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Topic 9: The atmosphereArne HendenDirector, AAVSO arne@aavso.org
1BasicsBeneficial to life, detrimental to astronomyAbsorbs incident lightScatters incident lightEmits radiationProvides weatherDegrades seeing2The life story of a photon . . .1000 lyDeep space
100 kmAtmosphere
1 mTelescope
1 mmFilter (optional)
10 m CCD detector
CCD camera
CCD readout electronics
ComputerLight reddened and absorbed by dust
Blue photons preferentially scatteredBackground photons from skyglow added
Photons absorbed, reflected and scattered in opticsPhotons at edge collide with telescope
Only photons of selected get through
Photons absorbed by dust particles on glassPhotons not recorded by detectorSome pixels more efficient than othersat making electrons
Electrons added by noise in electronicsElectrons from each pixel collected and turned into numbers by ADU
ADU counts used to calculate a magnitudeCredit: D. BoydAtmospheric absorptionBlue edge from ozone (O3)Red edge from water vaporOptical window not completely transparent (extinction, airmass)4temp
Model atmosphere
Air mass
8
Across a 15 arcmin field9
10Peterson&kieffaber 1973
Sky at H-band credit: CTIO
Extinction coefficientsKv = 0.12mag/X at 2300mKbv = 0.16mag/XKub = 0.25mag/XKvr = 0.04mag/XKri = 0.04mag/XAt sea level, Kv = 0.25mag/X13Extinction vs wavelength
Atmospheric scatteringScatters incident lightRayleigh from atmospheric gasesMie from water droplets/particlesNon-selective (large particles, haze)15Rayleigh scattering
Atmospheric emission - 1Twilight 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)17PEP measurements near sunset
Atmospheric emission - 2Night 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) 19Atmospheric emission
21Prescott aurora March 31, 2001
Color of Night SkyLyutyi & Sharov (1980)(B-V) = 0.95 (solar: 0.653) ~K5(U-B) = -0.35 (solar: 0.166) ~B5Late evening
23Weather statistics/monitoringhttp://www.ctio.noao.edu/site/last_r.php All-sky cameras now inexpensive; give you a handle on clouds, especially for automated systemsTypical southwest 30/30/3024World insolation map
U.S. insolation map
Good site comparisons
Scintillationhttp://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.
28ScintillationRadu Corlan tables: http://astro.corlan.net/gcx/scint.txt 29Scintillation
30seeingTypically better on mountaintopBest sites ~0.5arcsecSea level sites ~2-3arcsecMost seeing ground-basedRecommend 2 pixels per fwhm or moreExample: USNO winter31
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