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.
Which telescope do I need? Ask us below
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. That is a different job from aiming the telescope: a handheld beam shows a group where to look, but it says nothing about where a separate telescope is pointed. Aiming is a finder's job, and a finder only helps once it is aligned with the main telescope, as Celestron's own instructions for its StarPointer red dot finder put it: point, look through the eyepiece, and if properly aligned the object is centred. Some observers do mount a pointer on the tube as a finder, and it needs the same alignment. In every case 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. Aiming the telescope itself is a finder's job, and a pointer mounted on the tube only does it once it is aligned with the main telescope.
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.