Optics

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Chapter 6

• Optics
• and
• Telescopes

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Optical telescopes

• Two types
• refractors (use lenses)
• reflectors (use mirrors)
• Both focus light from a large opening to a
  smaller opening (eyepiece), with magnification.

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Refracting telescopes

• The first telescope ( a refractor) was invented
  in early 17th century.
• Refracting telescopes makes use of a lens to
  collect light.
• All lenses make use of a physical phenomena
  called Refraction.
• Light travels at a slower speed in a dense
  substance.
• Speed of light in a vacuum is 3.0 x 108 m/s.
• Speed of light in glass is less than 2 x 108 m/s.
• Therefore, when light travels from a rare medium
  to a denser medium, light bends!
• The study of Light is Optics

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Refraction and Lenses

• Previous figure showed the refraction (or
  bending) of a beam of parallel light passing from
  a vacuum to a glass.
• The amount of bending depends on the speed of
  light in glass.
• If the glass is curved instead of flat..
• When parallel rays of light falls on a convex
  (glass) lens, refraction cause all the rays to
  converge at a point called the focal point.
• The distance from the lens to the focal point is
  called the focal length.

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Refraction and Lenses
When a beam of parallel light rays pass through a
convex lens, refraction causes the rays to
converge to a point - focal point.
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Image from a Lens

• Light rays from a point source radiate in all
  directions.
• If the lens is at a great distance from the
  source, the rays arriving are essentially
  parallel.
• The rays will then converge onto the focal point
  giving rise to an image at this point.

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Image of an extended object

• Consider an extended object - an object with a
  larger angular size.
• Light rays from each point of the object is
  brought to focus at its individual point on the
  focal plane of the lens - a plane that includes
  the focal point.
• An extended image will form on the focal plane.

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Uses of Lenses Camera
Focal plane
Photographic film
lens
Focal length

• Light rays from a distant object fall parallel
  onto the lens.
• Then the rays will converge onto the focal
  plane.
• If you keep a photographic film at the focal
  plane, an image of the object will form on the
  film.

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Uses of Lenses Refracting Telescope

• Instead of getting the image on a film, if you
  want to observe the image with your eyes, you
  would use another lens to magnify the image
  formed at the focal plane.
• This arrangement of two lenses is called a
  refracting telescope.

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Uses of Lenses Refracting Telescope

• The larger lens at the front is called the
  Objective lens - Large Diameter, Longer Focal
  Length.
• The smaller lens at the back is called the
  Eyepiece lens - Smaller diameter, shorter Focal
  length.

• Place the eyepiece at a distance from the focal
  plane of the objective that is equal to the focal
  length of the eyepiece.

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Uses of Lenses Refracting Telescope
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A Refracting Telescope
40-in refractor at Yerkes Observatory near
Chicago.
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Magnifying power (m)
Characteristics of Refracting Telescopes

• Telescopes magnify distant objects.
• Moons angular diameter when observed with your
  naked eyes 0.50
• When observed by Galileo through his telescope
  the angular diameter of the Moon 100 .
• He saw craters, valleys, mountain ranges, etc.

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Characteristics of Refracting Telescopes
Magnifying power (m)

• Definition of magnification or magnifying power
  

• Example Galileos telescope
• m 100 / 0.50 20 times or 20X

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Characteristics of Refracting Telescopes
Magnifying power (m)

• What determines the magnification of a telescope
  ?

• the focal lengths of the lenses!

• In order to increase magnification

• increase the focal length of the objective
• decrease the focal length of the eyepiece

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Characteristics of Refracting Telescopes
Light Gathering Power

• Larger diameter lenses capture more light
  produces brighter images.
• It is important for astronomers that telescopes
  have large diameter objective lenses.

• light-gathering power ? area of the objective
• ? (Diameter of objective)2

• If you double the diameter, the light gathering
  power increase by a factor 22 2 x 2 4

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Characteristics of Refracting Telescopes
Light Gathering Power

• Light gathering power is so important that
  telescopes are described by the diameter of the
  objective
• Example 90-cm refractor on Mount Hamilton in
  California.

• This telescope has 900 time the light gathering
  power as Galileos 3-cm refracting telescope!

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Characteristics of Refracting Telescopes
Light Gathering Power

• For astronomers magnifying power is not the most
  important factor in a telescope.
• The light gathering power is more important.
• The reason is that there is a limit to how sharp
  sharp an image can be.
• for Earth base telescopes this is determined by
  the atmospheric disturbances.

• Magnifying a blurred image gives a bigger, but
  still a blurred image.

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Characteristics of Refracting Telescopes
Light Gathering Power
Andromeda galaxy as seen from two telescopes. We
can see the effect of doubling the diameter of
the objective.
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Characteristics of a Refracting Telescope
Disadvantages

• Passing of light through lenses causes several
  effects
• chromatic aberration lens acts slightly like
  prism.
• The lens bends different color light by different
  amounts.
• some light absorbed by glass.
• UV absorbed by glass.

• Chromatic Aberration is corrected by adding a
  second lens made from a different kind of glass.

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Characteristics of a Refracting Telescope
Disadvantages
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Characteristics of a Refracting Telescope
Disadvantages

• It is impossible to produce a large lens that is
  entirely free of chromatic aberration.
• Since you can only support a lens around its
  edge, the lens tends to sag under its own weight
  and distort the image.

• Nowadays astronomers avoid these problems by
  building Reflecting telescopes that use mirrors
  to collect light.

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Characteristics of reflecting telescopes

• Incoming light reflected by several mirrors to
  eyepiece.
• Light weight construction, since mirrors are
  lighter than lenses.
• Reflection from mirror has several advantages
• Light not absorbed.
• UV not absorbed.
• No chromatic aberration.
• Can be fully supported.

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Characteristics of a Reflecting Telescope
Principle of reflection

• All modern telescopes form images using the
  principle of reflection.
• If i and r are the angles that the incident
  reflected rays make with the perpendicular

• i r

Flat mirror
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Characteristics of a Reflecting Telescope
Reflection by a Concave Mirror

• Parallel rays of light incident on a concave
  mirror reflect and converge at point - focal
  point.
• The distance between the reflecting surface and
  the focus is the focal length.

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Characteristics of reflecting telescopes
Advantages

• Because light reflects off the silver coated
  surface and does not pass through the glass, the
  defects in the glass does not effect the image.

• No chromatic aberration (no refraction). All
  color light converge to the same focus.

• The mirror can be supported by a bracket on its
  back since light does not pass through.

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Characteristics of reflecting telescopes
Designs for reflecting telescopes

• Problem Since the focal point is in front of the
  mirror how can you view the image?
• Your head will block part of the light

• To get around this problem in 1668 Newton placed
  a small mirror at a 450 angle in front of the
  focal point - Newtonian Focus.

• For Reflecting telescopes the magnification is
  defined as
• Magnification focal length of the primary
  mirror
• focal length of the eyepiece

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Characteristics of reflecting telescopes
Designs for reflecting telescopes

• Subsequently different designs of refracting
  telescopes were introduced.

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Characteristics of reflecting telescopes
Designs for reflecting telescopes

• Prime Focus Astronomers place their recording
  instruments at the prime focus.

• Newtonian Focus Reflected light from the
  primary mirror is deflected by 900 by a secondary
  mirror, usually to an eyepiece at the side of the
  telescope.

• Cassegrain Focus An astronomer wanting to place
  a heavy piece of instrument that is too big to be
  place at prime focus can use a Cassegrain where
  the light from the primary is reflected back.

• Coude Focus A more complex cassegrain type
  that uses two secondary mirrors.

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Reflecting Telescopes

• At the moment there are 8 reflecting telescopes
  with primary mirrors of diameters greater than
  26.2 feet( 8meters)
• Photograph shows the 10-m Keck I Telescope on
  Mauna Kea, Hawaii.
• Hole in the middle of the primary is for the
  Cassegrain focus.

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Characteristics of Reflecting Telescopes
Spherical Aberration

• One problem with reflecting telescopes is
  Spherical Aberration.
• Light from different parts of the mirror
  converge at different focal points due to the
  shape of the mirror.

• This problem is corrected by two methods
• Use a parabolic mirror instead of a spherical
  one.
• Use a correcting lens in front of the mirror.

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Characteristics of Reflecting Telescopes
Spherical Aberration
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Characteristics of Telescopes
Resolving Power

• Another advantage of large telescopes is their
  finer Angular Resolution.
• The ability of the telescope to form, distinct,
  separate images of two objects that are close
  together, or small angular separation.

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Characteristics of Telescopes
Resolving Power

• Finer the resolution the more details we can
  see.
• Angular Resolution
• ? ? (wavelength of light) / (Diameter of
  the primary)
• Larger the D, smaller the ang. resolution, and
  hence better.
• However atmospheric turbulence makes it
  impossible for a telescope to have the desired
  angular resolution.
• These effects are corrected by using a technique
  known as - Adaptive Optics.
• mirror shape is corrected every few seconds.

