Thanks for the advice, but how does one know which ones do not contain
propellants?


Hi:

All the commonly available canned air products contain propellants. This is not
a problem as long as you're careful to hold the can upright. As I said,
however, I would not use this on first-surface mirrors.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
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