TELESCOPES

• Intercept and concentrate the light from distant objects

• Greatly magnify the image

• Can collect and record light over a period of time (photographic plates, electronic detectors, e.g., video camera-like devices)

• Images can now be electronically intensified, stored, and processed by computers to enhance certain features under study.

Refractors

Objective lens refracts the light.

• Good contrast, but can not make very large (40in. largest built) because lens will sag in the middle

• Chromatic aberration. Faint colored haloes around objects.

Reflectors

• Utilize concave primary mirror to reflect incoming light and bring it to a focus.

• Primary mirror supported f rom behind - can be made very large.

All have secondary obstructions due to additional mirrors in system.

• Some loss of contrast and light (trivial)

• Keck 400 in. (2) Mauna Kea - largest

• The 5m (200in.) @ Mt. Palomar

• Soviet Union - 6m

• Mauna Kea, Hawaii - 13,800 ft. elevation - one of best sites in world

Resolution of Telescopes:

• Separation of closely spaced objects, discernment of fine detail - "Sharpness" of image
• Resolution proportional to diameter of objective

Light gathering power - determine brightness of image - area proportional to area of objective (diameter)2

Photographic speed (f ratio): (Affects exposure time.)

Magnification M

(Not as important as light gathering power)

Very good optics can handle 50x - 10ox per inch of objective diameter under excellent seeing conditions.

Seeing - measure of how steady the atmosphere is

Very good seeing usually 0.5 - 1.0 arcsecond. Bad . seeing 5 arcsecand (or more).

Other Types of Telescopes

• Infra red: ground based and orbital

• Can see cold objects - invisible in visible light

• IRAS- very successful satellite

• X-ray; orbital

extremely hot gasses, black holes

• Ultraviolet: Orbital

solar studies

Radio Telescopes

• 24 hr. operation, many applications, very important

• quasars

• radio galaxies

• mapping the Milky Way - 21 cm radiation CHI regions) in spiral arms, molecular hydrogen clouds

• poor resolution used alone

Arecibo, Puerto Rico - 1,000 ft. dish - largest

• Radio Telescopes are large - receiving antennas would have to be enormous (miles in diameter) to achieve resolution of optical telescopes. But, antennas can be electronically combined -

Interferometer

• V.L.A. (Very Large Array) in New Mexico: 27 - 82 ft. out in "Y" pattern equivalent to as large as 15 mile antenna.