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Telescopes: Portals of Discovery
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How do light and matter interact?
• Emission
• Absorption
• Transmission– Transparent objects transmit light– Opaque objects block (absorb) light
• Reflection or Scattering
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Reflection and Scattering
Mirror reflects light in a particular direction
Movie screen scatters light in all directions
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Reflection of light with mirrorsThe flat surface enables an incoming light beam to perfectly bounce
Angle of Incidence = Angle of Reflection
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Reflection and Scattering with Matter
Interactions between light and matter determine the appearance of everything around us
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Thought QuestionWhy is a rose red?
a) The rose absorbs red light.
b) The rose transmits red light.
c) The rose emits red light.
d) The rose reflects red light.
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Thought QuestionWhy is a rose red?
a) The rose absorbs red light.
b) The rose transmits red light.
c) The rose emits red light.
d) The rose reflects red light.
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Refraction (occurs during transmission)
• The universal speed of light is measured in a vacuum.
• The speed of light is slower in different substances.
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Refraction• E.g. Light travels
slower through glass or water than through air.
• Refraction is the bending of light when it passes from one substance into another
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Focusing Light
• Refraction can cause parallel light rays to converge to a focus
• The speed of light through the liquid in your eye is slower than the speed of light in air.
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Image Formation
• The focal plane is where light from different directions comes into focus
• The image behind a single (convex) lens is actually upside-down!
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What have we learned?
• How do light and matter ineract?
– 4 forms of interaction, including reflection and transmission
– Refraction is when light slows and bends when travelling through a substance
• How does your eye form an image?– It uses refraction to bend parallel light rays
so that they form an image.– Cameras focus light like your eye and record
the image with a detector (CCDs in digital cameras).
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Telescopes: Giant Eyes
• Our goals for learning
• What are the two most important properties of a telescope?
• What are the two basic designs of telescopes?
• What do astronomers do with telescopes?
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What are the two most important properties of a telescope?
1. Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time.
2. Angular resolution: Telescopes that are larger are capable of taking images with greater detail.
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Light Collecting Area
• A telescope’s diameter tells us its light-collecting area: Area = π(diameter/2)2
• i.e The larger the aperture size, the better the telescope
• The largest telescopes currently in use have a aperture diameter of about 10 meters
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Bigger is better
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Thought QuestionHow does the collecting area of a 10-meter telescope compare with that of a 2-meter
telescope?
a) It’s 5 times greater.
b) It’s 10 times greater.
c) It’s 25 times greater.
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Angular Resolution
• Distant objects look very close together; i.e. a small 'angular separation'
• Angular resolution is the minimum gap between objects that the telescope can distinguish.
• Eventually the car is so far away that you see the headlights as one light.
-you have reached the limit of your eye to resolve the angular separation.
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Angular Resolution• Ultimate limit to
resolution comes from interference of light waves within a telescope.
• Larger telescopes are capable of greater resolution because there’s less interference for the amount of light
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What are the basic designs of telescopes?
• Refracting telescope: Focuses light with lenses
• Reflecting telescope: Focuses light with mirrors
• Catadioptric telescope: Focuses light with both lenses and mirrors
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Refracting Telescope
• Refracting telescopes need to be very long, with large, heavy lenses
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Basic Refractor Telescope 1608
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Galileo’s Telescopes 1609
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Chromatic Aberration• Light focuses imperfectly in a lens because the
light must transmit though glass• All lenses suffer from chromatic aberration (slight blurring caused by the short wavelength light
focusing before the long wavelengths)
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Reflecting Telescope
• Reflecting telescopes do not suffer chromatic aberration• Reflecting telescopes can have much greater diameters.• Most modern telescopes are reflectors
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Mirrors in Reflecting Telescopes
Twin Keck telescopes on Mauna Kea
Segmented 10-meter mirror of a Keck telescope
The largest mirrors in the world are in the Keck telescope on Hawaii
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Designs for Reflecting Telescopes
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Newtonian Telescope 1672
One of the most common amateur telescopes for deep sky observing.
Modern version with a Dobsonian mount
Early Newtonian used by Herschel
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Cassegrain Reflector Telescope, 1672
More efficient, but requires a more complex secondary mirror.(hyperbolic rather than flat)
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Some Cassegrain Correctors -Catadioptrics
Schmidt-Cassegrain Telescope 1930
Maksutov-Cassegrain Telescope 1941
One of most common amateur
telescopes
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Catadioptric Telescopes
Schmidt-Cassegrain telescope on an
equatorial mount
Corrector lens
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What do astronomers do with telescopes?
• Imaging: Taking pictures of the sky
• Spectroscopy: Breaking light into spectra
• Timing: Measuring how light output varies with time
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Imaging
• Early astronomers had no choice, they observed by eye and recorded with descriptions and drawings.– Visible light only.
• Camera - permanent record, non-subjective, reproducible world-wide in publications.– Can be built to detect any wavelength.
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Imaging
• Astronomical detectors generally record only one color of light at a time
• Several images must be combined to make full-color pictures
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Imaging
• Astronomical detectors can record forms of light our eyes can’t see
• Color is sometimes used to represent different energies of nonvisible light
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Imaging• True color image
– The colors are the same as if you were viewing with your own eyes
• False color image– The colors have been
arbitrarily assigned to assist image interpretation
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The famous image of the Eagle nebula is false color!
(Nebula in true color)
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Spectroscopy• A spectrograph
separates the different wavelengths of light before they hit the detector
Diffractiongrating breakslight intospectrum
Detectorrecordsspectrum
Light from only one starenters
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Spectroscopy• Graphing
relative brightness of light at each wavelength shows the details in a spectrum
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Timing
• A light curve represents a series of brightness measurements made over a period of time
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What have we learned?• What are the two most important properties of a
telescope?– Collecting area determines how much light a
telescope can gather– Angular resolution is the minimum angular
separation a telescope can distinguish• What are the basic designs of telescopes?
– Refracting telescopes focus light with lenses– Reflecting telescopes focus light with mirrors– Catadioptric telescopes focus light with both.– The vast majority of professional telescopes are
reflectors
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What have we learned?
• What do astronomers do with telescopes?– Imaging– Spectroscopy– Timing