Telescopes

1. Telescopes

2. Basic function of a telescope: extend human vision • Collect light from celestial object • Focus light to create image or spectrum of the object • Use larger aperture than the human eye • Expose for longer than the human eye • Observe at wavelengths the eye is not sensitive to (i.e. beyond 400 – 700 nm)

3. Light Hitting a Telescope Mirror huge mirror near a star small mirror far from 2 stars In the second case (reality), light rays from any single point of light are essentially parallel. But the parallel rays from the second star come in at a different angle.

4. Light rays from a distant source, parallel to the "mirror axis" all meet at one point, the focus.

5. Image Formation Light rays from a distant, extended source are all focused in the same plane, the "focal plane" creating an image of the source. "focal plane"

6. Optical telescopes Kinds of optical telescopes: 1) Refractor – uses a lens that light passes through, to concentrate light. Galileo’s telescope was a refractor.

7. Problems with Refracting Telescopes image at focus <-- object (point of light) - Lens can only be supported around edge. - "Chromatic aberration". - Some light absorbed in glass (especially UV, infrared). - Air bubbles and imperfections affect image quality.

8. Chromatic Aberration Lens - different colors focus at different places. white light blue focus red focus Mirror - reflection angle doesn't depend on color.

9. Largest Refracting Telescope Built Yerkes 40-inch (about 1 m).

10. Solution: 2) Reflecting telescope use concave mirror (shape is ideally parabolic), not lens, to focus light. Big, modern research telescopes are reflectors. Gemini South 8-m reflector.

11. focus options or Nasmyth focus

12. Nasmyth focus platforms

13. Reflector advantages • Mirrors can be large, because they can be supported from behind. • No chromatic aberration • Less light lost and fewer image quality problems Largest single mirror built: 8.4 m diameter for the Large Binocular Telescope

14. There are 10 m telescopes, but in segments Keck 10-m telescope

15. Reasons for using telescopes • Light gathering power:  area, or D2 Main reason for building large telescopes! Image with telescope of twice the diameter, same exposure time. Image of Andromeda galaxy with optical telescope.

17. Reasons for using telescopes, cont. • Magnification: angular diameter as seen through telescope/angular diameter on sky • Typical magnifications 10 to 100 • Field of View: how much of sky can you see at once? Typically many arcminutes – few degrees. • Resolution: The ability to distinguish two objects very close together. Angular resolution:  = 2.5 x 105 /D where  is angular resolution of telescope in arcsec,  is wavelength of light, D is diameter of telescope objective, in same distance units. • Example, for D=2.5 m, λ=500 nm,  = 0.05”

18. Two light sources with angular separation larger than angular resolution vs. equal to angular resolution

19. But, “seeing” limits resolution for ground-based optical telescopes * Air density varies => bends light. No longer parallel Parallel rays enter atmosphere No blurring case. Rays brought to same focus. Sharp image on CCD. * CCD Blurring. Rays not parallel. Can't be brought into focus. Blurred image. resolution limited to about 1”

20. fuzziness you would see with your eye. detail you can see with a telescope on ground.

21. Example: the Moon observed with a 2.5 m telescope 1" => 2 km 0.05" => 100 m Hubble Space Telescope image, 0.05” resolution Ground-based telescope image, 1” resolution

22. Detectors Quantum Efficiency = how much light they respond to: • Eye  2% • Photographic emulsions  1-4% • CCD (Charge coupled device)  80% • Can be used to obtain images or spectra CCDs also provide data directly in digital form – easier to process.

23. Photographic film CCD Same telescope, same exposure time!

24. Spectrographs: light spread out by wavelength, by prism or “diffraction grating”

26. Some future optical telescopes Large Synoptic Survey Telescope (LSST): 8-m telescope with large field of view (3.5°). Will survey entire sky repeatedly. Site in Chile. Thirty Meter Telescope (TMT): segmented design, like Keck.

27. Radio Telescopes Large metal dish acts as a mirror for radio waves. Radio receiver at prime focus. Surface accuracy not so important, so easy to make large one (surface shouldn’t have irregularities that are larger than 1/16 ). But angular resolution is poor. Remember:  = 2.5 x 105 /D Effelsberg 100-m (Germany) Andromeda galaxy – optical D larger than optical case, but  much larger (cm's to m's), e.g. for  = 1 cm, diameter = 100 m, resolution = 20". Andromeda radio map with Effelsberg telescope

28. Parkes 64-m (Australia) Jodrell Bank 76-m (England) Arecibo 300-m telescope (Puerto Rico) Green Bank 100-m telescope (WV)

29. So how can we get better resolution? • Interferometers – e.g., VLA Use interference of radio waves to mimic the resolution of a telescope whose diameter is equal to the separation of the dishes

30. Interferometry A technique to get improved angular resolution using an array of telescopes. Most common in radio, but also limited optical interferometry. D Consider two dishes with separation D vs. one dish of diameter D. By interfering the radio waves from the two dishes, the achieved angular resolution is the same as the large dish.

31. Example: wavelength = 1 cm, separation = 2 km, resolution = 1" Very Large Array (NM). Maximum separation 30 km (only about 1km in this configuration). Very Long Baseline Array. Maximum separation 1000's of km VLA and optical image of Centaurus A

32. Atacama Large Millimeter Array • 18,000 ft elevation plateau in Chile • USA/Europe/Japan collaboration • Starts observing in 2011 with a few dishes • 66 dishes eventually

33. UNM is building its own array for =3-10m: the Long Wavelength Array (LWA) • Far larger than the VLA, to give same resolution. “Stations” of 256 antennas, to be spread across NM

34. Square Kilometer Array, currently being designed, will be 50 times collecting area of VLA, with baselines to 1000’s of km

35. Optical-to-mm-wave Telescope Sites • Site requirements • Dark skies (avoid light pollution) • Clear skies • Good “seeing”, stable atmosphere • High, dry mountain peaks are ideal observatory sites, for optical to mm waves.

36. USA at night

37. Mauna Kea Observatory, Hawaii Kitt Peak National Observatory, Arizona

38. Radio Telescope Sites • Away from radio interference is most important. Radio astronomy can be done in cloudy weather, day or night.

39. Telescopes in space Pros – above the atmospheric opacity so can work at  impossible from ground, above turbulence, weather, lights on Earth Con – expensive! Repairs difficult or impossible.

40. Spitzer Space Telescope - infrared Infrared allows you to see radiation from warm dust in interstellar gas.

41. Infrared also allows you to see through dust. Dust is good at blocking visible light but infrared gets through better. Trifid nebula in visible light Trifid nebula with Spitzer

42. FERMI – Gamma Ray Telescope Gamma rays are the most energetic photons, tracing high-energy events in Universe such as “Gamma-ray Bursters”. Latest mission is Fermi – now launched and taking good data.

43. Hubble Space Telescope and its successor-to-be: the James Webb Space Telescope Advantage of space for optical astronomy: get above blurring atmosphere – much sharper images. Center of M51: HST (left; 0.05” resolution) vs. ground-based (right; 1” resolution)

44. The JWST Mock-up of JWST Will have diameter 6.5 meters (vs. HST 2.5 meters) – much higher resolution and sensitivity. Will also observe infrared, whereas Hubble is best at visible light. Expected launch 2013.