The new open access Solar Physics article The Daniel K. Inouye Solar Telescope – Observatory Overview is the first of several papers about the new state-of-the-art solar observatory. For more info see Phys.org's Solar telescope releases first image of a sunspot

Figure 4 of the paper is shown below. It is the 4 meter primary mirror viewed at an oblique angle with several people in clean room garb standing behind it.

We can see their reflections in the mirror as well. The geometry is similar to people standing on the opposite side of a reflecting pool.

What struck me is that this differs in an important way; the people are not upside down!

  1. Why not exactly?
  2. Would the people see the camera upside down then?

It looks like the photo was taken from fairly close to the mirror based on the strong perspective scaling from foreground to background objects.

The 4-meter primary mirror (M1) after coating at the AMOS coating facility on Haleakalā.

(Figure 4) The 4-meter primary mirror (M1) after coating at the AMOS coating facility on Haleakalā.

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    $\begingroup$ Since it's the telescope's primary mirror, it's going to be concave, rather than the geometry of a reflecting pool, and I suspect the light-path is hitting the mirror twice before it reaches the camera. $\endgroup$
    – notovny
    Dec 7, 2020 at 6:07
  • $\begingroup$ @notovny I'm not sure twice is necessary; this particular off-axis mirror has quite a short focal length and perhaps there's an intermediate focus somewhere along the way. $\endgroup$
    – uhoh
    Dec 7, 2020 at 13:16
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    $\begingroup$ given that the mirror has a focal length of only 8m (the paper describes it as F/2), I think it's very possible to have an intermediate image plane between the people and the side of the mirror furthest from them, as @uhoh suggested. Interesting to note, you can also see the scientist 2nd from left also has the top of his head reflected upside-down on the side of the mirror close to him. $\endgroup$ Dec 8, 2020 at 3:49

1 Answer 1


I think this seems legit. Why not?

enter image description here

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    $\begingroup$ It's best if you add some more to this so that the average reader here can see how this actually answers the question. Each SE site works differently; in Math SE one can leave a hint as an answer and everybody is happy, but here in Astronomy SE it's expected to take some time and explain the answer to readers. Maybe check the focal length of the mirror in question and see if it works within the constraints of the photo? Thanks! $\endgroup$
    – uhoh
    Dec 7, 2020 at 16:06
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    $\begingroup$ It's also necessary to show that they appear fairly well in focus, something that this first order cartoon doesn't address. $\endgroup$
    – uhoh
    Dec 7, 2020 at 16:57

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