1Class 13The Doppler effect;and an introduction to telescopes The Doppler effect; redshifts and blueshifts Basic principles of astronomical telescopes Atmospheric transmittance The diffraction limit Optical telescopes Telescopes at other wavebandsI : The Doppler shift23 The Doppler effect The wavelength of light is affected by the motion of thesource relative to the observer Source moving towards observer… Light waves compressed Spectrum of object shifted to shorter wavelength Blueshift Source moving away from observer Light waves stretched Spectrum of object shifted to longer wavelength Redshift Quantitatively… if wavelength λ is shifted by an amountΔλ then velocity is given byThis assumesv<<cII : Telescopes & atmospherictransmittance4 Modern astronomers study the Universeutilizing the full electromagnetic spectrum… But…many part of the spectrum are severelyattentuated (or even completely blocked) byEarth’s atmosphere Long wavelength radio waves… reflected by theEarth’s ionosphere mid-infrared to microwaves… absorbed by variousmolecules in the atmosphere Ultraviolet to X-rays… absorbed by various atoms inthe atmosphere Gamma-rays… absorbed through interactions withatomic nuclei in the atmosphereAtacama desert (Chili)56III : Resolution and diffraction limit Definition : The angular resolution of a telescope isthe smallest angular separation x for which the twopoint-like objects (e.g. stars) are still individuallydistinguishable. The resolution of any telescope is limited by thediffraction limit… If the telescope has an aperture size D and is observingradiation with wavelength λ, the diffraction limit is In practice, real telescopes may not reach thediffraction limit due to… Inability to make focusing system (e.g., γ-rays) Imperfections in lensing system (e.g., X-rays) Atmospheric effects (e.g., many ground-based optical)78IV : Optical telescopes Is what way is a telescope better than the human eye? Larger aperture ⇒ gathers more light Larger aperture ⇒ better resolution More sensitive detectors (+spectrometers, detectors thatwork outside optical band etc.) oh yeah… and it magnifies Two main types of optical telescope Refractor… uses lenses to focus incoming light Tend to have a long “focal length” Length of telescope + weight of lenses makes it hard to builda really big refracting telescope (largest has 1m aperture) Reflector… uses mirrors to focus incoming light All large telescopes are reflectors… easier to build largemirrors and can fold optical path (largest has 10m aperture)91011V : Telescopes and observatoriesoutside of the visible band Current astronomical observatories operatefrom long wavelength radio waves (λ≈4m) tovery high-energy gamma-rays (λ≈10-18m) The challenges… Each part of the e/m spectrum presents its ownchallenges when it comes to building a focusingsystem (some wavelengths cannot yet be focused!) Each part of e/m spectrum presents challenges interms of building detectors Many parts of the e/m spectrum require going tospace (or very special places on the Earth) We then need to understand the results…12ParkesVery Large Array (New Mexico)13CARMA (Combined Array for Millimeter Astronomy)SpitzerJames Webb Space Telescope14Hubble Space Telescope(near-IR, optical, and near-UV)15Compton Gamma-Ray