telescopes read pages 68-88. galileo’s telescope
TRANSCRIPT
Telescopes
Read Pages 68-88
Galileo’s Telescope
Galileo’s Telescope
Spherical Aberration
Spherical aberration in lenses
Perfect Lens“Thin”
Chromatic Aberration
Chromatic aberration: A problem of lenses
Reducing Chromatic Aberration
The Yerkes 40-inch Refractor
Purposes of Telescopes
• Gather and concentrate - Brighten• Reveal greater detail – Resolution• Make larger- Magnify
Brightness
• Gather and concentrate - Brighten– Light gathering power proportional to area– A= π r2
– Double the radius 4x gathering power
What do telescopes do?
Angular Resolution
• Reveal greater detail – Resolution– Angular resolution- the angle between two
adjacent objects that can be distinguished as two object.
– Resolution varies with the diameter of the primary lens or mirror.
– Double the size and halve the angle that can be distinguished.
Magnification
• We usually express this as 200X
Magnification = focal length of primary focal length of eyepiece
Magnification = 100 cm 0.5 cm
= 200
Better Telescopes
• To make telescopes better you make them bigger.
• Brightness and Resolution- bigger diameter• Magnification – longer focal length, longer
telescope
Huygens’ 123-foot-long refractor
Huygens 123 foot long telescope
Yerkes 40 inch (diameter) refractor
The Yerkes 40-inch Refractor
Refracting vs Reflecting
Refracting• Large lenses are heavy to
support only from the edge• A lens must be flawless-
bubbles block light• Chromatic aberration in
lenses• Light is dimmed traveling
through the glass
Reflecting• Mirrors can be supported
from the back• Only the surface of the
mirror must be perfect• No chromatic aberration in
mirrors• Light does not travel through
the glass of a mirror• Giant mirrors can be madeup
of smaller segments.
Reflection vs Refraction
Get to the root of it Refraction by lenses vs. reflection by mirrors
Newton to the Rescue (again)
Newton used a metal primary mirror to capture light and a secondary mirror to direct the light out the side of the telescope. Newton avoided the problem of chromatic aberration by using a mirror instead of a lens, but he could not clear up the blurry images caused by the spherical shape of the mirror. This problem, called spherical aberration, occurs in both spherical mirrors and lenses.
Parabolic shape eliminates spherical aberration
Parabolic Mirrors
Atmospheric Distortion
Hale TelescopeMount Palomar
Last Big Single Mirror ReflectorYear completed: 1948 Telescope type: Reflector
Light collector: Aluminum-coated glass mirror
Mirror diameter: 200 inches(5.0 m)
Light observed: Visible
•Discovered visible evidence of quasars — very bright objects at very great distances that were later found to be supermassive black holes at the centers of distant galaxies.
Multi Mirror Telescopes
Keck IYear completed: 1993
Telescope type: Reflector
Light collector: 36 aluminum-coated glass mirror segments
Mirror diameter: 400 inches(10 m) total
Light observed: Visible
•Discovered the first visual evidence of a brown dwarf, a failed star. With its light-gathering power, found planets around other stars.
Atmospheric Windows (P 68)
Radio Telescopes
Radio telescope design
Pictures from Radio Waves
False Color
Hubble in Space
Hubble HighlightsYear launched: 1990 Telescope type: Reflector
Light collector: Aluminum-coated glass mirror
Mirror diameter: 94.5 inches(2.4 m)
Light observed: Infrared, visible, ultraviolet
Discovery Highlights:•Helped determine the age of the universe and the way galaxies form. Revealed extraordinary details about the process by which Sun-like stars end their lives as planetary nebulae.
Chandra X-Ray Observatory
Chandra HighlightsYear launched: 1999 Telescope type: Reflector
Light collector: 8 iridium-coated glass mirrors
Mirror diameter: Each 32.8 inches(83.3 cm)
Light observed: X-ray
Discovery Highlights:•Has allowed astronomers to study energetic events such as black holes, supernovae, and colliding galaxies. Has found new stars that may have planet-forming disks around them.
Spitzer (follows the earth)
Spitzer Highlights
Year launched: 2003 Telescope type: Reflector
Light collector: Beryllium metal mirror
Mirror diameter: 33.5 inches(85 cm)
Light observed: Infrared
Discovery Highlights:•Has seen through dust clouds in our galaxy to better allow the study of star formation and black holes.
Webb 2013
Webb HighlightsYear to be launched: 2013 Telescope type: Reflector
Light collector: Gold-coated beryllium mirror
Mirror diameter: 255.6 inches (6.5 m)
Light observed: near- to mid-infrared
Discovery Highlights:•Telescope has not launched.
Human Eye as a Detector
• The human eye is a • sophisticated, • auto-focus, • auto-exposure, • electrical camera system. • However, for all its versatility and importance
in everyday life, it is a seriously limited astronomical detector:
Limitations of Human Eye
• eye is small, both brightness and resolution are improved with bigger diameter.
• maximum integration time only about 0.1 secs, • eye has low sensitivity We cannot see in dim
light, but cats can.
• Astronomers have long sought more capable detectors to use with telescopes.
Photographic Film or Plates• Detects only 1-2% of incident photons but allows long
integrations (hours)
• Requires chemical development of image after exposure
• Provides permanent storage of info, though not digital
• Large formats (up to 20" square for astronomy)
• Was the main astronomical detector used between 1900 and 1980.
Charged Coupled Devices• Solid state electronics; widely used now in video cameras & TV
• The CCD surface is composed of thousands of independent, light-sensitive pixels. After exposure, pixel contents are shifted in 2 dimensions across the surface to an output amplifier and storage device.
• Astronomical applications pioneered during development of Hubble Space Telescope (1974-85).
• Works well at both very short (TV) and very long (astronomy) exposure times
• 50-100x more sensitive than film
• Digital image storage for immediate computer processing
• Small formats (2-in typical) but can "mosaic" CCDs to create large areas
• Now are the standard detectors used in astronomy