PowerPoint Presentation
Topic 9: The atmosphereArne HendenDirector, AAVSO
[email protected]
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
