Using a green laser pointer for astronomy and stargazing

Ask any stargazer which laser pointer they reach for and the answer is almost always green. A green pointer is one of the simplest tools for enjoying the night sky, whether you are learning the constellations on your own or showing a group where to look. It is also widely misunderstood: the beam does not light anything up there, and on the wrong night you will barely see it at all. Here is what a green pointer actually does under a British sky, and how to get the most out of one.

Why green is the stargazer’s colour

The trick with astronomy is that you are not pointing at a surface. You are pointing at open sky, and a dot needs something to land on. What you see instead is the beam itself, lit by the dust and moisture it passes through on its way up, and whether that shows at all depends almost entirely on colour.

Human daylight vision peaks close to 555 nm, in the yellow-green. A 532 nm green laser sits right beside that peak, which is why a modest green beam reads as bright. Red at 650 nm sits far down the same curve, so a red pointer of similar output looks dim and its beam never appears at all. Violet at 405 nm is further down still, at the very edge of what the eye registers. That is the whole reason astronomy runs on green: not because green is more powerful, but because your eye is built to notice it.

There is more on how the three colours compare in our guide to which laser pointer colour to choose.

What you can and cannot see with one

It is worth being blunt about this, because the marketing around laser pointers rarely is.

What a green pointer does well:

• Traces a constellation shape in the air so a group can follow it. The curve of the Plough, the line from the Plough to Polaris, the three stars of Orion’s belt.
• Turns “the bright one to the left” into something everyone can see at the same moment.
• Acts as a rough finder. Line the beam up with the Moon, a planet or a bright star, then lift your binoculars along the same sight line and you are already close.
• Settles arguments quickly at a public evening, where half the group is looking at the wrong star.

What it does not do:

• It does not light up anything in the sky. The beam appears to end somewhere overhead because it has stopped scattering enough light to see, not because it has arrived anywhere.
• It will not help you find a faint galaxy or nebula. For anything you cannot see with the naked eye, the pointer is no use as a finder.
• It does not touch the star. You are sighting along a line, in the same way you would sight along a stick, and the parallax means the person standing behind you sees the line landing somewhere slightly different.
• It does not work in a lit street, under a full Moon or in bright twilight. In those conditions the beam is simply not there.

When the beam shows best

Three things decide how good the beam looks, and none of them is the pointer.

Dark-adapted eyes. Give yourself twenty to thirty minutes away from white light before you judge a pointer. The difference between a freshly arrived eye and a dark-adapted one is larger than the difference between two pointers.

A dark sky. A moonless night away from street lighting is worth more than any specification. Under a full Moon, or from a lit back garden in town, even a good green beam looks thin.

A little moisture in the air. The beam is visible because of what it bounces off. A still, slightly humid night shows a stronger line than crisp, bone-dry air, which is why the beam often looks better on an ordinary damp British evening than on the clearest night of the year.

Cold nights and batteries

Most disappointing sessions come down to tired cells rather than a tired pointer. Alkaline AAA batteries lose a noticeable amount of their usable capacity once the temperature drops towards freezing, and a beam that looked fine in the hallway can look weak twenty minutes into a January field.

• Carry a spare pair of AAAs in an inside pocket, where body heat keeps them workable.
• Let a pointer that has been in a cold car warm in your hand for a minute before you decide it has failed.
• Do not leave the beam running between targets. Short presses use very little, a beam left on for ten minutes does not.
• Take the batteries out if the pointer is going back in a cold shed or a car boot for the winter.

Every Emax pointer arrives with two AAA batteries in the box, so a spare set is a supermarket purchase rather than a special order.

Using one at a star party without annoying anyone

A green pointer is the fastest way to get thirty people looking at the same star, and the fastest way to make yourself unpopular at an observing site. The difference is habit.

• Ask the organiser first. Most clubs have a house rule, and it is usually a sensible one.
• Short bursts. A second or two to draw the line, then off. A beam left burning across the field is a distraction for everyone.
• Never across an eyepiece or an open shutter. Someone is always part-way through a long exposure, and a beam through the frame ruins it. Look before you point.
• Point up, and keep it well away from people. Aim at sky, not at the horizon, and never at anyone’s face.
• One person holds it. Passing a pointer round a dark field is how it ends up switched on in a pocket.
• Keep white light off. The pointer is useless to a group whose night vision you have just destroyed with a phone screen.

Which green pointer to take out

Any of our 532 nm green models will trace a constellation. The choice is really about how you hold it.

• Emax green laser pointer, £19.99. The default, and what most people should buy. A momentary button, two AAA batteries in the box, nothing to charge.
• Green laser pointer with on/off switch, £24.99. A latching switch, so one press holds the beam on. Useful if you are talking a group through a shape and do not want to keep a button down, and the model to choose if you give astronomy talks. It is also the one to be careful with in a pocket.
• Green laser pointer with five caps, £24.99. The same green pen with five pattern caps and a case. The caps are a party trick rather than an observing tool; take them off and it is the standard pointer.

The green line laser is the one model to leave at home. It projects a line onto a surface, which is exactly the wrong shape for pointing at a star.

Common questions

Do I need a green laser pointer for astronomy, or will red do?

Green, if you want the beam to show. Red at 650 nm sits far from the eye’s peak sensitivity, so a red pointer gives you a dot on a wall but no visible line against the sky. Red has its place indoors and around pets; for the night sky it is the wrong colour.

Why can I hardly see the beam some nights?

Usually moonlight, street lighting or dry air, in that order. The beam is only visible because of what it scatters off, and a bright background swamps it. Try again on a moonless night away from town, after your eyes have had half an hour to adjust.

Can I mount one on a telescope as a finder?

Plenty of people do, and for the Moon, the planets and bright stars it is quick and intuitive. It will not replace a proper finder scope for faint targets, because you cannot line up on something you cannot see.

How long do the batteries last?

Longer than a single observing session, as long as you use short presses rather than holding the beam on. Cold is the bigger factor, so on a winter night judge the pointer by how it behaves after a spare warm pair have gone in.

Ready for a clear night?

If astronomy is your plan, green is the only colour worth considering. Take a look at our green laser pointers, all of which ship free across the UK in one to three working days, or check lead times on the delivery page. Pick one up, wait for a moonless night, and the sky becomes a lot easier to share.

Tim Buikema runs Emax Laser Pointers, the UK shop of Emax Lasers. He has supplied green pointers to astronomy clubs, schools and outdoor groups since 2011 and opened the UK shop in 2017.