Red Flashlights For Astronomy: A Not-So-Secret Night Vision Trick

Since ancient times, people have been looking up at the starry sky, studying and admiring its beauty and mystery. Over time they developed tools to make stargazing easier. One such tool is the red flashlight.

Here’s the point of red flashlights for astronomy

A red flashlight helps you see star charts and equipment in dark surroundings with minimal impact to your night vision. White light disables the rods in our eyes, which makes it harder to see in the dark. Red light affects the rods far less, so it’s preferable for illuminating star charts and equipment in low light.

The red flashlight is an excellent addition to your stargazing equipment and a must-have if you want to have a full and unobstructed experience while observing the stars.

However, let’s start by looking at why ordinary flashlights won’t work very well. We’ll then dig deeper into the science of red light and your eyes, and end with some slick DIY alternatives.

But what’s wrong with a normal flashlight?

Normal flashlights use white or yellow light, which mimics daylight, thus having a negative impact on our night sight. But how does that happen?

To properly observe the night sky, you need to find a place with almost no light pollution, preferably with none at all and give your eyes the necessary time to adjust to the new conditions.

You need to keep in mind that it takes 15 to 30 minutes until your eyes have completely adjusted to the darkness so that you can see the faintest objects in the sky. If you use a normal flashlight while looking at the star chart after your eyes have just adapted to the darkness, the white light will affect your night sight, and you won’t be able to observe the night sky at your full capacity.

You will lose precious time waiting for your eyes to adapt to the darkness all over again, and you might even miss on some dim sky objects.

How would a red flashlight help me stargaze?

Our eyes are adjusting to darkness through two independent processes, and both of them are important when it comes to stargazing.

The first is correlated to our iris’s size, and it happens rather quickly, in a few seconds. When we are exposed to light, during the day, for example, our iris narrows, while at night, it opens up so that more light will be let in. You can observe this process by turning on and off the light while looking in the mirror.

The second one is far more complex, and it requires a lot more time. Our retina is susceptible to light through cones, which help us see colors, and rods, that are responsible for night vision.

When the rods are exposed to bright light, such as the one emitted by a normal flashlight, they become blind and stop functioning. It takes up to half an hour, or even more until they chemically reset so that you will be able to see in the dark again correctly.

But fortunately, the rods are less sensitive to red light, and using a red flashlight won’t ruin your night sight. This way, you can observe your star chart and adjust your telescope or binoculars without risking to alter your night vision.

Can I make one myself?

Yes, you can! Even though a red flashlight is not really expensive, it can mean more money invested in the equipment and money that you may not have for a beginner astronomer.
So here are some tips on how to make your own red flashlight!

1. Red plastic

You will need a piece of red plastic, a pair of scissors, and a marker. The plastic must be clean, without any writing on it. You can look for sheets of red plastic at a nearby craft store if you like, but any other piece of plastic will work just fine.

Remove the head of the flashlight and get the lens out. Place it on the red plastic, trace its form on it using the marker, and then cut it out. Try to see if the red piece fits where the former lens was and if not, cut it a little more until it fits.

Finally, put the red lent you made behind the original lens, put both of them back into the flashlight, and reassemble it. You should get a deep red color, but if this is not the case, you might want to try again with a different type of plastic.

2. Red Cellophane

This may be the easiest way of making a red flashlight on your own. All you need is a normal flashlight, a rubber band, and a piece of red cellophane. I suggest you pick a darker shade of red so that you won’t need that many layers.

Fold the cellophane two or three times over the flashlight so that you will cover the lens completely and fasten it with the rubber band. Close the lights and check to see if the light is red. If it is blue or pink, just add some extra cellophane layers until the light turns red.

4. A bottle cap

If you have a mini flashlight and you want to turn it into a red flashlight, this is the perfect tip for you. All you need is a red bottle cap, preferably with no inscriptions on it.

Just put the bottle cap directly over the lens. If it is too big, you can roll a piece of paper on the inside walls of the bottle cap so that you provide enough friction in order to stay put.

This hack is perfect if you want to have both a normal flashlight and a red flashlight in handy when you are stargazing. The cap can be easily removed and then placed back on.

3. Red gel filter

You will need red gel filters that you can easily find in most of the craft stores for this hack. Be sure you buy three or more pieces, just in case you will need more to get to the perfect shade of red.

Once you have them, remove the head of the flashlight and use its original lense to cut a perfectly round shape. Place the red gel filter behind the original lense and reassemble the flashlight. Check to see if the light is a dark shade of red and if not, just add more layers of gel filter.

Tips for using a red flashlight when stargazing

Red light is a lot friendlier to your night sight than white or yellow light, but it still has some impact.

You need to use it judiciously and not keep it on all the time, as it can lower your chances of observing faint deep sky objects. To minimize that impact, turn it on when you need to check the star chart or to adjust your telescope or binoculars, but leave it off otherwise.

Do not look right into the lenses as the light will appear stronger and will eventually alter your night vision, but project the light on the star or your point of interest.

In case you use a homemade red flashlight, you need to remember that long exposure to light can discolor the gel filter or the cellophane, thus altering the quality of the light. In this case, just replace the gel filter with a new one. It is better and safe to do this from time to time, according to how much you use your flashlight.

Stargazing can be extremely challenging, but it becomes a lot easier if you have the right equipment and tools. Using a red flashlight will not only preserve your dark sensitivity but will also allow you to experience stargazing at its best.