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This excellent answer by @MarkAdler to the question Opportunity's last tau was 10.8; what does that mean and how is tau defined and measured? explains that Curiosity uses its PanCam to image the Sun's disk to perform photometry (measuring the Suns apparent brightness) in order to analyze the opacity of Mars' atmosphere due to dust, and includes the first GIF below from Emily Lakdawalla High tau for Spirit and Opportunity.

I stumbled upon the following second GIF below in the Wikipedia article Syzygy_(astronomy)

  • Do these cameras have adjustable apertures (irises) in order to look directly at the Sun safely and yet still work on Mars near dawn and dusk?

  • If so, what range of f/no does each camera have?

enter image description here Source

MarsCuriosityRover-MercuryTransitsSun-20140603.gif Source

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    $\begingroup$ Mildly Related: Would the sun still be able to hurt your eyes if staring at it from Mars? Wonder how much difference the added distance makes. $\endgroup$
    – Magic Octopus Urn
    Commented Mar 21, 2019 at 15:49
  • $\begingroup$ @MagicOctopusUrn No, it isn't. "...in order to look directly at the Sun safely and yet still work on Mars near dawn and dusk?" That ratio is independent of distance from the Sun. $\endgroup$
    – uhoh
    Commented Mar 21, 2019 at 21:38
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    $\begingroup$ Yes, you would injure your eyes looking directly at the Sun. $\endgroup$
    – Mark Adler
    Commented Mar 27, 2019 at 17:27

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They use neutral-density filters to look at the Sun, which reduce the light by a factor of 100,000. The two Pancam cameras each have one neutral-density filter, with the left one filtering blue and the right one filtering red.

per comment: no, no irises.

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  • $\begingroup$ Nope. No irides. $\endgroup$
    – Mark Adler
    Commented Mar 27, 2019 at 4:58
  • $\begingroup$ How many shades of sunglasses is that? $\endgroup$
    – Muze
    Commented Mar 27, 2019 at 20:42
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    $\begingroup$ Sunglasses vary tremendously, and usually block more blue than red. In any case, if I assume 40% VLT as typical, then about twelve sunglasses stacked. Or two pair of polarized sunglasses with the second pair rotated 89.8 degrees relative to the first. $\endgroup$
    – Mark Adler
    Commented Mar 27, 2019 at 21:48
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    $\begingroup$ To get a reduction of a factor of 100,000 you need 4 sunglases with 5.6% transmission stacked. Or a stack of 8 glases with 23.7 %, or 12 glases with 38.3%, or 16 glases with 48.7 %, or 32 glases with 70 %. There are sunglases with about 80 % transmission down to only 4 %. $\endgroup$
    – Uwe
    Commented Mar 29, 2019 at 11:22
  • $\begingroup$ A light reduction by a factor of 100,000 is not possible using an iris anyway. The hole within the iris should be smaller than possible and would harm the resolution of the images too much. A f-stop of 1:316 would be necessary (for a lens with 1:1 maximum aperture, 1:885 for a lens with 1:2.8). But usual f-stop ranges from 1:16 to 1:64 (used by old large format cameras only). For a lens with a focal length of 50 mm 1:885 is an iris hole diameter of 56 µm only. Using neutral-density filters is only one solution. $\endgroup$
    – Uwe
    Commented Mar 29, 2019 at 12:17

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