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Published byGabriel Bates Modified over 9 years ago
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Topic 9: The atmosphere Arne Henden Director, AAVSO arne@aavso.org
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Basics Beneficial to life, detrimental to astronomy Absorbs incident light Scatters incident light Emits radiation Provides weather Degrades seeing
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The life story of a photon... 1000 lyDeep space 100 kmAtmosphere 1 mTelescope 1 mmFilter (optional) 10 μm CCD 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
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Atmospheric absorption Blue edge from ozone (O3) Red edge from water vapor Optical window not completely transparent (extinction, airmass)
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temp
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Model atmosphere
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Air mass
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Across a 15 arcmin field
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Peterson&kieffaber 1973
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Sky at H-band credit: CTIO
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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
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Extinction vs wavelength
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Atmospheric scattering Scatters incident light Rayleigh from atmospheric gases Mie from water droplets/particles Non-selective (large particles, haze)
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Rayleigh scattering
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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)
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PEP measurements near sunset
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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)
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Atmospheric emission
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Prescott aurora March 31, 2001
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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
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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
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World insolation map
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U.S. insolation map
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Good site comparisons
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Scintillation http://www.youtube.com/watch?v=e1Sd C9KnsGghttp://www.youtube.com/watch?v=e1Sd C9KnsGg scint = (0.09 * A 1.75 ) / (D 0.66 * sqrt(2 * t)) Where A is the airmass, D is the aperture in cm and t is the integration time in seconds.
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Scintillation Radu Corlan tables: http://astro.corlan.net/gcx/scint.txt http://astro.corlan.net/gcx/scint.txt
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Scintillation
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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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