Optics and Telescopes

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Optics and Telescopes
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Optics and Telescopes Guiding Questions

• How do reflecting and refracting telescopes work?
• Why is it important that professional telescopes
  be large?
• Why do most modern telescopes use mirrors rather
  than lenses?
• Why are observatories in such remote locations?
• Do astronomers use ordinary photographic film to
  take pictures of the sky? Do they actually look
  through large telescopes?
• How do astronomers use telescopes to measure the
  spectra of distant objects?
• Why do astronomers need telescopes that detect
  radio waves and other non-visible forms of light?
• Why is it useful to put telescopes in orbit?

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Two Basic Telescope Designs

• Refractors
• Use lenses to concentrate incoming light at a
  focus.
• Reflectors
• Use mirrors to concentrate incoming light at a
  focus.
• The goal is always the same gather as much
  light as possible and concentrate it at a focus.

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Refractor
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Reflector
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Refraction Bending of light when propagating
into material with higher refractive index (e.g.
glass)
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A refracting telescope uses a lens to concentrate
incoming light at a focus.
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Image Formation
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Image Formation
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Functions of a telescope

• Magnify

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A refracting telescope actually uses two a
lenses an objective and an eyepiece.
The two lenses are separated by the sum of their
focal lengths.
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Magnification of a 2-lens refracting telescope
Fo
Fe
Eyepiece
Objective lens
Example FO 1 m (1000 mm) FE 25
mm Magnification FO/FE 1000 mm/25 mm 40x
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Functions of a telescope

• Magnify
• magnification (objective lens focal length /
  eyepiece lens focal length)
• Brighten

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Light gathering power Comparing two telescopes

• Effective collecting area area (d telescope
  diameter
• Compare two telescopes Hubble (d 2.4m) and
  Keck (d 10m)

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Refracting telescopes have drawbacks

• Chromatic aberration
• The index of refraction varies with the
  wavelength of light, so different colors are
  focused to different places

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Chromatic aberration
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Special achromatic compound lenses and lens
coatings can often fix this aberration.
Lens is achromatic if it bends light at same
angle independent of wavelength. Expensive! Very
difficult to make large achromatic lenses.
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The largest research telescopes in the world are
ALL reflectors. The Keck I telescope on Mauna
Kea on the Big Island of Hawaii uses 36 hexagonal
mirrors to make a total diameter of 10 m. (Note
the astronomers standing on either side of the
platform.)
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A reflecting telescope uses a mirror to
concentrate incoming light at a focus.
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The secondary mirror in the tube does not cause a
hole in the image. It does however make it
slightly dimmer because it reduces the total
amount of light reaching the primary mirror.
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• Drawback of Using Spherical Mirrors in Reflecting
  Telescope
• Spherical Aberration
• (can be corrected with a correcting lens)

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An electronic device is commonly used to record
the image at a telescopes focus

• A Charge-Coupled Device (CCD)

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Ordinary Photographs vs. CCDs
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Functions of a telescope

• Magnify
• magnification (objective lens focal length /
  eyepiece lens focal length)
• Brighten
• called light gathering power
• Proportional to the diameter of the objective
  lens.
• Resolve fine detail
• called angular resolution
• Proportional to the size of the telescope (array).

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Poor and Good Angular Resolution
Telescope images are degraded by the blurring
effects of the atmosphere and the telescope
resolution.
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Angular resolution
How close can two stars be before they blur into
one? Measured in angular unit, radians or
arcseconds.
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Diffraction
Waves are bent when they pass through a narrow
opening. This places a limit on the angular
resolution of any telescope.
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Computing the diffraction limit or angular
resolution of a telescope (ignores blurring of
atmosphere)
where ?, d are in same units (e.g. meters) and ?
is in radians
1 radian 57.296? 206,265 arcsec ()
Example HST, yellow light (? 500 nm)
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University of Iowas Rigel Telescope in Arizona
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Spectrographs record the spectra of astronomical
objects.
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Spectrographs record the spectra of astronomical
objects.
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Spectrum of Vega (Rigel Telescope)
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Electromagnetic spectrum
The spectrum of a particular star is how much
light it produces at each wavelength.
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How can we observe nonvisible light?

• A standard satellite dish is essentially a
  telescope for observing radio waves

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Radio Telescopes

• A radio telescope is like a giant mirror that
  reflects radio waves to a focus

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An example of an interferometer
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Very Long Baseline Array (VLBA)
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North Liberty Iowa VLBA Radio Telescope

• 82 ft (25m) diameter
• Built 1991-1992
• Total cost 3M
• Part of VLBA (10 identical telescopes spanning
  US)
• Operated from VLBA control center (Socorro NM)
• Operates 24/7
• Science research includes stars, black holes,
  pulsars, cosmology,

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Angular resolution of an interferometer
where D is the largest separation between
telescopes.
For example the VLBA has telescopes in Hawaii and
Virgin Islands (8000 km). At a typical radio
wavelngth 1 cm the angulat resolution is
This is the size of a newspaper at the distance
to the Moon (but cant read newspaper!)
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Observations at other wavelengths are revealing
previously invisible sights.
UV
infrared
Map of Orion region
Ordinary visible
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Telescopes at high altitude or in orbit around
the Earth detect radiation that does not
penetrate the atmosphere.
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IR UV Telescopes
SOFIA
Spitzer

• Infrared and ultraviolet-light telescopes operate
  like visible-light telescopes but need to be
  above atmosphere to see all IR and UV wavelengths

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X-Ray Telescopes

• X-ray telescopes also need to be above the
  atmosphere

Chandra
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X-Ray Telescopes

• Focusing of X-rays requires special mirrors
• Mirrors are arranged to focus X-ray photons
  through grazing bounces off the surface

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Gamma Ray Telescopes

• Gamma ray telescopes also need to be in space
• Focusing gamma rays is extremely difficult

Compton Observatory
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Supernova Remnant Cas-A at Three Wavelengths
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The Entire Sky at the Visible Wavelengths
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The Entire Sky at the 21-cm Wavelengths
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The Entire Sky at the Infrared Wavelengths
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The Entire Sky at the X-ray Wavelengths
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The Entire Sky at the Gamma Ray Wavelengths