Tripod or pedestal mount to hold tube assembly
Equatorial mount (tilted at angle, move in several directions)
Alt-Azimuth mount (move to side and up and down)
Refracting telescopes
First telescope 1604 Hans Lippershey
First used by Galileo 1609 to look at the sky
Bend light with lenses
- Objective lens
- Eyepiece lens (magnifying lens to magnify concentrated light)
Each lens has a focus
Distance between lens and focus is focal length
The smaller the eyepiece focal length, the greater the magnification
(Tiny hole, higher power)
Magnification = (fo) /(fe)
Problems:
aberration = fuzziness of images even with meticulous focusing
2 types
Chromatic - ring of color surrounds image
Spherical - lenses don't bring light to a sharp focus. Need a non-spherical
lens with less curvature (flatter lenses). But this increases focal length, causing
telescopes to have to be long and skinny.
Reflecting telescopes
Use curved mirrors instead of lenses
A solution to the problem of long telescopes.
First built by Newton in 1668. (50 years after Galileo)
- Newtonian reflectors
focus of mirror is to the side of tube
- Cassegrain reflector
has hole in primary mirror
reflects light back to rear of telescope (look straight through via mirrors)
Problems:
early mirrors tarnighed easily (silver) and didn't reflect well
spherical aberration
First useful reflector built by Hadley in 1721. Used a parabolic mirror.
Catadiopteric telescopes use lenses and mirrors.
Historic large telescopes
Larger telescopes allow observewrs to see fainter objects.
Since mirror or lens is a light collector, a larger mirror or lens collects more light.
Radio telescopes
Radio astronomy began in 1932 when Karl Jansky detected radio signals coming from
the Milky Way.
Types of radio telescopes:
- Steerable parabolic reflectors (dish antennas)
- Stationary arrays
Largest in world is Aricebo (305 m or 1000 ft) in Puerto Rico.
Major disadvantage is poor resolving ability.
Wavelengths of radio waves are 1000's of times larger than those of visible light.
Use arrays of 2 or more to increase resolution. May be miles apart.
36 km array near Socorro, NM has 27 25-m (82 ft) antennas.
Space telescopes
Necessary because:
Our atmosphere filters out much IR, UV, X-rays and gamma rays.
Atmosphere introduces distortions - stars twinkle.
Patches of warm air bend light rays.
Dust, clouds, and humidity diminish light intensity.
Lights on the ground interfere with observations.
Earth's gravtational field distorts large mirrors and lenses, and limits precision of
pointing of large instruments.
Earth is rotating, limiting what you can see and length of observations.
(Many telescopes are at high elevations to lessen problems)
Mauna Kea volcano in Hawaii at 4200 m (13,800 ft)
Can place in orbit or use from spacecraft (shuttle).
First space observations were from German rockets after WWII.
1957, Soviets launched Sputnik.
Hubble Space Telescope
Primary mirror 2.4 m diameter
13.1 m long
11,600 kg
Cost $1.5 billion (original projection was $500 million)
Planned lifetime 15 years
Launch was scheduled for 1983, but fell behind schedule. Completed 1985.
Challenger Tragedy occurred, setting space schedule behind.
Launched by Discovery April 24, 1990
Orbits at 613 - 615 km
Had problem with spherical aberration of primary mirror (made in 1981).
Has been corrected and works great, surpassing expectations.
Return to Astronomy Home Page
This page created by Pamela J. W. Gore
DeKalb College, Clarkston, GA
pgore@gpc.edu
Created February 6, 1996