Telescopes

1. Telescopes Chapter 6

2. Optical Telescopes • Used to gather and focus visible light • Two Main Types • Refracting • Reflecting

3. Refracting Telescopes • Uses a large lens to focus light • The lens bends light to create an inverted image. • Yerkes Observatory has largest refracting telescope.

5. Refracting Telescopes Advantages Disadvantages • More resistant to misalignment • Air currents and temperature do not affect the performance because lens is closed inside… steady, sharp images • Chromatic Aberration or “color distortion” • How well light passes through decreases as lens thickness increases • Difficult to make with no imperfections • Glass sags under its own weight

6. Reflecting Telescope • Uses a concave mirror to focus light • Preferred design for modern observatories

8. Reflecting Telescopes Advantages Disadvantages • No chromatic aberration • Can be HUGE • Less expensive • Easy to get out of alignment • Needs to be cleaned frequently • Secondary mirror may cause “star” effect

9. Hale Telescope at Palomar Observatory (California)

10. Reflecting Telescope Designs

12. Coudé • A third mirror deflects the light rays towards instruments that are not attached to the telescope because of weight restrictions. • This arrangement is known as the coude' focus telescope named after a French word meaning "bent like an elbow."

13. Cassegrain • This design has a convenient, accessible focal point. A hole is drilled directly through the center of the primary mirror, and a convex secondary mirror, placed in front of the primary image, reflects light rays back through the hole to an eyepiece for magnifying or to an instrument for recording data.

14. Radio Telescopes Some are made of a mesh material while others are solid

15. Radio Telescopes • Radio telescopes are much larger than optical telescopes because radio wavelengths are much longer than optical wavelengths. • The longer wavelengths means that the radio waves have lower energy than optical light waves. • In order to collect enough radio photons to detect a signal, the radio dishes must be very large. • Both optical and radio telescope reflectors use a parabolic shape to perfectly focus the light to a point. • Increasing the size of the radio dish is also necessary in order to improve the clarity of the radio images.

16. Parkes Observatory • Image of the Milky Way Galaxy taken by radio telescope at Parkes Observatory • The majority of the bright emission seen in the image is from hot, ionized gas, or is produced by energetic electrons moving in magnetic fields. The intensity of the emission is color-coded, with the brightest regions appearing red and the faintest areas appearing purple and black.

17. Interferometer • An interferometer consists of two or more separate telescopes that combine their signals almost as if they were coming from separate portions of a telescope as big as the two telescopes are apart. • Interferometry makes use of the principle of superposition to combine separate waves to create a single, sharper image. • Good for new discoveries. • The Very Large Array is an example.

19. New research carried out allowed astronomers to reconstruct a detailed picture of the disc of matter around a young star.

20. Vocabulary • The following words can be found and defined on pg. 114 of your book • Focal length • Primary lens • Primary mirror • Objective lens • Objective mirror • Eyepeice • Chromatic aberration • Achromatic lenses