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Topic 9: The atmosphere

Topic 9: The atmosphere. Arne Henden Director, AAVSO arne@aavso.org. Basics. Beneficial to life, detrimental to astronomy Absorbs incident light Scatters incident light Emits radiation Provides weather Degrades seeing. 1000 ly Deep space 100 km Atmosphere 1 m Telescope

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Topic 9: The atmosphere

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  1. Topic 9: The atmosphere Arne Henden Director, AAVSO arne@aavso.org

  2. Basics • Beneficial to life, detrimental to astronomy • Absorbs incident light • Scatters incident light • Emits radiation • Provides weather • Degrades seeing

  3. 1000 ly Deep space 100 km Atmosphere 1 m Telescope 1 mm Filter (optional) 10 μmCCD detector CCD camera CCD readout electronics Computer Light reddened and absorbed by dust Blue photons preferentially scattered Background photons from skyglow added Photons absorbed, reflected and scattered in optics Photons at edge collide with telescope Only photons of selected λ get through Photons absorbed by dust particles on glass Photons not recorded by detector Some pixels more efficient than others at making electrons Electrons added by noise in electronics Electrons from each pixel collected and turned into numbers by ADU ADU counts used to calculate a magnitude Credit: D. Boyd The life story of a photon . . .

  4. Atmospheric absorption • Blue edge from ozone (O3) • Red edge from water vapor • Optical window not completely transparent (extinction, airmass)

  5. temp

  6. Model atmosphere

  7. Air mass

  8. Across a 15 arcmin field

  9. Peterson&kieffaber 1973

  10. Sky at H-band credit: CTIO

  11. Extinction coefficients • Kv = 0.12mag/X at 2300m • Kbv = 0.16mag/X • Kub = 0.25mag/X • Kvr = 0.04mag/X • Kri = 0.04mag/X • At sea level, Kv = 0.25mag/X

  12. Extinction vs wavelength

  13. Atmospheric scattering • Scatters incident light • Rayleigh from atmospheric gases • Mie from water droplets/particles • Non-selective (large particles, haze)

  14. Rayleigh scattering

  15. Atmospheric emission - 1 • Twilight emission lines effect twilight sky flats around 7-10degrees solar depression angle. Mostly Na, but some oxygen. • Main contributor to night-sky brightness is man-made (sodium, mercury, incadescent)

  16. PEP measurements near sunset

  17. Atmospheric emission - 2 • Night airglow (primarily 100km, variable), primarily O (557.7), Na (589.2), O2 (761.9, 864.5), and OH- (mostly near-IR) • Aurora. Mostly O, H, N. (show aurora of 010331; tek1k)

  18. Atmospheric emission

  19. Prescott aurora March 31, 2001

  20. Color of Night Sky • Lyutyi & Sharov (1980) • (B-V) = 0.95 (solar: 0.653) ~K5 • (U-B) = -0.35 (solar: 0.166) ~B5 • Late evening

  21. Weather statistics/monitoring • http://www.ctio.noao.edu/site/last_r.php • All-sky cameras now inexpensive; give you a handle on clouds, especially for automated systems • Typical southwest 30/30/30

  22. World insolation map

  23. U.S. insolation map

  24. Good site comparisons

  25. Scintillation • http://www.youtube.com/watch?v=e1SdC9KnsGg • scint = (0.09 * A1.75) / (D0.66 * sqrt(2 * t)) • Where A is the airmass, D is the aperture in cm and t is the integration time in seconds.

  26. Scintillation • Radu Corlan tables: http://astro.corlan.net/gcx/scint.txt

  27. Scintillation

  28. seeing • Typically better on mountaintop • Best sites ~0.5arcsec • Sea level sites ~2-3arcsec • Most seeing ground-based • Recommend 2 pixels per fwhm or more • Example: USNO winter

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