Refracting Telescopes

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Refracting Telescopes
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Faint Light

• Astronomical objects are distant and faint.
• Effectively at infinity
• Light collection is more important than
  magnification.
• Refraction
• Reflection

• The Andromeda Galaxy (M31) is 3 wide.
• 6 times the moon
• Only visible to the unaided eye in very dark
  conditions

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Refraction

• Light is bent at the surface between two media.
• This bending is called refraction.

• Lenses use refraction to bend and focus light.
• focal length and a focal point
• Parallel rays from a distant source

focal point
focal length
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Concentrator

• A refracting telescope uses lenses to concentrate
  light from a distant object.
• Object light rays nearly parallel
• Final image rays also parallel

objective
focal point
eyepiece
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Aperture

• Lenses collect and concentrate light by
  refraction.
• The diameter (D) of the objective lens is the
  aperture.
• Measured in m or mm
• Larger apertures for fainter objects
• The light gathering power (LGP) is related to the
  area of the lens.
• Circular lens A (?D2)/4
• Relative to eye aperture 5 mm LGP D2/(5 mm)2

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F-Stop

• The brightness of an image is measured by the
  focal ratio of the focal length to the aperture.
• F-number or f-stop f/D
• Dimensionless quantity
• Denoted by f/
• Lower f-numbers are faster and need shorter
  exposure times.

Primary focal length
Aperture diameter x f-number
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Magnification

• Refracting telescopes magnify images with two
  lenses.

objective
focal point
eyepiece
Primary focal length
Magnification
Eyepiece focal length
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Chromatic Aberration
Tony and Marilyn Karp

• Real material refract colors differently.
• Blue light bends more than red
• Compound lenses can compensate for chromatic
  aberration.

Wikipedia
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Yerkes Refractor

• Worlds largest refractor is in Wisconsin.
• 40 inch aperture, f/19.
• 63 foot tube.

Yerkes 40 inch