telescope
•You shouldn’t go over about 100-200 times magnification in a
small telescope
•Low-budget reflector telescopes offer a better value than
refractors (mirrors are cheaper to make than lenses)
Atmospheric Blurring
•The theoretical limit of resolution for the 5 meter Hale
telescope on Palomar mountain is 0.02”
•This cannot be achieved in practice due to the turbulent
motion of the Earth’s atmosphere
•The turbulent motions limit the angular resolution to about 1”
•The resulting observational conditions are called the “seeing”
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Atmospheric Blurring
•We can defeat atmospheric blurring in several ways:
1. Place the telescope high up in the atmosphere
New GMU Observatory
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Atmospheric Blurring
•We can defeat atmospheric blurring in another way:
2. Place the telescope in space
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Atmospheric Blurring
•We can defeat atmospheric blurring in another way:
3. Use adaptive optics to adjust the mirror in real time,
guided by lasers…
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Multiwavelength Astronomy
The Milky Way at many wavelengths
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Atmospheric Blockage
•Radiation propagates forever unless it is absorbed by something
along the way
•The Earth’s atmosphere is opaque to most radiation
Atmospheric Blockage
•Radio waves are reflected by the ionosphere
•Microwaves and infrared radiation are absorbed by water
molecules in Earth’s atmosphere
•X-ray, ultraviolet, and gamma-ray radiation is blocked by the
ozone layer (these are harmful forms of high-energy radiation)
•Hence, due to atmospheric blockage, certain wavelengths can be
observed from space only
High-Energy Astronomy
•X-ray and gamma-ray astronomy can be performed only from
space due to atmospheric blockage
•High-energy emission tells us about very hot environments
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High-Energy Astronomy
•Using x-ray and gamma-ray astronomy, we can study violent
processes occurring near compact objects like black holes,
neutron stars, and white dwarf stars
High-Energy Astronomy
High-Energy Astronomy
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Compton Gamma-Ray Observatory
Focusing X-rays
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Infrared astronomy using SIRTF
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Advantages of Radio Astronomy
•Radio telescopes are almost completely unaffected by
atmospheric blurring
•This is because because radio waves have longer wavelengths
than optical waves
•Weather doesn’t matter much either
•“Seeing” is almost always at the theoretical limit
•Observations can be made even in daylight!
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Radio Interferometry
•The angular resolution of radio telescopes can be improved by
using them in groups called interferometers
•The radio waves reaching telescope 1 take longer to arrive than
those reaching telescope 2
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Radio Interferometry
The Very Large Array (VLA) in New Mexico
Radio Interferometry
•Information from two or more telescopes can be combined using
interferometry
•The resulting angular resolution is equal to that of a single
telescope with diameter equal to the spacing between the
telescopes (the baseline L)
•With widely-spaced telescopes, resolution of 0.001” can be
achieved – the best in all of astronomy
Radio Interferometry
•The angular resolution of the radio interferometer depends on
the baseline L according to the formula
a(" ) = 2.5 ×
λ (cm)
L(km)
where the wavelength is measured in cm and the baseline is
measured in kilometers
•This angle is the theoretical limit of resolution for the telescope
•Hence, for a baseline of about 10,000 km (Earth’s radius), we can
achieve an angular resolution of less than one milli-arcsecond!
•Note: we do not receive all the radiation that we would if we had a
single dish the size of the baseline
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Radio Interferometry
resolution of 1”
(separation of radio lobes
is 100 kiloparsecs)
resolution of 1”
(length is 74 parsecs)
resolution of 0.00015”
(130 light-days!)
length is 3 milliarcseconds
Radio morphology of Cygnus A (distance = 170 Megaparsecs)
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