Focussing?

[GrahamRounce]

Hello. I'm not ready to try it yet, but I'd like to get all my ducks in a row before I do.

How is a laser (say red) actually focussed? Whatever the size of the dot, it appears to the eye as a speckly blob. How can one see tiny variations in focussing accuracy? (Not to mention with an invisible IR beam!)

Thank-you,

 

[gazer101]

You pick an arbitrary focus distance (closer offers more power density, but too close means that smoke gets on the lens), and then keep turning the lens until the dot stops getting smaller/brighter, then turn backwards and repeat more finely. There is a limit to how focused it gets, and perfection isn't possible.

For my TEM03 IR, I just use a camera and some sort of thing to reduce exposure to get a feel for the size of the "dot". Burn tests are nice too