LearnInstruments & observingSome maths
Seeing
The blurring of a point source by atmospheric turbulence, measured as the full width at half maximum of the resulting star image.
Why it matters
It is the ceiling on resolution from the ground. A 2-arcsecond night makes a 200 mm and a 500 mm telescope resolve identically, whatever the Dawes limit says.
In practice
It is also why lucky imaging works: shoot thousands of frames faster than the turbulence changes, keep the few percent that got lucky, and beat the seeing outright.
Rests on
- Aperture and resolving powerThe finest detail a telescope can separate, set by diffraction and therefore by the diameter of its objective alone.
- Extinction and airmassThe dimming and reddening of starlight by the atmosphere, which increases with how much air it has to cross.
3 concepts in the full chain back to first principles.
Leads to
Nothing here builds on it yet.