TELESCOPES

OPTICAL TELESCOPES collect light

1. Reflectors

• (nearly) parallel rays from far away are reflected from a mirror and pass through a focus, or focal point
• usually have secondary mirrors to deflect the light to a more convenient location: Newtonian, Cassegrain, coudé

2. Refractors

• rays are bent (refracted) through focus by a glass lens

problems with refractors:

• different colors (wavelengths) bent by different amounts: chromatic aberration
• other forms of aberration
• deform under their own weight
• absorption of light
• have two surfaces that must be optically perfect

Images form when light from different points in space is focused at different points

• The image is usually small, so magnify with a lens
• Light can be monitored by eye, camera, photometer, spectrometer, CCD....

Size of telescopes

• A larger telescope gathers more light (more collecting area)
• angular resolution is limited by diffraction of light waves; also improves with larger telescope size. (Atmospheric turbulence also limits seeing, i.e. reduces resolution)

Largest earth-based telescopes:

• Hale telescope, Mt. Palomar, CA; 5 m (200")
• Russian 6 m telescope (not very high quality)
• Keck telescope, Mauna Kea, HI; 36 x 1.8 m hexagonal mirrors; equivalent to 10 m. Above most of atmosphere. Operating since 1992.

To escape atmospheric limitations:

• Hubble space telescope: 2.4 m, since 1990.
• use computer to correct optics during light gathering adaptive and active optics

RADIO TELESCOPES

Reflect radio waves with large metal dishes (like satellite receiver dishes)

• Radio emissions from Milky Way first detected by Karl Jansky, 1930's
• Radio telescopes have been operating since the 1950's.
• Largest radio telescopes: Green Bank, WV; Aricebo, PR

Disadvantages:

• Large wavelength of radio waves means diffraction is more important; angular resolution of radio telescopes is poor.
• Can be improved by interferometry; two distant radio telescopes operate as one very large radio telescope

Advantage: tunable over wide range of frequencies

ASTRONOMY AT OTHER WAVELENGTHS

• IR (infrared) since 1983; UV (ultraviolet) since 1978; X-ray, gamma ray since late 70's.
• typically require going into space because of atmospheric absorption
• IR, UV can be focused; X-rays, to some extent; gamma rays, not at all.

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