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[sci.astro] General (Astronomy Frequently Asked Questions) (2/9)
Section - B.12 Is it O.K. to look at the Sun or solar eclipses using exposed film? CDs?

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Author: Joseph Lazio <jlazio@patriot.net>,
	Steve Willner <swillner@cfa.harvard.edu>

This question appears most frequently near the time of solar eclipses.

The short answer is no!  The unobscured surface of the sun is as
bright as ever during a partial eclipse and just as capable of causing
injury.  The injured area on the retina may be a bit smaller, of
course, but that's no reason to risk damage.  Moreover, there are no
nerve endings in the retina, so one can do permanent damage without
being aware of it.

People have proposed a host of methods for viewing the Sun, including
exposed film and CDs.  These home-grown methods typically suffer from
two flaws.  First, they do not cut out enough visible light.  Second,
they provide little protection against ultraviolet or infrared light.

The only safe method for viewing the Sun directly is using No. 14
arc-welder filter or a metallicized glass or Mylar filter.  A local
hardware store or construction supply store should carry or know where
to obtain arc-welder filters.  Many astronomy magazines carry ads for
solar filters.

Whatever filter you use, inspect it to make sure it has not been
damaged.  Even a pinhole can let through enough light to cause injury.
If you use a filter over a telescope or binocular, make sure the
filter is firmly attached and cannot come off accidentally!  Never use
an eyepiece filter, which can overheat and crack.  Any filter should
cover the entire entrance aperture (or more precisely, any part of the
entrance aperture that isn't covered by something completely opaque).
If using only one side of a binocular, cover the other side.

An alternative way to view the sun is in projection.  You can use a
pinhole camera or a telescope, eyepiece, and screen.  Many observing
handbooks illustrate suitable arrangements.  This method is not only
safe, it can give a magnified image and make it easier to see details.

If you are lucky enough (or put in the advance planning) to see a
total solar eclipse, the total phase can be enjoyed with no eye
protection whatsoever.  In fact, experienced eclipse-goers often cover
one eye with a patch for several minutes before totality so the eye
will be dark-adapted during totality.  Just be sure to look away (or
through your filter again) the instant totality is over.

Additional information on the safe viewing of solar eclipses is at the
Eclipse Home Page, <URL:http://sunearth.gsfc.nasa.gov/eclipse/>.

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