Telescopes Amateur and Professional
Jan 01, 2016
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 havechromatic 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
Vienna
Lick
YerkesObservatory
Largest refractingtelescope with aone 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 Reflectingtelescope
Objective Mirror
Dome of 4 meterKitt 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 24-inch 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 / dR = resolution in seconds of arc
= 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 500nmOur 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 onthis side
Good seeingon this side
Electromagnetic Spectrum
Radio TelescopesArecibo
Very Large Array
Radio telescope resolution
= 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