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At mars.nasa.gov/mars2020/multimedia/raw-images I found Mars Perseverance Sol 258: SkyCam Camera which shows a sky camera (180° FOV fisheye view of the sky) image. I've included it below along with its complete caption.

The caption contains a SkyCam link to a page that does not even mention it.

Terrestrial sky cameras for weather research just let the Sun hit the sensor all day. Point-and-shoot cameras, including cell phones have no problem with the Sun being in the picture (at least for seconds or minutes) and outdoor security cameras are exposed to the Sun regularly. I don't know how but the problem of image sensor damage from even long term pointing at the Sun seems to have been solved.

So seeing this large black annulus in Perseverance's SkyCam's FOV blocking out a large range of elevation angles surprised me.

Question(s):

  1. Why is there a black donut in Perseverance's SkyCam?
  2. Is it always used to block the Sun (sky cameras on Earth don't) ?
  3. How does it work? Does it change inner and outer radius to track the Sun's motion?

NASA's Mars Perseverance rover acquired this image using the SkyCam, an upward-facing camera located on the top deck of the rover. This image was acquired on Nov. 10, 2021 (Sol 258) at the local mean solar time of 08:51:41. Image Credit: NASA/JPL-Caltech/CAB

NASA's Mars Perseverance rover acquired this image using the SkyCam, an upward-facing camera located on the top deck of the rover. This image was acquired on Nov. 10, 2021 (Sol 258) at the local mean solar time of 08:51:41. Image Credit: NASA/JPL-Caltech/CAB

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    $\begingroup$ It's a puzzle. Perseverance is at 18* latitude, so the donut does not correspond to the ecliptic. In its present location, the donut is only blocking the sun because it is morning. $\endgroup$
    – Woody
    Nov 12, 2021 at 3:03

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The skycam is located inside the "black hole" in the middle of the Mars Environmental Dynamics Analyzer as shown in this picture by NASA/JPL: Perseverance MEDA

The full description of this instrument can be found in: J. A. Rodriguez-Manfredi: "The Mars Environmental Dynamics Analyzer, MEDA. A Suite of Environmental Sensors for the Mars 2020 Mission"

Citing from there:

On the window above the optics, an ND coating was added to reduce light by 5 orders of magnitude. This coating was designed such that, for any reasonable tilt and season, the Sun would spend 60 minutes per sol transiting the annulus before and after noon.

"ND" stands for "Neutral Density", i.e. a darkened area that reduces the amount of light by 99.999%. It allows to take images of the sky without being overexposed by the Sun, and to take a picture of the sun itself at the same time: Looking at uhohs posted image, there's a tiny dot in the ring - that's not just a random white pixel, but the Sun itself.

Pointing out Sun in SkyCam picture

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    $\begingroup$ Oh, that is a clever bit of design. $\endgroup$
    – Andrew
    Nov 13, 2021 at 12:31

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