Re: Iridium flares magnified?

Date: Thu Jun 28 2012 - 00:49:27 UTC

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    George Roberts, you wrote:
    "So if you stare at it long enough (a few seconds maybe) you could have
    permanent eye damage.  However, this is not a danger because the flare is
    brief.  I've looked at the sun many times for as long as 2 seconds with no
    problems.  Iridium flares are similarly short.  Also because everything is
    moving (you, your eye, the iridium across the field of view) the spot is
    unlikely to hover on the same retinal cells.  So the heat will be spread
    I agree with that. But as you note, it's probably right at the limit. The difference between staring for two seconds and staring for "a few seconds", or even ten or twenty seconds, is not much, right?
    For context, I have been trying to think of ways of describing the appearance of the Sun (and whether it is safe to look at it) at various interplanetary distances. It's one of those fun "what ifs" that people often ask about. Many people, including some with considerable knowledge of astronomy, are under the impression that you could stare at the Sun for extended periods of time out at the distance of Neptune (or Pluto near perihelion), about 30 AUs from the Sun. At that distance the angular diameter of the Sun is 30 times smaller, making it around 1 minute of arc across, and therefore the angular area is 900 times smaller and therefore the Sun is 900 times fainter in total brightness. But the surface brightness or "specific intensity" (also known by other names) doesn't change. The Sun's face is just as bright out there. Equivalently, if you look at the Sun down here on Earth through a pinhole at a distance such that the angular diameter of the hole is one minute of arc (f!
     or example, a hole an eighth of an inch across from 14 feet away), you're seeing its actual appearance at Neptune. And if you try this, you should find that you get spots in your vision lasting for many minutes if you look at the "mini Sun" for as little as ten seconds. I wouldn't try it longer than that! The catch with such a setup is that the Sun has to be high in the sky so that there's no extinction. And if you want to attempt considerably greater equivalent distances (and smaller angular diameters), it's difficult to arrange a pinhole fifty or a hundred feet up in the air.
    It dawned on me a few weeks ago that another way to get a "mini" Sun would be to look at a small, highly quality mirror. Properly oriented, the reflected beam could be easily viewed at any distance and any angular diameter. The beam from the mirror can be horizontal with the Sun high in the sky. Then it occurred to me that this is very similar to what we have already with the Iridium satellites. In the case of the Iridium satellites, the mirrored panels subtend an angle of around 0.25 seconds of arc. That would the apparent diameter of the Sun at roughly 7000 AUs distance or around a tenth of a lightyear distance. Clearly we can look at Iridium flares without any consequences at unit magnification so if we were a tenth of a lightyear away from the Sun, we could look at it without any problem. No surprise there. But this small angular size is below the diffraction limit of resolution of the human eye as an optical system so the rule about constant specific intensity doesn't a!
     pply. At some level of magnification the apparent angular diameter of a flaring Iridium satellite would be within the resolution limit of the eye. That's somewhere around 0.5 minutes of arc so a magnification of 120x would just about do it. If you have a telescope that can track at that magnification, following an Iridium flare would be like seeing the Sun as it would appear from roughly 120 AUs distance, and it would indeed put a near full-intensity spot of sunlight on the fovea of the retina of the eye. Bright enough to do damage? Probably not quite but it's a close case. Or for another equivalent case, looking at an Iridium flare at 120x magnification would be like looking at Baily's beads with the naked eye immediately before/after totality in a solar eclipse. 
    My own telescope tracks only as well as I can shove it around by hand (and I do chase satellites that way!), but regardless I hope to get some of my own empirical data on this in the next few weeks.
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