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This SpaceX tweet (found in CNET's SpaceX camera captures incredible view of rocket part returning to Earth) shows a video taken by a camera in one half of the fairing on the recent STP-2 mission and it says:

View from the fairing during the STP-2 mission; when the fairing returns to Earth, friction heats up particles in the atmosphere, which appear bright blue in the video

I made a montage of four screen shots near the end of the video, where a purple donut appears. Combined with the patterns behind it it tends to make what looks like a peace sign.

What causes this purple donut?

SpaceX tweet https://twitter.com/SpaceX/status/1146546495241371649


update: Here's the new 05-July-2019 Scott Manley video. He sidesteps the donut, but he does mention that he suspects the IR filter is removed from SpaceX color engineering cameras, so near-infrared (but not thermal IR) results in a purplish color.

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  • $\begingroup$ Could this simply be the reflection from a diffuser of a ring light used to illuminate the parachute and it's line? The camera and ring light may be in the same enclosure causing some reflections? The purple donut does have 4 bright spots which are possible led positions. $\endgroup$
    – GittingGud
    Jul 4, 2019 at 8:44
  • $\begingroup$ @GittingGud it does look a lot like that, it's remarkably in-focus for something so close-range, but then with all that light then they can then use a high f/no lens with large depth of focus. Presumably the light is white, but the antireflection coating of the glass could have a strangely-shaped reflectance curve. (Thorlabs antireflection coating data for example broadband visible 350 - 700 nm: i.stack.imgur.com/0yWrH.gif) $\endgroup$
    – uhoh
    Jul 4, 2019 at 8:52
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    $\begingroup$ I assumed it was a reflection of the camera lens itself in a secondary protective filter above the camera. It relative brightness in the image does seem to change as you would expect based on the brightness of the background and illumination of the inside of the fairing. $\endgroup$ Jul 4, 2019 at 9:38
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    $\begingroup$ @uhoh I would've assumed that the intensity of the reflection is constant only the light sensitivity of the camera isn't. The same as you cannot see your own reflection of a window in a car/train (with interior lighting) in the sunlight but if you drive into a tunnel you see crystal clear and bright reflections. And if it is the reflection of the camera lens itself you would still need a light source for a reflection. $\endgroup$
    – GittingGud
    Jul 4, 2019 at 10:59
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    $\begingroup$ @GittingGud: it's something like that, yes. Notice that it is faint to nonexistent when there's sun in the lens or a light show going on, but becomes clear when the camera adjusts for the dimmer parachutes. It's an internal reflection of some kind, possibly a reflection of the lens housing of the camera itself. $\endgroup$ Jul 7, 2019 at 11:18

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