Convert a Flashlight for Astronomy in a Few Seconds




Introduction: Convert a Flashlight for Astronomy in a Few Seconds

About: Master in Electrical Engineering Background: Renewable energies. Hobbies: Astronomy, DIY projects, Flight sports, RC planes and drones

Flashlights are a very popular gadget. Everybody should have one, at least, at home or in the car. If you travel a lot during the night or if you spend some time at dark places, definitively, you already should have one!

But, if your purpose is observing the night sky, that's a different story!

As you know, there is a phenomena called "light pollution" around populated areas, like big towns. One story tells that, during a blackout, some New Yorkers called to 911 to report some strange lights on the sky... Well, they were just reporting what they were seeing: the stars light... because usually, they can't see them.

So, if you want to admire the night sky, you must move to a very dark place to avoid light pollution. It's also desirable you search for highlands to avoid the atmosphere diffraction effect.

Yes, as you can imagine, among other things, you will need a flashlight, but not a common flashlight.. What you will really need its a red flashlight!

Why red? There's a little explanation that will "open your eyes":

  1. Inside your eyes, there is a mechanism that regulate the amount of light that can enter, called diaphragm, like those you can see in a camera lens. Through that mechanism, the size of your pupil will differ from a dark place to a light place. You can try, front to a mirror, switching from a low to a strong light and you will see your pupil opening and closing accordingly.
  2. Some studies reveal that the human eye takes approximately 20–30 minutes to fully adapt from bright sunlight to complete darkness and become ten thousand to one million times more sensitive than at full daylight. In this process, the eye's perception of color changes as well. However, varying the wavelengths of the light, will also affect the dark adaptation curve. Long wavelengths, such as extreme red, create the absence of a distinct rod/cone break as the rod and cone cells have similar sensitivities to light of long wavelengths. Conversely, at short wavelengths the rod/cone break is more prominent, because the rod cells are much more sensitive than cones, once the rods have dark adapted.

That´s why you need a red light! Now, you can stop thinking that the astronomers are freaks when they use red lights...! You already know the reason!

So, if you are in the middle of nothing, waiting for your eyes to adapt to the darkness and you need to walk around to find a place to "take a leak" or if you just need to read something (such as a sky chart) when your eyes are already adapted to darkness, you always can use a red light to guide you. Please, don´t worry, because it's a red light, you will not loose your eye adaptation and start over again!

Because I intended to have a flashlight for astronomy, but also for my daily usage, I thought gather the best of both worlds: A convertible one!

This instructable will guide you, step by step, how to make a red flashlight from an ordinary one and how to easily convert from one to the other.

Step 1: Tools and Materials

What will you need?

Well, if you already have a basic flashlight, you will only need to find a red and semitransparent plastic film and a small rubber band - that´s it!

I found a nice piece of red plastic from an old beach ball, like the one at the picture above. You can also try with a red balloon, but it might be a little too opaque.


All that you will need is:

  • Scissors,
  • Pen,
  • One can (or something circular..)

Step 2: Cut the Plastic Film

Cut a red strip from the ball, using the scissors.

Find something circular with about the double (or even the triple) of the flashlight diameter to draw a circle - an empty can should be fine!

After drawing the circle, cut it out from the strip.

Simple, isn't it?

All of your building work it's done!

Step 3: Switch Your Flashlight to Red and Back to White

You must have noticed all that you need to turn your flashlight into red and back to white, it's simply done by placing or removing the red plastic from the light source.

To do that, use one hand to place the plastic strip around the flashlight and the other one to place a rubber band around it. If your rubber band it's too big like mine, give 2 or more turns until it gets tight enough.

I made a small video to show you how it's done. It took only a few seconds to completely convert from white to red and back to white.

Enjoy it!

Step 4: Some Extra Tips

Extra tips:

To prevent losing parts (rubber band and red plastic) while you are not using them, it's a good idea if you stow them, using the same idea, in the back of the flashlight.

Just in case, it's also a good idea if you have a spare rubber band... You can stow it around the tube.

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    3 Discussions


    4 years ago

    Hi, Thanks for your comments!

    Painting the glass with red nail polish, looks like a good idea .. always learning something new!


    4 years ago

    Great DIY Astronomy Flashlight! I've also heard of people painting the glass with red nail polish, to produce the red light.


    4 years ago

    Very nicely done!