There has been a lot of confusion about this but it is actually overexposed pixels deleted in software. From the lead author of the paper:

https://twitter.com/AndrasGaspar/status/1655680076103716878

Black Hole Sun? There was a camera that "suffered" this problem, but can't remember the model to look it up.

Film in general used to "solarize" in some situations, which would turn some over-exposed areas black.

Maybe, but there was a specific video camera that would do this which earned the black hole sun moniker that I was thinking of. I want to say it was in the DV era, but I was never a connoisseur of that format so it was all just fuzzy memory from hearing/reading about it. I do remember looking at DV shooters with side-eye jealousy at their tiny camcorders while I was heaving full sized broadcast cameras on my shoulder.

Thanks.

Silly me for believing NASA.

Why make overexposed pixels black instead of white? Just to make it extra clear that the pixel data exceeding overexposure threshold is not mistaken for valid data?

I'm baffled as well. I think it's fairly understood that any white pixel is always understood to be probably overexposed.

Astronomy often also deals with wide ranges of pixel exposure such that it can be hard to tell the difference between overexposed and "brighter than the max on the color scale". Better to be extra clear and remove all doubt.

Additional bits of information? You cannot see what shape is that area if it is surrounded with a light yellow and you will miss some info if you'll do it after JPEG conversion.

> Just to make it extra clear that the pixel data exceeding overexposure threshold is not mistaken for valid data?

Saturated pixels are not necessarily "invalid" the way cosmic ray bit flips and stuck pixels are, but setting them to zero does make the other bright but not saturated pixels much easier to identify.

Does the precise line dividing "bright but just barely not saturated" from "bright and saturated" actually matter?

Yes, because a saturated pixel might be 101% full scale, but it might also be a hundred million percent and you have no way to know. The same reason that a camera might substitute zebra stripes or a bright colour for a blown highlight in a preview as opposed to just making it white.

Though the actual value in the data isn't black (0,0,0), it'll be some obviously special value that won't otherwise happen, because black is a valid value.

Because that’s what the person who was dealing with the data did

An mspaint.exe adjustment

What's odd is even when downloading the full res image, the central colors are decidedly not all #000. There is a faint yet distinct radial gradient. It does not appear to be a compression artifact (format is png anyways).

images released to the press sometimes get another, um, aesthetic pass. I bet if you could get ahold of the actual image file it would have the proper sentinel sentinel value (if they're still using fits files it'd be something ridiculous like -9999.)

To be clear, they were using the coronagraph: https://arxiv.org/pdf/2305.03789.pdf

>The 23.0 µm coronagraph of MIRI uses a classic Lyot mask to achieve high contrast imaging near bright sources. The mask is rather large, providing an inner working angle (IWA) of ∼ 3.3λ/D, i.e., ∼ 2. Our data is the first complete reference PSF subtracted set of observations taken with JWST using this mode.

There's some residual light and diffraction effects around the Lyot mask which they have manually deleted.

Some images used a mask and some did not. The 25.5 micron image at the top of the article is one that did not use a mask.

Any idea why it's not a perfect circle? Aberration in focus? And would that be pretty much equivalent to the true size of the star's corona?