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Optics and Telescopes. Lecture 11. Why do we use telescopes?. Human eyes are lenses! Using larger lenses…  collect more light  magnification. Larger lens can make brighter and magnified images. Change in direction of travel. Refraction. light travels at the fastest

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why do we use telescopes
Why do we use telescopes?
  • Human eyes are lenses!
  • Using larger lenses…

 collect more light

 magnification


light travels at the fastest

speed (e.g., speed of light)

in vacuum.

parallel light rays from distant objects
Parallel light rays from distant objects
  • If a lens is located very far from the light source, only a few of the light rays are entering the lens.
  • These rays are essentially parallel.
extended object extended image
Extended object Extended image
  • A lens creates an extended image of an extended object.
  • each point on an extended object passes through a lens and produces an image of that point.
  • collection of point images = image of an extended object.
refractive telescope
Refractive Telescope

Objective lens (light-gathering) + eyepiece (making image)

Light-gathering power = area of the objective lens

magnification =

focal length of objective lens

focal length of eyepiece lens

disadvantages of refractive telescope
Disadvantages of refractive telescope
  • Hard to make defect free lenses (especially larger one)
  • Glass is opaque to certain wavelengths (UV is 100% blocked!)
  • Very difficult to make larger lens
  • Large lenses are heavy  gravitational distortion
law of reflection

Angle of reflection requals angle of incidence i

Law of Reflection


to mirror surface

incidence angle


reflection angle




light ray


light ray


different designs of reflecting telescopes
Different designs of Reflecting telescopes
  • Prime focus is good but inconvenient.
all modern telescopes are reflecting telescopes
All Modern telescopes are Reflecting telescopes

Gemini Telescope (8m)

Primary mirror

secondary mirror

Cassegrain focus

Large mirrors (nearly defect free : error is less than 8.5 nanometers) are much easier to make.

Hollowed mirror base (honeycomb)

secondary mirror making a hole in the image
Secondary mirror making a hole in the image?

Secondary mirror (or Cassegrain focus hole) does not make a hole in the focused image.

However, support structure creates a diffraction spike from a point source.

spherical aberration
Spherical Aberration

Different parts of a spherically concave mirror reflect light to slightly different points  image bluring

A solution

parabolic mirror (harder to make)

correcting lens

angular resolution
Angular resolution

Because of diffraction of light (light waves spread out from a point), there is a limit in angular resolution

Diffraction-limited angular resolution

θ = angular resolution in arcseconds

λ = wavelength of light, in meters

D = diameter of telescope, in meters

(example) Keck telescope, red light.

effect of earth atmosphere
Effect of Earth Atmosphere
  • Light = wave
  • Perfect waveform got deformed due to turbulence in atmosphere…


turbulence in atmosphere

adaptive optics
Adaptive Optics

Using a nearby star (e.g., point source), reshape the mirror so that it can become a perfect (diffraction limited) point source.

Active Optics

wind shakes

tip/tilt correction

in summary
In summary…

Important Concepts

Important Terms


Focal length


light pollution

Aberration (chromatic, spherical)

  • Refractive telescope
    • disadvantages
  • Reflective telescope
    • various designs
  • Angular resolution
  • Active Optics
  • Adaptive Optics
  • Chapter/sections covered in this lecture : sections 6-1 through 6-3