This is related to a question asked on Space.SE:

Is this purple dot really caused by Cassegrain optics and coherent backscattering?

Zoomed in:

Is this sort of artifact known in photography? If so, what is its cause?

(please, no guessing games. If that artifact is not something you can ever find in common photos, "no" is a perfectly valid answer and better than sending me on a wild goose chase after something that is similar-but-not-quite-that.)

  • 5
    \$\begingroup\$ No, this specific type of artifact is not common in photography. But then again, photography doesn't commonly use Cassegrain optics viewing the earth from the L1 Lagrange point. \$\endgroup\$
    – scottbb
    May 17, 2017 at 17:33

2 Answers 2


From reading the linked question and the answers to that question as well as the discussion in the comments there a picture of what created this artifact is fairly clear. It's not lens flare. The first answer to this question is correct when it states the following, but it also doesn't explain what is almost certainly the cause of the artifact.

These types of flares are not common in reflective telescopes.

Shooting a picture of the Earth from the L1 point between Earth and the Sun isn't common either.

  • From such a position the sun is always perfectly behind the camera when the camera is pointed at the Earth.
  • If the camera is pointed at the earth perfectly centered in the FOV, and if the point of the Earth's surface is level, then the surface at the point in the center of the image is perfectly perpendicular to the optical axis of the lens.
  • Since the optical axis in this case is 932,000 miles long, it also means any light reflected from the center of the FoV that reaches the camera is HIGHLY collimated. Even more collimated than most advanced lasers.
  • If the surface of the Earth at the center point is both level and highly reflective (such as the area in question in the example image that has a high amount of quartz on the surface), it will reflect a tremendous amount of the light energy from the sun falling on such a spot directly back at the camera.
  • Since the light from the spot in question will be many orders of magnitude brighter than the light reflected from the rest of the Earth, if the camera is set to expose most of the Earth properly, the bright reflection in the center of the FoV will be fully saturated and totally blown out with plenty of light energy to spare.
  • As indicated in the question linked above, the artifact is the size of one pixel at the exact center of the image.

It should be fairly obvious the artifact is the result of extreme overexposure of that single pixel and the way the image processing software dealt with the signal from that single pixel.

If you've ever observed an "Iridium Flare" in a bright daylight sky, you'll understand just how bright sunlight reflected from a smooth, flat, highly reflective surface can be, even compared to the sunlight reflecting off everything else in one's field of view.

  • \$\begingroup\$ I'm not sure I understand the sentence correctly, if "the nefarious conspiracy" is about my question, then please note, the thing that concerns me is the answer to which I have linked in my question which explains that the cause of the point is related to the Cassegrain secondary mirror and coherent backscattering. I'm questioning if the dot is really caused by those two things - Cassegrain optics and coherent backscattering. Photographers often use Cassegrain or similar, folded "mirror lenses". \$\endgroup\$
    – uhoh
    May 19, 2017 at 11:10
  • \$\begingroup\$ See for example this question addressing the optics issue in greater detail, and this question addressing coherent backscattering in greater detail. \$\endgroup\$
    – uhoh
    May 19, 2017 at 11:14
  • \$\begingroup\$ Maybe nefarious is not the correct word. Your description of a "wild goose chase" without any details as to what that chase is does leave open a wide number of possible interpretations. We do get questions here periodically about artifacts in photos that the OP wants to be able to say "proves" something about aliens or massive government conspiracies or some other such fringe views. It seemed to strike me that way when I first read it, even if that was not what you are hinting at. \$\endgroup\$
    – Michael C
    May 19, 2017 at 15:15
  • \$\begingroup\$ I'm aware of the phenomenon! Astronomy SE does a good job of helping 'triage' and explain unidentified lights in the sky questions, SXSE so far doesn't have very many of those. I did ask this one which turned out to have a really noteworthy answer that's worth reading. Which Apollo “mystery” was said to be finally solved by a better rendering engine? \$\endgroup\$
    – uhoh
    May 20, 2017 at 10:04

If it's an optical effect, it's a narcissus or ghost image. These types of flares are not common in reflective telescopes.

  • \$\begingroup\$ I'm not sure why your answer got a downvote, but can you define narcissus in relation to optics, or link to anything that uses it? It seems to be a fairly rarely used term, and I've only found references to it when paired with "or ghost image". I get the gist of the reference to the mythological figure, but is narcissus distinct in any way from ghost images? \$\endgroup\$
    – scottbb
    May 19, 2017 at 17:13
  • \$\begingroup\$ @scottbb it's just the term for an in-focus bit of stray light. \$\endgroup\$ May 21, 2017 at 13:33

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