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Lecture 7: Observing from Space

Lecture 7: Observing from Space. eyes only sensitive to photons with λ ~ 350 – 750 nm but Universe looks very different at other λ ! e.g. Objectives: Advantages of observing from space Disadvantages of observing from space Importance of exploiting full electro-magnetic spectrum.

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Lecture 7: Observing from Space

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  1. Lecture 7: Observing from Space • eyes only sensitive to photons with λ ~ 350 – 750 nm • but Universe looks very different at other λ ! e.g. Objectives: Advantages of observing from space Disadvantages of observing from space Importance of exploiting full electro-magnetic spectrum PHYS1005 – 2003/4

  2. PHYS1005 – 2003/4

  3. Electro-Magnetic Spectrum • Light has: • wave-like properties λע = c • particle-like properties (photons, E = hע) • usually measure photon E in electron-volts (eV) • (1eV = 1.6 x 10-19 J = E to raise one electron through 1V potential) • e.g. takes 13.6eV to ionise H (from its ground state) •  photons with E > 13.6eV strongly absorbed by interstellar gas •  Galaxy almost opaque for λ just below 91 nm PHYS1005 – 2003/4

  4. Advantages of Observing from Space: • No atmospheric absorption • No image motions (“seeing” or blurring) • No sunlit sky or “weather” • Lower sky background • air glows ≡ one 22nd mag star per square arcsec •  less (but not 0) in space (why?)  detect fainter stars • especially important in IR (at 10μ everything glows at T = 300K  like observing with the lights on!) • No atmospheric refraction (bending) of light • light bent by refraction by 0.5o at horizon! • limits positional astronomy for parallaxes • Can get closer to target (solar system only!) • No mechanical vibration (essential for gravity wave expt) • Avoid terrestrial interference (radio) Additional reading: Kaufmann (Chap. 5), Zeilik (Chap. 9) PHYS1005 – 2003/4

  5. Disadvantages of Observing from Space: • Expense! Hugely expensive e.g. HST ≈ £2B, 8m VLT ≈ £0.1B! • Radiation background. High (particle and radiation) background damages detectors, causes spurious events. • Maintenance. Difficult, dangerous, expensive, sometimes impossible. e.g. HST, JWST • Long development time. Space equipment often out-dated by time of launch. e.g. original HST • Risk! Things can go horribly wrong! e.g. CLUSTER, Challenger, Columbia. PHYS1005 – 2003/4

  6. 1) Atmospheric absorption effects: • Earth’s atmosphere allows a few spectral windows (optical, IR, radio) Note loss of UV, X, γ-ray (λ < 320 nm) and far-IR (>10μ) PHYS1005 – 2003/4

  7. 2) Atmospheric Motions (“Seeing”) • seeing limits resolution to ~ 0.3 – 1 arcsec • cf diffraction limit for VLT 8m at λ ~ 500nm is 0.016 arcsec! •  development of HST • N.B. higher angular resolution  can detect fainter stars! • e.g. HST has reached close to V = 30 PHYS1005 – 2003/4

  8. 3) No sunlit sky or weather • no air to scatter sunlight  can observe in “daytime” • can observe for » 8 hours (e.g. Chandra) • no clouds but beware: PHYS1005 – 2003/4

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