A green laser pointer throws a visible beam into the sky and is used to point at stars for other people. It is genuinely useful at a public session and it is the one accessory on this shelf with real legal restrictions attached.
Why the beam is visible
Air scatters short wavelengths, so a green beam appears as a line rather than just a dot at the far end. That is why green is used rather than red, and why the effect is stronger on humid or slightly hazy nights than on perfectly dry ones.
What it is legitimately for
Showing somebody else exactly which star you mean, which is otherwise surprisingly hard. Some observers mount one on the telescope as a finder. In both cases the beam is used briefly and pointed well away from anything but sky.
The restrictions, which vary by country
Several jurisdictions regulate the sale or possession of higher powered pointers, and pointing one at an aircraft is a serious offence essentially everywhere. Rules differ enough between markets that the only sensible advice is to check your own before buying, and never to point one anywhere near an aircraft or a flight path.
Practical care
Beams are stopped by cloud and reflected by nothing useful, so keep the beam moving and off the horizon where people and buildings are. In cold weather the batteries fade quickly. And never hand one to a child at a star party, which is how most incidents at public sessions begin.
Questions people ask about telescope
What is a telescope laser pointer used for?
Pointing at a specific star so somebody else can see which one you mean, and sometimes as a finder mounted on the tube.
Are astronomy laser pointers legal?
Rules vary by country and several restrict higher powers. Pointing one at an aircraft is a serious offence essentially everywhere, so check local law before buying.
Why are astronomy pointers green?
Air scatters green light more than red, so the beam itself is visible as a line rather than only the dot at the end.