SJAA Ephemeris January 2001 | SJAA Home | Contents | Previous | Next

Lasers in the Sky with Diamonds

Bill Arnett

As authorized by the board at the last meeting, I purchased a green laser pointer with the intent of using it for pointing out constellations at our star parties and classes. It does indeed work pretty well but there are a couple of caveats. First, it is badly affected by light pollution; it works much better in dark skies than in bright ones. In a bright sky the beam is easy to see if you're the one holding the pointer, but if you're more than a few feet away from the origin of the beam it can get pretty dim. In reasonable suburban darkness (i.e. my backyard) I can see the beam from 10 or 12 feet away from the origin; that should be fine for star parties. Secondly, it works better in damp weather. I guess the deal there is that it has more water droplets to reflect off of. How these two factors will balance out at Houge is anybody's guess. I think we'll just have to try it for a while.

Dark adaptation really matters, too. When you first go out from a bright room the beam is hard to see. But it only takes a few seconds before it starts to work. It's a little funny: at first it seems like it doesn't quite reach to the stars :-).

The beam itself is just gorgeous: a nice pure green line with bright sparkles that constantly dance around (dust motes, I guess). Lasers in the sky with diamonds!

The reason it works so much better than a red laser is slightly controversial. One theory is that the human eye is more sensitive to green light. That's true, but I don't think it's the right explanation. When I aim both red and green laser pointers at a distant white object they both make spots of about the same brightness. But there's a line in the air from the green one only. I think the answer is scatter from air molecules and especially dust and water droplets. That's more efficient for green light than red (same reason that the sky is blue; this would work even better for blue lasers but they're way too expensive).

What this thing really excels at is as a replacement for a Telrad. I used a couple of bungie cords to attach it to my dob and align it to the optical axis (not a good system, but adequate for a test). So when I want to point the dob I just swing roughly in the right direction, push the button and move until the beam is in the right place. It's easy to get within a fraction of a degree of the intended target (assuming of course, that you know where it is :-) and much easier than trying to peer thru a telrad. And way more fun! As soon as the price comes down (a few years, I would guess) I think it will be the end of the line for Telrads. A green laser is a lot better. (Unless there are photographers around; the laser would almost certainly mess up their images. Telrads may survive for those cases.)

Maybe the best thing is when you're looking at something and someone comes up and says, "Hey, that's neat, where is it?" you can just flash on the beam and the best imaginable indication of location flashes into the sky!

You can see the beam through the eyepiece, too. That's useless except for initially aligning, but it's kind of fun. It might be useful to attach one of these to a binocular for finding and pointing out dimmer objects.

The backscatter beam is not bright enough to affect one's dark adaptation. But as with all lasers, one must be careful to avoid getting the beam directly in someone's eye. While it isn't as bad as looking at the sun with a scope, it's still not a good idea. At public parties we'll have to be careful about children.

The cost is $235 + shipping; see

Mail to: Bill Arnett
Copyright © 2001 San Jose Astronomical Association
Last updated: July 19, 2007

Previous | Contents | Next