Unit 3

Telescopes

Optical Telescopes

• Two basic types– Refractors– Reflectors

Refractors

• Uses lenses to gather and concentrate light

Refractors• Advantages:– Refractor telescopes are rugged– Lenses inside the tube is sealed so it rarely needs

cleaning– images are steadier and sharper

• Disadvantages:– chromatic aberration – Lens supported only at the ends, lens will sag – Expensive, heavy

Refractor

Reflectors

• Uses a curved mirror to focus incoming light

Reflectors• Advantages:– Do not suffer from chromatic aberration – Can be made very BIG!– Less expensive, lighter

• Disadvantages:– Optics need more frequent alignment– Open to the outside, need frequent cleaning– Secondary mirror can produce diffraction effects

Reflector

Types of Reflectors• Gregorian

• Newtonian

• Cassegrain

Schmidt-Cassegrain

Telescope magnificationMagnification = focal length of telescope / focal length eyepiece

e.g. focal length of telescope = 1000 mm focal length of eyepiece = 30 mm

magnification = 1000 mm/ 30 mm

= 33

Brightness• The observed brightness of an object is

directly proportional to the area of the telescope mirror, e.g. a 5 meter telescope will have a light gathering capacity 25 times (52) greater than a 1 meter telescope

History of Large Telescopes

• William Herschel (late 1700s)

• Mt. Wilson (1917)– 100 inch

• Hale (1948)– 200 inch

Space Telescopes

• outside the distortion of Earth's atmosphere• extremely sharp images • almost no background light

Space Telescopes

• Hubble

• Spitzer

• Chandra

Kepler Space Telescope

• Designed to discover Earth-like planets

Herschel Space Observatory

•Launched in 2009 by the European Space Agency•Sensitive to Infrared radiation

Image Acquisition & Processing

• Charge-coupled devices (CCDs) computer (very efficient)

Radio Astronomy

How to pick a telescope

• Start by using your bare eyes • Next, move up to binoculars• When you’re ready for a telescope, read/

research

Choosing the right telescope

• Performance• Purpose• Portability• Light Pollution• Price

• All other things being equal, the greater the mirror/ lens diameter the greater the light gathering ability

• Primary Mirror Size Square Inches of Aperture

4" 12 6 28 8 50 10 78 12 113

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