geckzilla wrote:I suppose the background was masked and excluded from the inversion technique. I wouldn't have suspected that it had been done.
Yes - that seems like a possibility - but in other articles on his technique he does consistently mention "inverting" the image.
I tried some things in photoshop with his own image, and when you invert the raw Ha image, after converting it to grayscale, it takes on most of the qualities of his image except the limb brightening is relatively subdued and the sunspots are glaringly white and look 'wrong' - and go against the desired effect of a glowing, normal looking sun. The area around the sunspots (inverted view) does match pretty well his final image, though, so I no longer think there is some kind of inverted brightness function - but instead I think the sunspots are specifically altered after the inversion.
I think what must be happening is that the image is inverted and stretched a bit, and then the sunspots are manually colored in to turn them black again. This removes the main thing that gives it a boring "negative" look - and instead looks like a normal sun with a lot of detail and a glowing limb - and black, natural looking sunspots.
As an aside - there is extensive literature on measuring and "flattening" the limb darkening - since it can help model the atmosphere of the sun, and flattening is needed for consistent photometry of objects on the surface. But I don't know anyone who previously did the rendering this way - Instead of flattening the limb you make it "glow," but with black sunspots. I think the visual impact is real - but I also think there is an added step to "fix" the sunspots that isn't mentioned and is purely artistic - and makes it hard to write a caption describing the physics of the scene.
The details of the processing are also somewhat confusingly described, with cross-out corrections and a mea culpa, on Phil Plait's blog - but the detail of black sunspots that should be white, if it were a simple inversion, is not mentioned:
http://blogs.discovermagazine.com/badas ... pting-sun/
If it isn't a manual operation on selected sunspots - I think there is still some kind of "fix" in place to make the sunspots dark that is not a simple inversion of the scene.
It's interesting that all you have to do is make the sunspots black after inversion - and now the sun looks fairly normal despite the inversion. If you did the same to the pupils (manually darkened them) in an inverted image of a person's face - I don't think that would work so well - but it would probably help.