2

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

uhoh
  • 148,791
  • 53
  • 476
  • 1,473
  • 1
    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. – Magic Octopus Urn Mar 21 '19 at 15:49
  • @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. – uhoh Mar 21 '19 at 21:38
  • 1
    Yes, you would injure your eyes looking directly at the Sun. – Mark Adler Mar 27 '19 at 17:27

1 Answers1

6

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.

uhoh
  • 148,791
  • 53
  • 476
  • 1,473
Mark Adler
  • 58,160
  • 3
  • 171
  • 251
  • Nope. No irides. – Mark Adler Mar 27 '19 at 04:58
  • How many shades of sunglasses is that? – Muze Mar 27 '19 at 20:42
  • 1
    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. – Mark Adler Mar 27 '19 at 21:48
  • 1
    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 %. – Uwe Mar 29 '19 at 11:22
  • 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. – Uwe Mar 29 '19 at 12:17
  • @MarkAdler I've just quoted you in Puzzler: help understanding these amazing Curiosity eclipse GIFs – uhoh Apr 05 '19 at 02:37