Review Worksheet



Name __________________________________ Date ____________ Per. ______

Unit 4 Telescopes: Review Worksheet Solutions

1. Draw a sketch showing how your eye focuses light. Label the lens, focus, retina and focal plane.

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2. Describe the advantages CCD cameras have over the human eye and over film.

CCD vs film: CCDs are more sensitive to light so they detect higher percentage of photons, have wider range so they can record both dim and bright light at the same time, and the digital image can be manipulated to bring out detail.

CCD vs eye: CCDs record images more reliably than the eye & detailed drawings based on observations (think Galileo), they have a longer exposure time, the exposure time can be controlled, and they can “see” a greater range of wavelengths.

3. What are they two key properties of a telescope and why is each important?

Light collecting area: the larger the surface area of the telescope mirror the more light can be collected.

Angular resolution: the better the angular resolution is the more detail we can see in the image.

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4. What is the diffraction limit, and how does it depend on the telescope’s size and the wavelength of light being observed?

This is the natural, physical limit on the best resolution a given telescope can have. The behavior of light (interference) puts a limit on the angular resolution. There’s less interference with a bigger telescope, so a larger telescope has a smaller diffraction limit.

5. How do reflecting telescopes differ from refracting telescopes? Which is more commonly used by professional astronomers and why? Sketch how each forms an image.*

These telescopes differ in how they focus light. Reflecting telescopes are more commonly used by professional astronomers because large mirrors are easier to support than large lenses.

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6. What are the three basic categories of astronomical observation and how is each conducted?

Imaging: Telescope functions like a camera. Take pictures of the sky

Spectroscopy: Breaking light into spectra using a spectroscope/diffraction grating.

Timing: Measuring how light output varies with time.

7. What do we mean by spectral resolution? Does telescope size affect spectral resolution?

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Spectral resolutions measures how fine the details are that we can see ina spectrum. With better spectral resolution we can make out features in the spectrum that are closer together in wavelength. High spectral resolution requires longer exposures than we need for images.

8. Describe how Earth’s atmosphere limits astronomical observations.

The effects of the atmosphere include:

– twinkling due to turbulence

– light pollution (man – made light is scattered)

– blocks all but visible and radio wavelengths

9. Explain how adaptive optics works.*

Adaptive optics corrects the distortions caused by the atmosphere by changing the shape of a deformable mirror to compensate for the distortions. In order make the corrections, astronomers have to monitor the a bright star in their field of view. If no bright star is visible, they have to make an artificial star using a laser. The laser excites atoms high in the atmosphere which give off light creating a “star”. A computer monitors the distortions of the star’s image and calculates the changes that have to be made to the shape of the mirror to compensate.

10. Explain what an interferometer is and give an example.*

An interferometer is a system of two or more telescopes that are linked in order to combine their signals and therefore achieve a better angular resolution. There are several well known interferometers including the VLA radio telescopes in New Mexico and the Twin Keck optical telescopes in Hawaii.

11. Problem: Two binary stars are separated by a distance of 350 million kilometers. The system is located 35 light-years away. Can we study the stars individually with the Hubble Space Telescope’s angular resolution of 0.05 arc second?*

separation distance = 350,000,000 km

distance = 35 ly

convert distance to km

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Since Hubble’s angular resolution is smaller than this it will have no problem with seeing the stars.

12. For each major wavelength band (visible, IR, UV…etc.) describe what type of telescope is used (give examples).

Radio – like a satellite dish but very large (10 m diameter), ex Arecibo Radio telescope

Infrared – can be focused with mirrors like visible light, ex Spitzer, SOFIA

UV – t can be focused with mirrors like visible light, ex Hubble

X-Ray – need special “nested” mirrors so that light can come in at low angles and gradually funneled to a focus, ex Chandra,

Gamma Ray – light can’t be focused, requires massive detectors to capture the photons because they easily pass through most materials, ex. Compton, Swift, Integral

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