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Chapter 6 Telescopes: Portals of Discovery. 6.1 Eyes and Cameras: Everyday Light Sensors. Our goals for learning How does your eye form an image? How do we record images?. How does your eye form an image?. Refraction.

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Chapter 6 telescopes portals of discovery l.jpg

Chapter 6Telescopes: Portals of Discovery


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6.1 Eyes and Cameras: Everyday Light Sensors

  • Our goals for learning

  • How does your eye form an image?

  • How do we record images?



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Refraction

  • Refraction is the bending of light when it passes from one substance into another

  • Your eye uses refraction to focus light


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Example: Refraction at Sunset

  • Sun appears distorted at sunset because of how light bends in Earth’s atmosphere


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Focusing Light

  • Refraction can cause parallel light rays to converge to a focus


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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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Focusing Light

Digital cameras detect light with charge-coupled devices (CCDs)

  • A camera focuses light like an eye and captures the image with a detector

  • The CCD detectors in digital cameras are similar to those used in modern telescopes


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What have we learned?

  • How does your eye form an image?

    • It uses refraction to bend parallel light rays so that they form an image.

    • The image is in focus if the focal plane is at the retina.

  • How do we record images?

    • Cameras focus light like your eye and record the image with a detector.

    • The detectors (CCDs) in digital cameras are like those used on modern telescopes


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6.2 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?

  • Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time.

  • 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

  • This is called the telescopes “aperture.” This is one of the two important characteristic which is a number that specifies a telescope.

  • The largest telescopes currently in use have a diameter of about 10 meters



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Thought QuestionHow does the collecting area of a 10-meter telescope compare with that of a 2-meter telescope?

  • It’s 5 times greater.

  • It’s 10 times greater.

  • It’s 25 times greater.


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Thought QuestionHow does the collecting area of a 10-meter telescope compare with that of a 2-meter telescope?

  • It’s 5 times greater.

  • It’s 10 times greater.

  • It’s 25 times greater.


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Angular Resolution

  • The minimum angular separation that the telescope can distinguish.


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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


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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


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Angular Resolution

  • The rings in this image of a star come from interference of light wave.

  • This limit on angular resolution is known as the diffraction limit

  • Angular resolution is aperture depenent.

Close-up of a star from the Hubble

Space Telescope


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Angular resolution versus wavelength and aperture formulae.

q=1.22l/D

The resolution angle is q, the wavelength is l, and D is the diameter of the objective, the aperture.


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What are the two basic designs of telescopes?

  • Refracting telescope: Focuses light with lenses

  • Reflecting telescope: Focuses light with mirrors


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Refracting Telescope

  • Refracting telescopes need to be very long, with large, heavy lenses


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Reflecting Telescope

  • Reflecting telescopes can have much greater diameters, because mirrors can be supported from the back, where lens can only be supported by the edges.

  • Most modern telescopes are reflectors



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Mirrors in Reflecting Telescopes

Twin Keck telescopes on

Mauna Kea in Hawaii

Segmented 10-meter mirror of a Keck telescope


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What do astronomers do with telescopes?

  • Imaging: Taking pictures of the sky

  • Photometry: measuring of total photon count. We will do this in the CLEA “Photoelectric Photometry of the Pleiades” project.

  • Spectroscopy: Breaking light into spectra

  • Timing: Measuring how light output varies with time (generally done with photometry, but it can be done with images or spectroscopy, too.


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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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Spectroscopy

  • A spectrograph separates the different wavelengths of light before they hit the detector

Diffraction

grating breaks

light into

spectrum

Light from

only one star

enters

Detector

records

spectrum


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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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Want to buy your own telescope?

  • Buy binoculars first (e.g. 7x35) - you get much more for the same money.

  • Ignore magnification (sales pitch!)

  • Notice: aperture size, optical quality, portability, focal length or f number is also important.

  • Consumer research: Astronomy, Sky & Telescope, and Astronomy clubs like NCA and NOVAC.


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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 two basic designs of telescopes?

    • Refracting telescopes focus light with lenses

    • Reflecting telescopes focus light with mirrors

    • The vast majority of professional telescopes are reflectors


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What have we learned?

  • What do astronomers do with telescopes?

    • Imaging

    • Photometry

    • Spectroscopy

    • Timing

    • Interferometry (mainly done in the radio, will eventually be done in the visible and in the far future in the x-ray and the gamma rays perhaps.


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6.3 Telescopes and the Atmosphere

  • Our goals for learning

  • How does Earth’s atmosphere affect ground-based observations?

  • Why do we put telescopes into space?


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How does Earth’s atmosphere affect ground-based observations?

  • The best ground-based sites for astronomical observing are

    • Calm (not too windy)

    • High (less atmosphere to see through)

    • Dark (far from city lights)

    • Dry (few cloudy nights)


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Light Pollution observations?

  • Scattering of human-made light in the atmosphere is a growing problem for astronomy


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Twinkling and Turbulence observations?

Turbulent air flow in Earth’s atmosphere distorts our view, causing stars to appear to twinkle

Same star viewed with Hubble Space Telescope

Star viewed with ground-based telescope


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Adaptive Optics observations?

Rapidly changing the shape of a telescope’s mirror compensates for some of the effects of turbulence

Without adaptive optics

With adaptive optics


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Calm, High, Dark, Dry observations?

  • The best observing sites are atop remote mountains

Summit of Mauna Kea, Hawaii



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Transmission in Atmosphere observations?

  • Only radio and visible light pass easily through Earth’s atmosphere

  • We need telescopes in space to observe other forms


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What have learned? observations?

  • How does Earth’s atmosphere affect ground-based observations?

    • Telescope sites are chosen to minimize the problems of light pollution, atmospheric turbulence, and bad weather.

  • Why do we put telescopes into space?

    • Forms of light other than radio and visible do not pass through Earth’s atmosphere.

    • Also, much sharper images are possible because there is no turbulence.


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6.4 Eyes and Cameras: Everyday Light Sensors observations?

  • Our goals for learning

  • How can we observe nonvisible light?

  • How can multiple telescopes work together?


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How can we observe nonvisible light? observations?

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


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Radio Telescopes observations?

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


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IR & UV Telescopes observations?

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

SOFIA

Spitzer


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X-Ray Telescopes observations?

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

Chandra


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X-Ray Telescopes observations?

  • 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 observations?

  • Gamma ray telescopes also need to be in space

  • Focusing gamma rays is extremely difficult

Compton Observatory



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Interferometry observations?

  • Interferometery is a technique for linking two or more telescopes so that they have the angular resolution of a single large one


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Interferometry observations?

  • Easiest to do with radio telescopes

  • Now becoming possible with infrared and visible-light telescopes

Very Large Array (VLA)


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Future of Astronomy in Space? observations?

  • The Moon would be an ideal observing site


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