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Light from the sun works the same way as light from a nebula - it just
happens to be at the other extreme of the range of brightness we astronomers
have to deal with. Let's compare a 6x30 finder with naked eye - let's
assume a 5mm pupil (the solar astronomer's will be smaller than this, the
stellar astronomer's probably larger). The 30mm aperture of the finder is 6
times larger in diameter or 36 times the area of the naked eye pupil. It
therefore accepts 36 times as much light. However, when using the telescope
the image of the sun on the retina will also be 6 times larger in diameter
or 36 times the area of the image when observing naked eye. So 36 times as
much light spread over 36 times as much area gives the same intensity per
unit area. What if we reduce the magnification of the scope to say 3x?
Then the exit pupil of the scope will be 10mm so 1/4 of the light coming
from the scope will be blocked from entering the eye by the pupil and we end
up with the same intensity.

Finally - the welding filters are designed for use with arc-welding which
generates copious amounts of UV and IR. They are guaranteed safe naked eye.
And, of course, the UV and IR are both affected the same way as visual light
as far as aperture and magnification goes.

Therefore if a filter is safe for naked eye viewing it will also be safe in
front of any scope.

Note that all this works the same for nebulae. A nebula can never have a
higher intensity of light on the retina than when observing naked eye. So
why does the telescope help? By increasing the size of the nebula at the
same intensity the eye/brain combo is able to better detect the photons
arriving over a larger area - they don't ignore it as noise. For more
detail check out, for example, Mel Bartel's web pages on optimal visual
magnification - . All
this doesn't work for stars though. A star is essentially a point object so
it doesn't look any bigger in a 60x300 telescope than a 6x30 so it does get
much brighter in the larger scope.

Rick