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
19 th century: epoch of the large refractors
Refracting telescopes Vienna Lick
Yerkes Observatory Largest refracting telescope with a one meter objective
20 th 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 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 / d R = 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 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 = 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 = seconds
Observing from space No clouds Perfect seeing Can see wavelengths of light blocked by the earth’s atmosphere
Hubble Space Telescope
Rooftop telescopes