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Observational Astrophysics I

Observational Astrophysics I. Nikolai Piskunov Oleg Kochukhov Sara Lindgren. Requirements to pass:. Attend lectures (9 lectures) Do home work and report it in the class Do telescope lab Take an exam: oral or written Book: Kitchin: Astrophysical Techniques, IoP, 5th edition

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Observational Astrophysics I

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  1. Observational Astrophysics I Nikolai PiskunovOleg Kochukhov Sara Lindgren

  2. Requirements to pass: • Attend lectures (9 lectures) • Do home work and report it in the class • Do telescope lab • Take an exam: oral or written • Book: Kitchin: Astrophysical Techniques, IoP, 5th edition Check the schedule carefully!

  3. Why do we need telescopes? Kitchin, “Astrophysical Techniques”, 4th edition, pp 45-89 • Collect photons and create image of a region on the sky (Field of View) • See small details (angular resolution) • Track objects on the sky (Tracking) • Feed light to multiple instruments

  4. Why telescopes can help?(Tracking objects) © Istituto e Museo di Storia della Scienza Telescope mounts: • Equatorial • German mount • Fork mount • English mount • Alt-Azimuth Zelentchuk 6m BTA, Russia

  5. Primary Nasmyth Cassegrain Coudé Focussing light (telescope focii) LBT ? DAO

  6. Telescope mounts: equatorial versus alt-azimuth • Gravity center location and flexurein alt-azimuth mount the support force passes precisely through the gravity center thus canceling any torque: very important for large and heavy telescopes • FoV behavior while trackingIn any focus located on the tube of an equatorially mounted telescope the field of view does not rotate ESO VST ESO 3.6m La Silla

  7. Home work: • Look at the BWT and answer the following questions: • While tracking does one need to change the azimuthal velocity? • If yes, when the azimuthal velocity is largest? • Which way the field of view rotates? • Look at the optical scheme ofcoudé train of an equatoriallymounted telescope shown hereand figure out how the field ofview rotates

  8. Guiding Tracking is not good enough for long exposures… Telescope FOV Possible locations of the pickup mirror Offset guide pickup mirror … we need offset guiding! Telescope focal plane

  9. Collecting photons

  10. Future: • European ExtremelyLarge Telescope(E-ELT, 42m) • The Thirty MeterTelescope (TMT) • The Giant MagellanTelescope (GMT, 24.5m)

  11. Optomechanics • Tracking must be very smooth • Sources of vibration: mechanical noise and wind • Higher vibration frequencies have lower amplitude • Important design goal: make resonance frequencies of the telescope as high as possible (>10 Hz)

  12. Angular resolution ESO OWL 100m design • Angular resolutiongoes aswavelength/diameteror baseline • Interferometers

  13. Little bit of history Galileo Galilei (1564-1642) Telescope description published in Sidereus Nuncius (Starry Messenger) 1610 With permission of the Master and Fellows of Trinity College Cambridge

  14. Refractors Refractors are based on lensesEasy to make, can combine several elements Chromatic aberrations: Largest refractor (1897):Yerkes Obs. 40”, f/19 1.2 m

  15. Reflectors Lots of options: from basic single mirror to Newtonian and Cassegrain

  16. Summary: refractors • Axial symmetry • Combination of multiple elements • Compact • Cheap for small sizes • Chromatism • Difficult making many meter size lenses • Heavy • Impossible to make segmented lenses

  17. Summary: reflectors • Light (high surface/weight ratio) • Large choice of materials (e.g. temperature insensitive materials) • Can be made in large sizes • Can be made segmented • Shape can be adjusted (“flexible” mirrors) • Difficult to combine several mirrors • Hard to make axial systems (vigneting)

  18. Specialized telescopes: • Wide field (Schmidt camera combining reflective and refractive elements) • Infra-red (coatings, thermal control) • Automatic/robotic telescopes (complex telescope control system) • Solar telescopes (heat) • With fixed primary (Large&Cheap) Home work: find one example of each specialized telescope in the list above and prepare a short description of what is different, why and how it is done?

  19. Conclusions: • Binoculars, photo and video cameras, small telescopes – refractors • Intermediate size telescopes – combined reflectors/refractors • Large telescopes - reflectors

  20. Next time… • A bit more about telescope optics • Image distortions (aberrations) • Active optics • Adaptive optics • Coatings of optical surfaces

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