Re: could Iridium flashes cause eye damage?

Leigh Palmer (
Sun, 14 Dec 1997 20:01:10 -0800

>	At the local astronomy club meeting I gave a short presentation on
>the Iridium flashes, to see if anyone else was interested in observing one.
>One fellow with a whopper telescope (20-inch diameter) wanted to know what
>would happen if he were unlucky enough to be looking at something at the spot
>in the sky just where the satellite happened to flash. Certainly it would be
>dazzling. Could it conceivably cause eye damage? If not in that size
>what about in an even larger one?
>	My first answer was "of course not", but then he told me that even in
>his size scope it is dangerous to look at the moon without a filter. To back
>this up he pointed out that he can make a piece of paper start smoking by
>holding it at prime focus while his scope is pointed at the full moon.

Nonsense! The surface irradiance won't be as great as the same piece of
paper being exposed to direct sunlight. It will be down by a factor of
four or more at all points due to the albedo of the Moon alone (which
varies from 0.07 to 0.24), and reaching that irradiance would require
a numerical aperture considerably beyond the realm of reason. If this
man's claim were true then his telescope could be the basis for a
perpetual motion machine of the second kind. My advice is to bet him
money that won't happen.

>	Hmmm. You can't make an extended source of light any brighter by
>magnifying it. However, if the satellite makes a perfect mirror, then at SOME
>(implausibly high?) level of magnification all you will see is the
>of the sun, and thus looking at the satellite should become, if for only an
>instant, approximately equivalent to looking at the sun. That is certainly not
>something you want to do.
>	Has anybody tried "running the numbers" on this out of curiosity?
>Granted, it would take extraordinarily bad luck to point at an Iridium flare
>with a huge amateur telescope at high power by accident.

Let's do a worst case calculation. Imagine the observer has a daylight
adapted pupil of diameter 1 mm. The telescope has an aperture about 500
times as great. This corresponds to a factor of 500**2 = 250,000 in
light grasp. Ideally this might increase the apparent magnitude of an
object by 13.5 magnitudes. Thus if the iridium flash is of magnitude -6,
the apparent magnitude through the scope would be equivalent to a flash
of magnitude -19.5 at most. The Sun itself is 500 times brighter than
this, and I've actually looked at it very briefly without damaging my
retinas. Thus the irradiance on the retina would be no greater than that
due to 1/500 of the Sun's surface exposed to a daylight adapted eye. I
have seen a larger fraction of the Sun's surface emerging from a total
solar eclipse, and that through somewhat dilated pupils. Of course I
didn't continue to stare at it, and there are no lesions on my retinas
as a result (I work with lasers and so I have my eyes checked for such

I can't say what would happen to an observer stupid enough to
gaze steadily at a very bright light source, but Darwin had something
to say about such people. I conclude there is no significant hazard.