telescopes read pages 68-88. galileo’s telescope

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