Introduction: Astronomy: Hacking a Redlight

About: I'm a professor of physics and astronomy at Northwestern University. I do a lot of hobbies, including amateur astronomy, woodworking, and Lego modeling among many others.

One of the most difficult parts of getting started in a new hobby is having the right equipment. Stargazing and amateur astronomy are particularly susceptible to sucking money out of you -- even modest equipment can be a drain on your funds. Fortunately, there is a strong DIY community in the amateur astronomy world, and hobbyists enjoy all aspects of designing and creating their own equipment, from star charts, to camera equipment, to ladders and chairs, to telescopes!

With the arrival of warmer weather every spring, I start to think about stargazing! Stargazing is easy -- you go outside and look up! But if you want to know what you are looking at, then at a minimum, you need a simple star chart, and a light to read by. This instructable is about making a serviceable red flashlight that will help get you out under the stars and observing the Cosmos.

Any of these hacks will work perfectly well until you take the dive and buy a commercial LED flashlight for astronomy, but many of us still tote around and use the first flashlight we hacked when we started out in the hobby. These hacks are also excellent options for guests when you are out guiding your family, neighbors, or local classroom on a sky tour.

Our cardinal rule that you should never forget: "Don't let the perfect get in the way of the good!" Any red flashlight is better than no red flashlight! You can use any of these hacks, but you may be inspired to invent your own. If you have a good one, please share it!

Step 1: Your Eyes, the Cosmos, and Light to See By

We are predominantly daytime creatures, so our eyes are well adapted to seeing in well lit conditions. If you use an ordinary white flashlight to read starcharts, the light will blast your eyes out, making it difficult to see anything. When you are out stargazing, the need for light is at odds with the need to be immersed in darkness to see the diaphanous glow of the Cosmos.

Amateur astronomers use red flashlights to address this issue. If kept dim, they allow your pupils open up and to remain dilated, allowing your eye to gather more light. Additionally, the red color does not dramatically interfere with your low-light sensitivity; this has to do with the chemistry of the cells in your eye (the rods and cones), and how they respond to the brightness and color of light).

There are lots of red-light options on the market, especially with the advent of LEDs, but if for some reason you find yourself without a redlight, you can hack one. Here are several tricks.

Step 2: Hack 1 -- Red Cellophane

This is the time honored and traditional method for making a quick and dirty redlight. If you've ever attended a star party at your school or library with your local astronomy club, you've probably encountered this technique. You'll need:

  1. White flashlight (any white flashlight will work)
  2. Red cellophane (gift wrap, tail light film -- the darker the better, the larger the better)
  3. Rubber Band

Fold the cellophane over the end of your flashlight, completely covering the lens. Secure it in place with your rubber band, and you're done! Almost. :-)

Step 3: Checking the Cellophane Redness

To get the full benefit of having a red light, the light needs to be really red. A common mistake with the cellophane hack is not using enough layers of cellophane, so the light is not very red -- it is pinkish or white. In the picture above, the blueness of the LEDs really shows up!

The solution is to have more layers of cellophane; I use the rule of thumb that I fold the cellophane over on itself (or use multiple sheets) until I cannott read a starchart through the red cellophane.

Step 4: Deep Red Color

With enough layers of cellophane you cannot see through them, the light produced is usually a deep, rich red that will go a long ways to protecting your night vision.

To test your results (with any of the hacks presented here), take your starchart into a darkened room, and give your eyes a few minutes to dilate open and adjust from being in the bright light.

When you shine your new redlight on a starchart, you should be able to read it when it is held at the same distance you would normally read a book, or slightly closer. It will not be bright -- you don't want it to be, otherwise the pupil in your eye will close up too much.

Step 5: Hack 2 -- Red Plastic

For this redlight hack, you'll need:

  1. White flashlight (any white flashlight will work)
  2. Red plastic (any thick, red plastic will work, for example from a resealable food can; your local craft store may also have sheets of red plastic)
  3. Marker
  4. Scissors or Hobby knife

For some brands of flashlights, you can buy colored plastic lenses to insert into the flashlight to change the projected color of light. This is a DIY version of that approach. For this hack, you will need to first disassemble your flashlight, and remove the clear plastic lens that protects the bulb.

Step 6: Cutting a Red Filter

Using the lens as a template, trace its outline on your plastic with your marker, then cut out the red piece. The final piece should closely match the clear lens. Trim it up so the two are as close the same size and shape as possible.

Step 7: Insert Lens Into Flashlight

Putting the red plastic behind the clear lens, insert both back into the flashlight. Usually the main reflector around the bulb holds the lens in place, and should be adequate to keep both the lens and the new red insert in place when you reassemble the flashlight.

You may need to experiment with finding the right red plastic to use; remember you want a deep red color, without a noticeable white or pink color in the center when the light is turned on.

Step 8: Hack 3 -- Pop Bottle Cap

This technique works best with Mini-maglight style flashlights, or flashlights of similar size. It relies on a plastic bottle cap fitting directly over the end of the flashlight. This hack is great for two reasons:

  1. It is usually pretty easy to find a bottle cap!
  2. It allows you to quickly convert your flashlight back to white if you need it to.

If the cap doesn't want to stay on, a folded strip of paper wrapped around the inside of the cap is usually enough to provide enough friction to keep it on your flashlight.

Step 9: Checking the Redness

When you look directly at the illuminated cover, you may be fooled into thinking the red color is not deep enough. But the true test of the light is to use it in the dark and try it out on a starchart.

You may have to experiment with some different red caps to find one that works well and gives the deep red color we are looking for, but I find the color from a Coke bottle works great.

Step 10: Starcharts

You may also need some skycharts to use with your new flashlight!

  1. Each month, magazines like Sky & Telescope and Astronomy publish a starchart showing what stars and sights there are to see in the night sky.
  2. You can download and print a starchart each month from
  3. A planisphere is an all sky chart that can be set to anytime of night on any night of the year.

Most modern smarphones can also show you the sky, and help you navigate. Remember if you use one that smartphone screens can also ruin your ability to see in the dark! Make sure to cover their screens (the red cellophane trick can work well here), or use the "redlight mode" that many stargazing applications have built in.

Happy stargazing!