On 2006-07-07, Arnold wrote:

Do we know if Galileo looked at the sun using his early telescopes?
What kind of filters / shielding did he use to prevent eye damage?

Galileo famously observed the sun. He projected an image of the
sun onto a piece of paper for most of his observations, or
observed the sun directly when it was low on the horizon and
the light and heat were attenuated by the atmosphere. He
left a detailed description of his projection technique. By
plotting sunspots on the paper he could preserve an accurate
record of their positions. This allowed him to prove that
sunspots were not the silhouettes of satellites orbiting the
sun.

Bud