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Optical Telescope. Faint Light. Astronomical objects are distant and faint. Effectively at infinity Light collection is more important than magnification. Refraction Reflection. The Andromeda Galaxy (M31) subtends 3 ° . 6 times the moon

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faint light
Faint Light
  • Astronomical objects are distant and faint.
    • Effectively at infinity
  • Light collection is more important than magnification.
    • Refraction
    • Reflection
  • The Andromeda Galaxy (M31) subtends 3°.
    • 6 times the moon
    • Only visible to the unaided eye in very dark conditions
  • Light is bent at the surface between two media.
    • Index of refraction n
  • Refraction is governed by Snell’s law.




radius of curvature
Lenses shaped like parts of spheres are easy to make.

Easy to calculate rays

Use Snell’s Law on a small part of a sphere.

Radius of curvature R

Focal length f

Index for air is 1

Radius of Curvature



refracting telescope
Refracting Telescope
  • A refracting telescope is designed to concentrate light from a distant object.
    • Object light rays nearly parallel
    • Final image rays also parallel


focal point


  • Lenses collect and concentrate light.
  • 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
    • Intensification relative to eye aperture 5 mm: LGP = D2/(5 mm)2
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.
airy disk
Fraunhofer patterns are symmetric around the opening.

A circular hole produces rings around a central maximum.

84% of energy in center

Airy Disk
angular resolution
The limit of resolution is set by the aperture.

The Rayleigh criterion is calculated from the first minimum of the Airy disk.

Aperture radius a

Wavenumber k

Bessel function J1

Angular Resolution
tube length
Tube Length
  • The intermediate image at the focal point is a real image.
    • Long tube accommodates long focal length
    • Parallel ray image related to the focal length


focal point


  • The eyepiece magnifies the intermediate image.
  • The total magnification is the product from both lenses.


focal point


yerkes refractor
Yerkes Refractor
  • The world’s largest refractor is in Wisconsin.
    • 40 inch aperture, f/19
    • 63 foot tube

Yerkes 40 inch

chromatic aberration
The index of refraction depends on the wavelength.

Longer wavelengths - lower indexes

Blue light bends more than red

Compound lenses can compensate for chromatic aberration.

Chromatic Aberration
  • Air n(589 nm) =1.00029
  • Crown glass 1.52
  • Flint glass 1.66
spherical aberration
A spherical surface does not focus all parallel lines to the same point.

Aspheric lenses can be used to correct the aberration .

Spherical Aberration


curved mirror
Light that begins at one focus of an elliptical mirror converges at the other focus.

A parabola for a focus at infinity


Curved Mirror


parabolic mirror
Parabolic Mirror
  • A perfect parabolic mirror has a focal length like a lens.
  • All wavelengths are focused to the same point.
    • No chromatic aberration
  • The size of the mirror dish is the aperture.

focal length

focal point

newtonian reflector
Newtonian Reflector
  • For viewing ray should be parallel on exit.
    • Combined primary mirror and eyepiece
  • The reflecting telescope is cheaper, because a mirror is easier to make than a lens for a given size.

secondary diagonal mirror

primary mirror


schmidt cassegrain reflector
Schmidt-Cassegrain Reflector
  • A Cassegrain focus uses a flat mirror to make the tube up to three times longer.
    • Spherical aberration from extra mirror
    • Aspheric Schmidt lens corrects aberration

Schmidt corrector lens


keck reflector
Keck Reflector
  • World’s largest reflector is in Hawaii.
    • 400 inch aperture, f/1.75
    • Focal length 57.4 feet.
    • Telescope height 81 feet.

Keck Observatory

  • Parabolic mirrors focus precisely for rays parallel to the central axis.
  • The distortion for off axis objects is called coma.
    • Greatest for low f-numbers
  • Lenses can correct for the coma.


atmospheric absorption
Atmospheric Absorption
  • The atmosphere absorbs radiation, except at visible light, infrared, and radio frequencies.
adaptive optics
Adaptive Optics
  • The moving atmosphere disturbs images.
    • Wavefront distortions
  • Real time corrections are made by feedback to a deformable mirror.
    • Sample wavefront from beam splitter
    • Measure distortion
    • Compute necessary compensation for mirrors
telescope advantages

Superb resolution

Good for detail

Rugged alignment

Transports well


Inexpensive optics

Large aperture

Good for dim objects

Uniform treatment of colors


Portable size

Combines best optical qualities

Good for photography

Telescope Advantages
altazimuth mount
Altazimuth Mount
  • Telescope mounts should permit two directions of motion.
  • Altazimuth mounts directly control altitude and azimuth.

altitude control

azimuth control

equatorial mount
Equatorial Mount
  • Altazimuth mounts do not track with the star’s movement.
  • Equatorial mounts are oriented to the pole.
  • Allows control of declination and right ascension.

declination axis

polar axis

charge coupled device
Charge-Coupled Device
  • The CCD is an array of photosensitive semiconductor capacitors.
    • Charge stored proportional to light intensity
    • Transfers charge as a shift register
    • Amplifier on last capacitor converts charge to voltage


telescope ccds
Telescope CCDs
  • CCDs are sensitive to light from ultraviolet to infrared.
  • CCDs are very efficient.
    • Can be sensitive to individual photons
  • Sensitivity to thermal noise and cosmic rays can blur an image.
  • Multiple exposures are averaged to get correct image.
    • Dark frame closed shutter
hubble space telescope
Hubble Space Telescope
  • The Hubble is an orbiting reflector telescope.
  • It has no atmosphere to peer through.
  • The onboard computer gives it enhanced optics.
  • There are four different
  • cameras for different views.
infrared and ultraviolet
Infrared is absorbed by water vapor.

Observe at high altitude

Satellite telescopes avoid the atmosphere.

IRAS (1983) - first evidence of planets around other stars

Spitzer Space Telescope (2003-9).

Ultraviolet is largely absorbed by the atmosphere.

Requires satellites


Infrared and Ultraviolet

M81 from GALEX