Introduction to Astronomical Telescopes

1. Introduction to Astronomical Telescopes • Main functions of an astronomical telescope • Types of astronomical telescopes • Components of a typical optical telescope • Choosing a telescope • Maintenance of telescopes

2. I. Main functions of telescopes

3. Main functions of telescopes • ‘telescopes’: collect signals from afar • ‘detectors’: receive the signals and transform them into meaningful information An astronomical CCD camera A refractor

4. diameter d optical axis incoming light objective Focus焦點 Focal length f 焦距 Collection of distant signals • Signals become weaker for longer distance  spread out over larger area • collect and concentrate signals from a large area • Image intensity (影像亮度)  collection area  (diameter of main mirror/lens)2 焦距比 Focal ratio= f/d

5. diameter of human iris瞳孔 diameter of main mirror Interference patterns telescope Basic telescope characteristics • Light collection power  (d/7mm)2 eg. 0.3m telescope in CC Observatory: images are ~2000x brighter than naked eyes • Resolution: finite aperture diffraction of light entering the telescope  point source diffraction rings

6. 解像度 Resolving power of a telescope telescope Point sources Diffraction rings 繞射環 for =550 nm Resolving power: angular separation of two stars at which their diffraction disks half cover each other eg. theoretical resolution of 0.3m telescope ~ 0.47” (1”=1 arcsecond=1/3600o=一角秒)

7. Atmospheric seeing (大氣穩定度) • atmospheric turbulence distortion of starlight, twinkling of stars best resolution on ground ~ 0.5 arcseconds only • Stellar image smeared and dances around • Typical seeing in Hong Kong: 2-3” http://op.ph.ic.ac.uk/ao/overview.html#adaptive

8. Atmospheric seeing M20 M27 poor seeing better seeing

9. Common misconceptions • A.: How far can one see with this telescope? • Q.: In principle  even with naked eyes. In practice how far one can see depends on the object brightness, atmospheric conditions, detector sensitivity and resolution, telescope,… • A.: How much can one magnify with this telescope? • Q.: In principle , depending on the focal length of the eyepiece used (m=f.l.(telescope)/f.l.(eyepiece)). In practice, useful magnification depends on the object brightness, atmospheric conditions, detector sensitivity and resolution, telescope,…; typically < 200-300

10. Common misconceptions For telescopes, the bigger is better In ideal situations, yes. image brightness  d2 resolving power  1/d In practice, many problems with big telescopes: optical quality, heavy mounts, precision of mounts, more seeing problems, thermal equilibrium (eg. ex-Soviet telescope), often limited by atmosphere

11. II. Types of Astronomical telescopes

12. Types of telescopes • Types of signals collected: Electromagnetic waves (電磁波) - visible light (optical光學望遠鏡) - radio (射電望遠鏡) - Infra-red (紅外線), microwave (微波), X-ray, gamma ray (伽瑪射線), … Non-EM waves - neutrinos (中微子) - gravitational waves (重力波) - cosmic rays (宇宙射線)

13. Keck I and II 10m telescopes Photos from Keck Observatory webpage http://www2.keck.hawaii.edu:3636/index.html

14. 305m diameter Arecibo Radio Telescope, Puerto Rico photo credit: NAIC-Arecibo Observatory

15. Pulsar planets (脈沖星行星系統) • First extrasolar planetary system discovered (1991): 2 Earth-size, 1 Moon-size • Make use of extraordinary precision of pulsar period to detect small periodic deviations fit planetary orbits Wolszczan, A. & Frail, D., 1992, Nature, Vol. 255, pg. 145, "A Planetary System around the Millisecond Pulsar PSR1257+12". http://www.astro.psu.edu/users/pspm/arecibo/planets/planets.html

16. Radio waves interferometry VLA, New Mexico Photo credit NRAO Photo credit KH Kong http://www.aoc.nrao.edu/vla/html/VLAhome.shtml

17. Examples of recent discoveries: X-ray astronomy • X-ray sources: high temperature/energy • X-ray telescope: must be out of atmosphere • High energy astrophysics: neutron stars, black holes, violent collisions/explosions, … Chandra X-ray Observatory http://chandra.nasa.gov/

18. http://chandra.nasa.gov/ figures credit NASA Chandra X-ray Observatory • Launched in 1999; high orbit (1/3 to Moon) • resolution ~0.5”, 8 nested cylindrical mirrors 1.2m diameter • Improvement: 8x in resolution, 20x in sensitivity

19. Visible light Radio http://chandra.nasa.gov/ photos credit NASA X-ray 3C295 The majority (90%) of ‘ordinary’ matter in the universe is in the form of hot gas surrounding galaxies

