Whilst checking the latest images of the E-21 flyby returned by the Cassini probe, I've noticed that some of them show interesting optical phenomena that I'm not sure I can explain;

For example, on this one, there are two spotlight-like beams of light at the top part of the image, crossing each other just above the Enceladus' plume at the image center:

   Cassini Solstice Mission image of Enceladus plume

  Cassini Solstice Mission image N00250060, taken on October 28, 2015 and pointing towards Enceladus (non-filtered)

My thinking is that the beam going from the top left towards image center is forward scattered light of Saturn shine through the E Ring, and the beam going from top right towards image center could be coherent backscatter amplified reflection of the Sun by Saturn's main rings, but the latter makes less sense to me as coherent backscatter should be visible when the spacecraft is directly between the Sun and the surface reflecting its light, so maybe there's some other type of light amplification going on there.

So, since I fail to explain it better and I doubt these beams are actual features of the E Ring as they don't appear in all subsequent images (e.g. they're not visible here), neither do I think these are lens flares or other image artifacts as they still appear on several other images (e.g. this one only shows one of them, but not the other), here's my question:

Could these optical effects be explained with the help of Cassini's position relative to Saturn and the Sun when images they appear on were taken? Or are they some other optical phenomena? Or are they actual tangible features, perhaps the E Ring tendrils?


1 Answer 1


The very structured nature of the features strongly suggests stray light. In fact, in-flight calibration has an example of this (ref. Figure 18). From the text:

Also, when a bright object is located near to the corners of the NAC frame, a bright streak can be seen to extend diagonally across the field of view

  • $\begingroup$ OK but stray light from where? The two diagonal and crossing beams obviously have a different source. They're not orthogonal to each other or symmetrical with respect to the image frame which would indicate single source reflecting off the camera. Note that there are also horizontal bands on the image which could be attributed to stray light from Saturn and its rings. In other words, there's either two distinct optical phenomena going on here or there's two more bodies off which these stray rays reflect. I wasn't able to find flyby geometry that would explain it (tho one could be off Dione). $\endgroup$
    – TildalWave
    Nov 17, 2015 at 16:13
  • $\begingroup$ The paper goes into it: Some of the most prominent stray-light artifacts occur when relatively bright, compact sources (like moons or nearly edge-on rings) lie just outside the camera’s field of view. These stray-light artifacts can possess a great deal of fine-scale structure that changes as the off-image object moves relative to the field of view. For the NAC, a bright object that lies just off the edge of the frame gives rise to bright streaks extending perpendicularly to the relevant edge, as well as more diffuse arc-like pattern $\endgroup$
    – Dave
    Nov 17, 2015 at 16:39
  • $\begingroup$ As for the exact source for that particular image, you'd need to make an educated guess based upon its pointing and what was near the field of view. But based on the paper and the image, I would bet that both the diagonal streaks as well as the diffuse background are stray light from an extended source across the FOV, which would have to be an edge-on ring or the daylight side of the planet. $\endgroup$
    – Dave
    Nov 17, 2015 at 16:43
  • $\begingroup$ I've been trying to match position of Saturn's inner moons with respect to Cassini and on the date and time of the flyby here but I just don't find any moment where on top of Saturn's rings (Saturn itself was nearly in conjunction during the flyby from spacecraft's point of view) producing horizontal streaks, there would be two additional bright objects in positions and bright enough (again considering Sun's position) to produce the diagonal streaks. I would prefer not to make educated guesses but to learn how to extract such information. $\endgroup$
    – TildalWave
    Nov 17, 2015 at 17:23
  • $\begingroup$ I've never used that simulator before; that is a neat link. $\endgroup$
    – Dave
    Nov 17, 2015 at 17:43

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