Quote Originally Posted by vk3ukf View Post
I can't find anything that specifically (I may have missed it) mentions in an atmosphere, vacuum, or both regardless.
It's hard to quote, because I found it as a Google book sample, but "Optical communications rules of thumb" By John Lester Miller, Edward Friedman, offers the similar theta = lamda/d as above, and: "This rule is widely used to estimate the size of a laser beam that has propogated through a vacuum and is also used as a first estimate even in atmospheric applications. [...] This rules provides quick estimations of minimum beam divergence."

I take that one can reduce the divergence as much as one wants by enlarging the beam diameter, but to reach zero divergence requires... an infinite budget (to produce an infinite diameter beam).