One popular, legitimate use for lasers outdoors is for pointing out stars in the sky. This needs to be done with caution. It is an application that requires relatively strong lasers, so that viewers can see the beam shaft (not just the end dot). Further, the beam is aimed at glowing objects in the sky. Because these objects can be aircraft, outdoor laser users need to be careful.
Select green
For star pointing, green is best. The human eye sees green much better than red or blue, so a 5mW green laser appears to be 5 to 10 times brighter than a 5mW red laser.
Looking at a green beam in the sky should not adversely affect night vision. To keep your night vision, avoid looking directly at the laser dot on a nearby or light-colored surface.
5 mW works fine
In some jurisdictions, there are limits on laser power. For example, in the U.K. it is not allowed to sell laser pointers over 1 mW. In the U.S. it is not permitted to sell lasers for pointing applications over 5 mW. You may be restricted, therefore, in the laser pointers which are available to you.
However, you may be in a country without laser restrictions. Or you may obtain a laser which was not sold or intended for pointing but which nevertheless could be used outdoors. The question then is “how much power is needed?”
For yourself or a small group under most conditions, 5 mW is sufficient. For a larger group, or where the air is especially clean and dry, slightly higher power such as 10 to 25 mW will be better. The absolute limit for this application should be about 50 mW. There is no objective reason to need more than 50 mW for astronomical pointing applications.
Here is one user’s description of the visibility of his 4.92 mW laser pointer:
- “The first time I used it, I was in a rural area, although not very far from the city, and there was a setting gibbous moon. Limiting magnitude was around 5.0, maybe 5.5. The laser was bright and easy to see.... can you see it from a dark spot in the worst light-polluted sky imaginable? Yes, you can see it. Just for perspective, I used it about 45 minutes after sunset. The sky was still quite bright, with 20 minutes of nautical twilight left, and an hour of astronomical twilight. Limiting magnitude was perhaps 3.5. The beam was visible in these conditions. Dim, but unmistakably visible.”
Don’t point; circle instead
Aircraft at a distance can look like stars. This is especially true if they are moving towards you, since there will be little apparent motion.
For this reason, NEVER point directly at a dim or unknown “star”. Instead, move in a circular motion around the object. When doing the circular motion, or when “drawing out” a constellation, keep the beam moving and keep it away from any stars. The “stars” may be aircraft.
Use sparingly
Use the laser only long enough to point out the object. Once it is identified, leave the laser off. (After all, this is how people observed the sky for thousands of years, before laser pointers were invented!)