ls12 - AST 3722C summary for lecture on tuesday april 1,...

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AST 3722C summary for lecture on tuesday april 1, class #12 ------- 19.7 - Atmosphere - what does it do? - extincts light - this is a fn of wvln though! - distorts light - turbulence. Here're some more details about the airmass extinction A lot of time at the telescope is spent understanding k for your particular night at your particular location. it varies from day to day, night to night, and is different for different locations on the planet. so to be a good observer you need to measure it every night. in principle it can cahnge over the course of the night but that's usually a small effect -- not always though! what contributes to k? in other words what really is the cause of extinction? specifically let's worry about visible wvln since that's the most common. Rayleigh scattering by molcs in the air -- not really depednent on composition of air, of molecules per se, more on the size of the molecules themselves. But in reality since most of the atmos is N2 and O2 everywhere -- it depends on those molecules and probably doesn't vary too much. what it depends on is atmospheric pressure. it also depends on lambda^-4, so most of the extinction happens at the bluer wavelengths. water and ozone -- from previous handouts, notice that water and ozone absorb some of the visible light reaching us from space. it can be highly variable -- think of humidity. even in dry spots on earth you can have lots of water above that's high up. so while you're humidity might be low right at your telescope, you could still be potentially looking at blobs of wet air above you. aerosol scattering -- solids or liquids suspended in the air. smoke, dust, salt, pollutants - these are all aerosols. this of course is variable too. If there're brush fires, if there's a lot of pollution, all this stuff can change your extinction. how does it change with wvln? weakly, approx like 1/lambda. so again it is worse in blue wavelengths. OK - other concept is atmospheric turbulence. figure 19.7 -- explains what's going on with light waves passing into atmosphere. turbulence messes up the wavefronts. there are temperature gradients in various cells of air, so index of refraction changes a little bit, which changes photon directions. so a blob of air redirects some of the photons. there are actually two effects here: - scintillation or twinkling - rapid changes in brightness. it's not because the extinction is fickering, it's just that photons get redirected.
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- seeing: the image will jiggle around, come in and out of focus, get blurred out. you see a 'seeing disk' instead of airy rings at the diffraction limit. "scintillation" your eye notices the twinkling because the photons don't enter your eye. you're actually losing light. what about a telescope? same thing, although if the telescope is big then you can wind up collecting those redirected photons anyway and so the effect is minimized. the problem is when your collecting area is comparable to
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ls12 - AST 3722C summary for lecture on tuesday april 1,...

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