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There are many different types of aurora. The same aurora can look different
from different directions. The viewing geometry of an all-sky imager is shown in the figure. A typical auroral
arc is shown in which light is emitted from a thin sheet that is aligned with the magnetic field
which is almost vertical at Halley. If the arc is seen
from the side then it looks like a sheet or "curtain". If the arc is seen
from below then it looks like a "corona".The view of the imager is also
distorted by the fish eye lens. The recorded image is made up of roughly equal angular segments of the night
sky. So, for a given emission altitude light comes from about a 5 km square overhead but from about a 150 km
square at 800 km away from the camera. In other words, aurora occurring a long way away will be squashed up
with worse spatial resolution compared to those occurring much nearer.
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