ASTRONOMY 161 Introduction to Solar System Astronomy
Class 11 Telescopes Friday, February 2 Telescopes: Key Concepts
(1) Telescopes use either a lens or a mirror to gather light. (2) The main purposes of a telescope are to gather light and resolve detail. (3) Radio and microwave telescopes use a reflecting dish to focus waves. (4) Telescopes in orbit avoid the distorting effects of the atmosphere. (5) Ultraviolet, X-ray, gamma ray, and most infrared light is visible only from orbit. (1) Telescopes use either a lens or a mirror to gather light.
Refracting telescopes use a lens to bend (or “refract”) the path of light. Light with a short wavelength is bent through a larger angle than light with a long wavelength.
(This is why prisms spread light into a spectrum.) A convex lens (thicker in the middle) focuses light to a point:
Light from a large area is funneled into a small area. Reflecting telescopes use a mirror to change the path of light.
Angle of incidence (I) equals angle of reflection (R), regardless of wavelength. A mirror shaped like a parabola focuses light to a point:
Light from a large area is funneled into a small area. Lenses and mirrors (if shaped correctly) produce an accurate image of an object. Reflecting telescopes use a number of designs: (2) The main purposes of a telescope are to gather light and resolve detail.
A telescope is sometimes called a “light bucket”. Number of photons collected per second is proportional to the area of the lens/mirror: Area = π/4 x D2 where D = diameter of the lens/mirror. A bigger telescope is able to resolve finer detail.
Two stars are resolved if they are seen as two separate points. Smallest angle resolved is proportional to 1/D. BIGGER IS BETTER Larger diameter for your lens or mirror means more light, higher resolution. The world’s biggest telescopes are reflectors (mirrors), not refractors (lenses).
The problem with lenses: 1) Lenses absorb light. 2) Lenses sag. 3) Lenses have chromatic aberration: colors do not focus at same point. The world’s largest refracting telescope:
Yerkes Observatory, Wisconsin 1 meter diameter Completed 1897 Soon to be the world’s largest reflecting telescope (2x8.4 m):
(3) Radio and microwave telescopes use a reflecting dish to focus waves.
Parabolic “dish” of a radio telescope acts as a mirror, reflecting radio waves to the focus. Radio telescopes can be huge, because they do not have to very smooth. Mirrors must be smooth compared to the wavelength of light observed: Visible light: λ ∼ 0.0005 mm Microwaves: λ > 1 mm Radio waves: λ > 100 mm. Resolution of a telescope: θ ∼ λ/D
(4) Telescopes in orbit avoid the distorting effects of the atmosphere. Turbulence in the air makes stars “twinkle” and limits resolution (so called “seeing”). City lights drown out faint stars.
Great idea: place a telescope in orbit, above the atmosphere. Earth at Night Hubble Space Telescope (launched 1990)
Diameter of mirror = 2.4 meters Angular resolution = 0.05 arcseconds (5) UV, X-ray, γ ray, and most IR is visible only from above the atmosphere.
Infrared (IR) light is absorbed by water vapor in the Earth’s atmosphere. The Spitzer Space Telescope, launched in 2003, uses a 0.85 meter mirror.
Ultraviolet (UV) light is absorbed by ozone in the Earth’s ozone layer, 25 km up.
The Hubble Space Telescope could detect UV light with λ = 100 nm. X-rays are absorbed by most molecules in the Earth’s atmosphere.
Chandra X-ray Observatory, launched in 1999, uses “grazing incidence” mirrors to focus X-ray light.
Gamma rays are absorbed by just about everything. Making gamma ray mirrors is beyond current technology.
Compton Gamma Ray Observatory, launched in 1991 and “deorbited” in 2000, had several instruments. Sky: Optical Sky: Radio Sky: Microwaves Sky: Infrared Sky: X-ray Telescopes: Few closing questions:
1) If you double the diameter of a telescope, how much more light will it collect? 2) If you double the diameter of a telescope, how will its resolution change? 3) Which has better resolution: 1 meter diameter optical telescope, or 30 meter radio dish? 4) What is the largest X-ray telescope on Earth? 5) How much ($$$) is a 10 meter optical telescope?