The next big lunar eclipse is coming up this weekend, on September 28. If you’re on the east coast of North America, or in South America (NASA has a handy map), you’ll be able to see the whole thing. One essential piece of gear for looking at the heavens: a night-vision flashlight, which you can make yourself.

When stargazing, it’s important to let your eyes adjust to the darkness. This takes a few minutes, and using a normal white-light flashlight can reset the process. But sometimes you need enough light to read a star chart, adjust your telescope, or find your way down the path to the star party. This is where you need a special light.

A night-vision-safe flashlight produces only red light, the spectrum of light that will allow you to see your surroundings without sacrificing your natural night vision. Of course, night vision flashlights can be purchased wherever telescopes, astronomy supplies, or camping gear is sold. But they’re easy to hack together, so we’d encourage you make your own. That way, you can your favorite flashlight or lantern, and not be limited by the seller’s options.

What you’ll need

• A flashlight with an old-school incandescent bulb, not a CF bulb or LED array.
• Red “holiday” plastic wrap or cellophane. The kind used to wrap food is perfect.
• Rubber bands (the thicker the better!) or duct tape

How to Proceed

Step 1: Cut a piece of the red plastic, making a strip that’s very long, but about two inches wider than the bulb-end of your flashlight on both sides.

Step 2: Fold the plastic over itself into a square that’s large enough to cover the lens of the flashlight. Be sure to keep that two inches of extra width on all four sides of the square. Keep folding until your square is 1/8th of an inch thick. Trim the rest of the unfolded plastic strip.

Step 3: Secure the red square to your flashlight using either tape, rubber bands, or both. Secure the plastic against the sides of the flashlight, and not onto the lens itself, as not to obstruct the beam of light.

Step 4: Test it out. Turn on the flashlight to check that there are no leaks of white light, and make sure the red filter is still thin enough to allow sufficient lighting. If there are any leaks, tape them up nice and tight or re-secure your rubber band.