Topic 9: The atmosphere Arne Henden Director, AAVSO

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

The life story of a photon 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

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

temp

Model atmosphere

Air mass

Across a 15 arcmin field

Peterson&kieffaber 1973

Sky at H-band credit: CTIO

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

Extinction vs wavelength

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

Rayleigh scattering

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)

PEP measurements near sunset

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 ; tek1k)

Atmospheric emission

Prescott aurora March 31, 2001

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

Weather statistics/monitoring All-sky cameras now inexpensive; give you a handle on clouds, especially for automated systems Typical southwest 30/30/30

World insolation map

U.S. insolation map

Good site comparisons

Scintillation C9KnsGghttp:// 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.

Scintillation Radu Corlan tables:

Scintillation

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