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10, 1, 5, 1
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Image processing in Astronomy

• Instead of using photographic film to record
  images present day astronomers uses electronic
  detectors known as charge coupled devices (CCD).
• Data is stored on a silicon wafer that is divided
  into a 2-dimensional array of elements (pixels).
• CCDs are much more sensitive than film and
  therefore can detect much fainter objects.
• These images are then read by a computer and
  processed

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Image Processing

• A CCD

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Image processing
Photographic film vs. CCD a) Photographic film.
b) c) using a CCD with the same telescope.
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Limitations of optical telescopes

• Available sky
• can only see part of sky at any time on
  particular night
• Atmospheric distortion
• blurring of image caused by atmospheric heat
  waves which cause image to shimmer
• good seeing means less shimmering
• Light pollution
• growth of night time lights has distanced us from
  the night sky

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Atmospheric distortion
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Observing at other wavelengths

• Light and radio (some infrared) are only ground
  based observations possible
• rest must be space-based due to atmospheric
  absorption

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Radio Astronomy

• Until the mid 20th century our view of the
  universe was based on visible light.
• Since then astronomers have used other forms of
  electromagnetic waves to study the skies, and
  these observations have revealed startling
  aspects of the cosmos.
• Radio telescopes were the first telescopes built
  that used non-visible part of the EM spectrum.

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Radio Astronomy

• The curved metal dish, usually made of wire mesh
  captures cosmic radio waves and reflect them to
  the focus.
• A receiver at the focus collects the signals and
  directs them to a computer.

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Radio Astronomy

• Optical vs. Radio view of Saturn.
• a) shows Saturn seen at 2-cm wavelength radio
  waves. This radio emission is caused by tiny
  charged dust particles moving in Saturns strong
  magnetic field. (Blue for weak and red for strong
  emission)

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Radio Astronomy

300-m Arecibo Radio Telescope in Puerto Rico.
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Earth orbiting telescopes

• Our atmosphere is mostly transparent to two
  wavelength regions, the optical window and the
  radio window.
• In order to observe the universe using other
  forms of light we have to place telescopes above
  the atmosphere.

• These invisible astronomies opens up a whole new
  window on the universe.

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Infrared Astronomy

• Water vapor in the atmosphere absorbs most IR.
• Infrared Astronomical Satellite (IRAS) was
  launched in 1983 on a 9-month mission.
• It mapped most of the sky.
• Discovered the presence of dust-disks around
  nearby stars, presenting us with the first
  evidence of planets orbiting other stars.

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Infrared Astronomy

• Infrared Space Observatory (ISO) was launched in
  1995 by ESA.
• Made ground breaking discoveries of very distant
  galaxies.

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Ultraviolet Astronomy

• Observing at UV wavelengths has given us valuable
  insight into hot stars, ionized clouds of gas
  between the stars and the Suns corona. All these
  emit a lot of UV radiation.
• International Ultraviolet Explorer (IUE) launched
  in 1978 was the first UV orbiting telescope.
• The Hubble (Optical) telescope is also a very
  good UV telescope.

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The Hubble Space Telescope
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The Hubble Space Telescope

• The HST was placed in 600-km orbit by the space
  shuttle Discovery in 1990.
• Has a 2.4 m (7.9-ft) primary mirror
• Designed to observe from near-infrared through
  visible light and into the UV region.
• better resolution in bigger telescopes
• Uses a CCD to record images and radio them to
  Earth.

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The Hubble Space Telescope

• Soon after being placed in orbit astronomers
  found a big problem with HST
• The primary mirror manufacturer had made a
  mistake and the mirror was suffering from
  Spherical Aberration
• Images were blurred.
• This problem was corrected by a 1993 space
  shuttle mission when the a set of correcting
  secondary mirrors were installed.

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The Hubble Space Telescope
Images taken before and after repair of HST of
Galaxy M100
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X-ray Astronomy

• A series of X-ray observatories have been
  launched since 1970s.
• The latest of these being NASAs Chandra X-Ray
  Observatory and ESAs XMM-Newton
• These telescopes have observed X-ray bursts
  coming from heated gas around compact massive
  objects, possibly Black Holes.

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Infrared Astronomy
Chandra X-ray Observatory and XMM-Newton
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Infrared Astronomy
The Compton Gamma ray Observatory (CGRO) launched
in 1991 by NASA

• Observing with Gamma rays can give us insight
  into extremely high energy phenomena, such as
  Supernova Explosions.

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Observing at other wavelengths