Chapter 5 Telescopes

5.1 Optical Telescopes

• Refracting Lens

• Reflecting Mirror

• Images can be formed through reflection or refraction

• Modern telescopes are all reflectors because:

  • Light traveling through lens is refracted differently depending on wavelength

  • Some light traveling through lens is absorbed

  • Large lens can be very heavy, and can only be supported at edge

  • A lens needs two optically acceptable surfaces; mirror needs only one

• Chromatic aberration: Light traveling through lens is refracted differently depending on wavelength

Discovery 5-1: The Hubble Space Telescope

• The Hubble Space Telescope’s main mirror is 2.4m in diameter and is designed for visible, infrared, and ultraviolet radiation

More Precisely 5-1: Diffraction and Telescope Resolution

• Diffraction is an intrinsic property of waves, and limits telescope resolution depending on wavelength and size

5.2 Telescope Size

• Light-gathering power: Improves detail

• Brightness proportional to square of radius of mirror

• Resolving power: the power to distinguish objects that are closer together

• Resolution is proportional to wavelength and inversely proportional to telescope size—bigger is better!

5.3 Images and Detectors

• Image acquisition: Charge-couple devices (CCDs) are electronic devices, can be quickly read out reset

• Image processing by computers can sharpen images

5.4 High-Resolution Astronomy

• Atmospheric blurring: Due to air movement

• Solutions:

  • Put telescope on mountaintops, especially in deserts

  • Put telescopes in space

• Active optics: Control mirrors based on temperature and orientation

• Adaptive optics: Track atmospheric changes with laser; adjust mirrors in real time

5.5 Radio Astronomy

• Radio Telescopes:

  • similar to optical reflecting telescopes

  • Advantages:

    • Less sensitive to imperfections (due to longer wavelength); can be made very large

• Advantages of radio astronomy:

  • Can be observe 24 hours a day

  • Clouds, rain, and snow don’t interfere

  • Observations at an entirely different frequency; get totally different information

5.6 Interferometry

• Disadvantage:

  • Longer wavelength means poor angular resolution

• Solutions:

  • Interferometry: Combine information form several widely spread radio telescopes as if they came from a single dish

• Interferometry:

  • Resolution will be that of dish whose diameter = largest separation between dishes

• Interferometry involves combining signals from two receivers; the amount of interference depends on the direction of the signal

• Can get radio images whose resolution is close to optical

• Interferometry can also be done with visible light but is much more difficult due to shorter wavelengths

5.7 Space-Based Astronomy

• Infrared radiation can image where visible radiation is blocked; generally can use optical telescope mirrors and lenses

• The spitzer Space Telescope, an infrared telescope, is in orbit around the Sun

• Ultraviolet observing must be done in space, as the atmosphere absorbs almost all ultraviolet rays

• X-rays and gamma rays will not reflect off mirrors as other wavelengths do; need new techniques

• X-rays will reflect at a very shallow angle and can therefore be focused

• Gamma rays can not be focused at all; images are therefore coarse

5.8 Full-Spectrum Coverage

• Much can be learned from observing the same astronomical object at many wavelengths