We have completed a robotic wide-angle imaging survey of the southern sky ( \delta = +15 \arcdeg to -90 \arcdeg ) at 656.3 nm wavelength , the H \alpha emission line of hydrogen .
Each image of the resulting Southern H-Alpha Sky Survey Atlas ( SHASSA ) covers an area of the sky 13 \arcdeg square at an angular resolution of approximately 0.8 arcminute , and reaches a sensitivity level of 2 rayleigh ( 1.2 \times 10 ^ { -17 } ~ { } erg~ { } cm ^ { -2 } ~ { } s ^ { -1 } ~ { } arcsec ^ { -2 } ) per pixel , corresponding to an emission measure of 4 ~ { } cm ^ { -6 } ~ { } pc , and to a brightness temperature for microwave free-free emission of 12 ~ { } \mu K at 30 ~ { } GHz .
Smoothing over several pixels allows features as faint as 0.5 rayleigh to be detected .