20. Blackhole (黑洞) Observation of X-ray hotspots on accretion disk helps to map out the curved space-time near a blackhole M82 http://chandra.nasa.gov/ photos credit NASA

21. Explosions Eta Carinae figures credit NASA http://chandra.nasa.gov/

22. penetrating Signals from stellar center Originated from stellar cores Neutrino Observatory • Neutrinos: weakly interacting, almost massless, produced in nuclear fusion • Neutrino detector: a large tank of water with PMT’s lined up on wall • Discovered deficit of solar neutrinos and neutrino oscillations

23. 50,000 tons pure water 41.4m 39.3m 11,200 PMT’s Super Kamiokande http://www-sk.icrr.u-tokyo.ac.jp/doc/sk/

24. III. Components of a typical optical telescope - Optical tubes (OPT) - Mount

25. Optical tubes (鏡筒) • Popular designs: refractors (折射式望遠鏡) – use lens to collect and focus light reflectors (反射式望遠鏡) – use mirrors collect and focus light catadioptrics (折反射式望遠鏡) – use both mirrors and correcting lens

26. Chromatic aberration Can be fixed by using expensive glass: APO refractors Optical axis Incoming light objective Focus for red light Focus for blue light Refractors (折射式望遠鏡) 小白 • Pros: clear aperture, sharp images • Cons: expensive, chromatic aberration (色差), long/heavy 6“ f/7 APO refractor

27. aperture reflected Optical axis secondary Incoming light objective mirror eyepiece Focal length Reflectors (反射式望遠鏡) • Pros: inexpensive, no chromatic aberration • Cons: central obstruction, long/heavy, tube currents, alignment of mirrors critical 5“ f/6 Newtonian reflector 傻仔

28. reflected aperture Optical axis focus Incoming light Corrective lens Main mirror Catadioptrics (折反式望遠鏡) • Pros: compact, inexpensive • Cons: central obstruction, image quality 小黑 11“ f/10 Schmidt-Cassegrain

29. 圖二十:目鏡:放大倍數=d/e 目鏡 焦點 入射光線 主鏡 目鏡焦距 物鏡焦距 e d Accessories (配件) • Eyepieces (目鏡): ½ of the telescope! commonly used: 1.25” or 2” focal length (shorter = larger mag.) FOV typically ~50o (/mag = true FOV) 視角

30. barlow diagonals Focal reducer Accessories • Barlows (巴羅鏡，放大鏡）- increases focal length (mag.) • Focal reducers (縮焦鏡) – reduces f.l. • Diagonals (稜鏡) – for convenience only

31. White-light filter Filters (濾鏡) • Select band-passes for enhancing images • Eg. Solar atmosphere: mostly Hydrogen →H-alpha filter Sun in H-alpha 2002/11/07

32. Prominences 日珥

33. Sunspots (黑子) Sunspot groups on 25/10/03 as seen from CUHK 101mm f/5.4 refractor + H-alpha filter Taken by MCC 180mm f/8 refractor + white light filter Taken by Delphi

34. Solar flares and sunspots on 30/10/03 70mm f/6.8 refractor + H-alpha filter Taken by MCC

35. Filters (濾鏡) Solar white-light filter Moon filter Deep sky filter Color filters

36. Mounts (支撐系統) • Provide support and control of optical tubes • clock drive: to cancel Earth’s movement • Computer control • Most popular designs: German Equatorial (德式) Fork (叉式) Dobsonian (多普信式)

37. German Equatorial Mount Dec RA Pros: precise Cons: complex, expensive

38. Fork Mount Pros: simple to set up and use Cons: heavy, less precise

39. Dobsonian Pros: inexpensive, simple Cons: not suitable for photography

40. IV. Choosing a telescope

41. Some tips on choosing a telescope • 1. Don’t be greedy on size! Consider portability and convenience of use. 90mm f/14 Maksutov 70mm f/6.8 refractor

42. Some tips on choosing a telescope • 2. What is the main usage? For planetary (bright, easy to find), small refractors are good enough. For deep sky objects (dim, hard to find), need large reflectors. • 3. Consider magnification/field of view For planetary, needs large mag (long f ). For deep sky, needs short f .

43. V. Maintenance of telescopes

44. Some tips on maintenance of telescopes • 1. Don’t rub the optical parts (lens/mirrors)! You would damage the coatings! • 2. Don’t mind dusts. • 3. Keep it dry! Stop using when dews form. • 4. Never touch optical parts! • 5. Recoat mirrors periodically.

45. Introduction to Astronomical Telescopes • Main functions of an astronomical telescope • Types of astronomical telescopes • Components of a typical optical telescope • Choosing a telescope • Maintenance of telescopes