Advances in Modern Telescope Design (1)

1. 0 Advances in Modern Telescope Design (1) Modern computer technology has made significant advances in telescope design possible: Segmented mirror 1. Lighter mirrors with lighter support structures, to be controlled dynamically by computers Floppy mirror

2. 0 Advances in Modern Telescope Design (2) 2. Simpler, stronger mountings (“Alt-azimuth mountings”) to be controlled by computers

3. 0 Adaptive Optics Computer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence

4. 0 CCD Imaging CCD = Charge-coupled device • More sensitive than photographic plates • Data can be read directly into computer memory, allowing easy electronic manipulations Negative image to enhance contrasts False-color image to visualize brightness contours

5. 0 Examples of Modern Telescope Design (1) Design of the Large Binocular Telescope (LBT)

6. 0 Examples of Modern Telescope Design (2) The Very Large Telescope (VLT) 8.1-m mirror of the Gemini Telescopes

7. 0 Interferometry Recall: Resolving power of a telescope depends on diameter This holds true even if not the entire surface is filled out. • Combine the signals from several smaller telescopes to simulate one big mirror  • Interferometry

8. 0 Radio Telescopes Large dish focuses the energy of radio waves onto a small receiver (antenna) Amplified signals are stored in computers and converted into images, spectra, etc.

9. 0 Radio Maps Radio maps are often color coded: Like different colors in a seating chart of a baseball stadium may indicate different seat prices, … colors in a radio map can indicate different intensities of the radio emission from different locations on the sky.

10. 0 Radio Telescopes Advantages 1. Can reveal clouds of cool hydrogen because the emit a radio signal. 2.They can penetrate dust clouds that obscure visible light. 3.They can detect the most distant objects in the universe.

11. 0 Radio Interferometry For radio telescopes, resolving power is a big problem since Radio waves are much longer than visible light. Use interferometry to improve resolution!

12. 0 Radio Interferometry (2) The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter. Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very Long Baseline Interferometry)!

13. 0 The Largest Radio Telescopes The 300-m telescope in Arecibo, Puerto Rico The 100-m Green Bank Telescope in Green Bank, WVa.

14. 0 NASA’s Space Infrared Telescope Facility (SIRTF) Infrared light with wavelengths much longer than visible light (“Far Infrared”) can only be observed from space.

15. 0 The Hubble Space Telescope • Launched in 1990; maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000’s • Avoids turbulence in the Earth’s atmosphere • Extends imaging and spectroscopy to (invisible) infrared and ultraviolet

16. 0 Optical Telescopes Astronomers use telescopes to gather more light from astronomical objects. The larger the telescope, the more light it gathers.

17. 0 The Powers of a Telescope:Size Does Matter • Light-gathering power: Depends on the surface area A of the primary lens / mirror. • Area depends on diameter. • More light collected means you can see fainter objects. D

18. 0 The Powers of a Telescope (2) 2. Resolving power: the ability to make out fine detail. The larger the telescope the more resolving power.

19. 0 Seeing Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images. Bad seeing Good seeing

20. 0 The Best Location for a Telescope Far away from civilization – to avoid light pollution

21. 0 The Best Location for a Telescope (2) Paranal Observatory (ESO), Chile On high mountain-tops – to avoid atmospheric turbulence (seeing) and other weather effects

22. 0 The Powers of a Telescope (3) 3. Magnifying Power = ability of the telescope to make the image appear bigger. The magnification depends on the ratio of focal lengths of the primary mirror/lens (Fo) and the eyepiece (Fe): A larger magnification does not improve the resolving power of the telescope!

23. 0 Refracting/Reflecting Telescopes Refracting Telescope: Lens focuses light onto the focal plane Focal length Reflecting Telescope: Concave Mirror focuses light onto the focal plane Focal length Almost all modern telescopes are reflecting telescopes.

26. 0 Secondary Optics In reflecting telescopes: Secondary mirror, to re-direct the light path towards the back or side of the incoming light path. Eyepiece: To view and enlarge the small image produced in the focal plane of the primary optics.

27. Limitations of Refractors • Lens must be made of high-quality glass with no imperfections • Larger lenses weigh a lot; lenses can be supported only around their rims • The lens will sag under its own weight • So, the largest refractor telescope anyone has built is about 1.1 m.

28. Usefulness of Reflectors • Objective (mirror) can be made of many things, even plastic or metal • Only one side of the mirror is polished • Mirror can be supported from its entire back, not the rim only • So, all large modern telescopes are reflectors because they are easier to make and maintain.

29. A Prime Focus Reflector (some light blocked)

30. A Newtonian Reflector (secondary mirror)

31. A Schmidt Cassegrain Reflector (hole in primary mirror) Example: Hubble Space Telescope

32. 0 Traditional Telescopes (2) The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona)