Corona is very hot (millions of degrees) as opposed to 6000 of the Sun's surface, therefore it has higher contrast over Sun's surface if you go to shorter wavelengths. The reason corona is still visible from Earth is because it you mask the main solar disk (during the eclipse).

It might be that the surface is much brighter in visible light than the corona rather than the corona emits no visible light (as anyone who witnessed the recent total solar eclipse can attest). Since the corona is made up of rarefied high energy particles I would expect it to emit less total, but more short wavelength light.

I’m guessing they mask out the edges. Maybe for exposure reasons or something?