Telescopes – Then and Now

1. Telescopes – Then and Now Pgs. 18 - 23

2. Optical Astronomy • A telescope is an instrument that collects electromagnetic radiation from the sky and concentrates it for better observation. • An optical telescope collects visible light for closer observations. • A telescope will use two lenses: one is objective for collecting light, the other is the eye piece which magnifies the size of the object.

3. Refracting telescopes • Refracting telescopes use a set of lenses that gather and focus light. • The objective lens is curved so that light is bent toward the eye piece. • The eye piece then allows the viewer to see the distant object. • The larger the objective lens is, the more you can see. • However, if it is too big the lens will sag and distort the image. • Refracting telescopes are not used by professional astronomers.

4. Reflecting Telescopes • Reflecting telescopes use curved mirrors to gather and focus light. • The telescope collects the light and focuses it to a focal point using a mirror to project that image to the eye piece. • Mirrors can be made very large. • Mirrors are polished so their images don’t show flaws. • Mirrors also reflect all colors of light to the same place, while refracting lenses don’t.

5. Very Large Reflecting Telescopes • Some reflecting telescopes at observatories around the world use more than one mirror to collect their image. • All the mirrors work together to focus the light on the same focal point. • Keck Observatory in Hawaii uses two telescopes that each have 36 hexagonal mirrors working together.

6. Optical Telescopes and the Atmosphere • The light gathered by telescopes on Earth is affected by the atmosphere. • The atmosphere blurs the light and has light pollution from large cities. • The optimal place for optical telescopes are in dry areas high in elevation. • Here water vapor is low and atmosphere density is low. • The image produced by these telescopes are much clearer.

7. Optical Telescopes in Space • In order to avoid the interference of the atmosphere all together, scientists put their telescopes in space. • The Hubble Telescope in space only has a mirror of 2.4 meters, but can produce images that are as good or better than the largest optical telescopes on Earth.

8. Non-Optical Astronomy • In 1800, William Herschel discovered a form of radiation that was invisible, called infrared. • We observe this as heat. • James Clerk Maxwell found that visible light was made up of different wavelengths to produce the colors we see. • In fact, he found that light was a small part of the electromagnetic spectrum.

9. Electromagnetic Spectrum • The electromagnetic spectrum is made up of all the wavelengths of electromagnetic radiation. • Humans can only see visible light. • The electromagnetic spectrum is blocked to us by the Earth’s atmosphere. • Only infrared, visible light, some UV, and radio waves are allowed through. • The rest are blocked.

11. Night Sky through different eyes • Astronomers are interested in all forms of electromagnetic radiation because different objects give off different wavelengths. • A different telescope needs to be used in order to collect each type of electromagnetic radiation.

12. Radio Telescopes • Radio telescopes receive and focus radio waves. • They need to be much larger than visible light telescopes because radio wavelength are about 1 million times longer than visible light waves. • The Arecibo radio telescope is 305 meters across.

13. Linking Radio Telescopes Together • Just like an optical telescope can use several mirrors, radio telescopes can be linked together to work like one giant radio telescope. • The Very Large Array (VLA) is a set of 27 radio telescopes linked together over 30 km. • The larger the area they link, the more data they can collect.

14. X-ray Vision • As you know, some of the Electromagnetic spectrum is blocked by Earth’s atmosphere. • To detect these blocked waves, scientists have put special telescopes into space. • A different one is used to detect UV, gamma rays, and X-rays.