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Telescopes
Amateur and Professional
Galileo 1609
The Moon as a World
Jupiter has Moons
Refracting telescopes
Long focus refractors were awkward but suffered less from
chromatic aberration
Isaac Newton’s reflecting telescope
Mirrors do not have
chromatic aberration
Reflecting telescope
Objective mirrors instead of lenses
Three Powers
• Magnifying
• Resolving
• Light Gathering
Magnifying Power
• Ability to make objects appear larger in
angular size
• One can change the magnifying power of
a telescope by changing the eyepiece
used with it
• Mag Power = focal length of objective
divided by the focal length of the eyepiece
Resolving Power
• Ability to see fine detail
• Depends on the diameter of the objective
lens or mirror
Light Gathering Power
• The ability to make faint objects look
brighter
• Depends on the area of the objective lens
or mirror
• Thus a telescope with an objective lens 2
inches in diameter has 4 times the light
gathering power of a telescope with a lens
1 inch in diameter
Herschel & Lord Rosse
19th century: epoch of the large
refractors
Refracting telescopes
Lick
Vienna
Yerkes
Observatory
Largest refracting
telescope with a
one meter objective
20th century Large Reflectors Come
of Age
Mount Wilson Observatory 1.5m (1908) and 2.5m (1918)
Palomar 5-m
(entered operation in 1948)
4 meter
Reflecting
telescope
Objective Mirror
Dome of 4 meter
Kitt Peak
Keck Telescopes
SOAR Telescope
4.1 meter
SOAR Telescope -- Cerro Pachon
SOAR Observing Room
SOAR Image of the planetary nebula NGC 2440
MSU Campus Observatory
Boller & Chivens reflecting telescope with a 24inch objective mirror
More on resolution
• Eagle-eyed Dawes
• The Dawes Limit
R = 4.56/D
Where
R = resolution in seconds of
arc
D = diameter of objective in
inches
More appropriate for visible
light and small telescopes
A more general expression for the
theoretical resolving power
• Imagine that star
images look like Airy
disks
Minimum Angle that can be
resolved
• R = 1.22 x 206,265 l / d
R = resolution in seconds of arc
l = wavelength of light
d = diameter of the objective lens or mirror
Note that the wavelength of light and the
diameter of the objective should be in the
same units
Examples
• For Visible light around 500nm
Our 24-inch telescope
R = 0.20 seconds
This may be compared with the Dawes limit of
0.19 seconds
But with large ground-based telescopes it is
difficult to achieve this
Astronomical “seeing”
• Blurring effect of looking
through air
• Causes stars to twinkle
and planetary detail to
blur
– At the SOAR site: good
seeing means stellar
images better than about
0.7 seconds of arc
– In Michigan, good seeing
means better than about 3
seconds of arc
– Not to be confused with
good transparency
Bad seeing on
this side
Good seeing
on this side
Electromagnetic Spectrum
Radio Telescopes
Arecibo
Very Large Array
Radio telescope resolution
l = 1m d = 100m
R = 2500 seconds = 42 minutes!
Even though radio telescopes are much
bigger, their resolving power is much
worse than for optical telescopes
Interferometric arrays get around this
Very Large Array
Interferometry
Size of array = 10 km for a VLA
This becomes the effective d
Now R becomes 25 secsec for a
1-m wavelength
For VLBI (very long baseline interfeormetry)
the d = 10,000km and R = 0.025 seconds
Observing from space
• No clouds
• Perfect seeing
• Can see wavelengths of light blocked by
the earth’s atmosphere
Hubble Space Telescope
Rooftop telescopes