8 Chapter 8 – How does a telescope work?
OpenStax Astronomy Chapter 6: Astronomical Instruments
Optical Telescopes
I. Telescopes
A. = Area that measures how much light is gathered
1. Size of , primary lens or mirror
2. Often given as the “size” of the
a. Example: a 1 meter telescope has a 1 meter
3. aperture gathers more light, see stars
B. : distance between lens or mirror and the image (eye)
1. focal length give a image, more magnified
II. – use
A. Large lens sits at opening to sky
1. Size of primary lens = size of telescope =
B. Lens (bends) incoming light
1. Curved lens allows light to be at eye or camera
C. Overall size limited by of big lenses and
1. Focal length is the same as the length of the
2. Refractor: 40 inch primary, 62 foot focal length, Yerkes Observatory (Wisconsin, 1897)
III. – use
A. mirror sits at of telescope
1. Size of = size of telescope = aperture
2. Reflects light toward mirror
B. Secondary – small mirror at , light back through telescope to eye
C. is the path of light reflecting off mirrors before reaches the eye
1. Allows for telescopes with focal length
D. All major telescopes are reflectors
1. Largest Reflectors: primary mirror
a. Example: Keck Observatory (Hawaii, 1992), 2 telescopes, 10 meter/33 foot primary, 57 foot focal length, 80 feet tall
2. and easier to build than refractors
3. Can use thin, light-weight or segmented for primary
a. Example: James Webb Space Telescope, primary with segmented mirror
What is one reason reflecting telescopes are designed with a secondary mirror in addition to a primary mirror?
A. To increase the aperture size and see brighter objects
B. To make the telescope shorter and easier to move around
C. To decrease the focal length and magnification
D. To allow for a sharper focus
CCDs
IV. – Detects light at the end of the telescope to make an
A. CCD = , better known as a digital
B. Silicon wafer divided into
1. First astronomical CCD: 100,000 pixels, Your phone: million, Research telescopes today: million or more
C. Light entering telescope on the CCD by lenses or mirrors
D. When a photon hits a pixel, it creates a
E. Charge becomes an electronic
F. Computer to interprets signals from the CCD by to form a picture
Why do astronomers want to build telescopes with bigger apertures?
Radio Telescopes
V. Telescopes
A. Use large to collect waves and direct them toward
1. Radio waves have wavelengths (inches to thousands of feet)
a. FM radio (90 MHz): about 10 feet
2. Need a collecting area (dish) for long waves
B. collects radio waves and sends signal to
C. Typical radio telescope dish is across
1. Largest Radio Telescope: Green Bank 100 meter (330 feet) in West Virginia
2. Largest Radio Telescope: 500 meter (1600 feet) FAST in China, built into the hills
Earth’s Atmosphere and Space Telescopes
VI. Earth’s and Light
A. Objects in space “shine” at wavelengths of the EM spectrum
B. The atmosphere a lot of this light
1. Light can be by atoms and molecules
C. light and most waves are blocked
1. Visible light can be “blurred” by bouncing off molecules in the atmosphere (why stars )
D. are almost completely
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- Need to use telescopes (satellites)
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E. is blocked by vapor in atmosphere
1. Need to go to upper atmosphere with or balloons or into space
Why would astronomers want to build telescopes on high mountains?
You are super rich and want to build a telescope. Which one is the best value for your money? Assume space telescopes are more expensive.
Tutorial Activity – TMT Video Response
Watch this video on the controversy surrounding building a new thirty meter telescope in Hawaii: https://www.youtube.com/watch?v=onERw0-ixLA
Video Response: Write at least 5 sentences based on the following prompt. Full credit is based on the length and content of your response. Responses should be thoughtful and respectful.
Imagine you are either a Native Hawaiian OR an astronomer. What would you say to the opposing side?
