You Don't Need a Lab to Stargaze Like a Pro: How to Run a Backyard Spectroscopy Session With $15 of DIY Gear

Last June, I showed up to my local astronomy club's quarterly star party 20 minutes late, hauling my $1,800 research-grade spectroscope I'd been itching to demo for the group's teen members. I fumbled with the hatchback of my SUV, patted my pockets three times, and realized I'd left the entire case on my kitchen counter. The 12 curious 14-to-18-year-olds who'd signed up for the "advanced astro tools" workshop were already circling my empty folding table, looking equal parts bored and disappointed.

I ditched the panic, rummaged through my recycling bin and the trunk of my car, and 10 minutes later had a working spectroscope built from a paper towel roll, a scrap of diffraction grating film I'd left over from a middle school science fair, a cereal box, and a roll of black duct tape. We spent the next three hours pointing it at streetlights, neon diner signs, and bright stars, identifying sodium in the local streetlamps, argon in the taco shop's neon "OPEN" sign, and the hydrogen absorption lines in Vega. By the end of the night, three of the teens had asked to build their own to take home.

That impromptu session taught me something I've carried with me ever since: spectroscopy doesn't have to be the domain of university labs or people with $1,000+ gear. It's one of the most accessible, fun, and scientifically rewarding backyard astronomy projects you can try, no fancy equipment required. All you need is a few dollars' worth of household supplies, a dark night, and a curiosity about the chemical makeup of the world (and galaxy) around you.

For the uninitiated: spectroscopy is the science of splitting light into its component wavelengths, which act as a unique chemical fingerprint for whatever the light is bouncing off of. A sodium streetlight will glow with two sharp, bright yellow lines at 589 nanometers, a star like Betelgeuse will have a deep red continuous spectrum dotted with dark absorption lines from ionized metals in its atmosphere, and a neon sign will have a signature bright red line from neon gas. Commercial handheld spectroscopes can cost anywhere from $100 to $1,000, but a DIY version works just as well for casual backyard use, and costs less than a large pizza to build.

Build Your $15 DIY Spectroscope First

All the parts you need are available at a dollar store, online for a few bucks, or lying around your house already. Here's what to grab:

• A 12-inch long sturdy cardboard tube (a cut section of wrapping paper tube works best, but a clean paper towel roll or empty Pringles can works in a pinch)
• A 2x2 inch sheet of transmission diffraction grating film (you can buy a 10-pack for $5 on Amazon, or salvage a small piece from an old cheap laser pointer or broken CD/DVD if you're in a pinch, though it will have lower resolution)
• A 1mm wide slit cut from black construction paper (or a sharp razor blade to cut a slit directly into the end of your cardboard tube)
• Black duct tape or electrical tape (regular silver tape lets in stray light that will wash out your spectrum, so black is ideal)
• A 2x2 inch piece of white cardstock (to make your viewing port)
• Optional: a cheap smartphone tripod mount, or just steady hands to hold your phone up to the eyepiece

Building it takes 10 minutes, no tools required:

1. Cut a 1mm wide, ½-inch long slit in one end of the cardboard tube, about 1 inch from the edge. If you're worried about the edges of the slit letting in stray light, cut the slit in a piece of black construction paper first, then tape the paper securely over the end of the tube so the slit edges are perfectly dark.
2. Tape the diffraction grating film over the opposite end of the tube, making sure the thin grating lines run vertically (this will make the spectrum spread horizontally, which is much easier to view and photograph). If you're using a DVD, scratch the shiny metallic layer off a small 1-inch square section, then tape that over the end of the tube as a low-budget diffraction grating.
3. Cut a 1-inch square hole in the side of the tube, about 2 inches away from the slit end. Tape the white cardstock over this hole, leaving the center open -- this is your viewing port, where you'll hold your eye or smartphone to see the spectrum.
4. For phone photography: cut a 1-inch circle in a small piece of black cardboard, tape it over the viewing port, then hold your phone's camera up to the circle to take clear shots of the spectrum. No fancy adapter needed, just steady hands.

What to Test During Your First Backyard Session

Start with easy, bright terrestrial targets first to get the hang of your new spectroscope, before you move on to fainter celestial objects. Even if you live in a light-polluted suburb, these targets will give you clear, easy-to-interpret results:

1. Local streetlights : The orange high-pressure sodium bulbs that power most neighborhood streetlights will have two distinct, bright yellow emission lines right around 589nm -- that's the signature of sodium vapor, the gas that makes these bulbs glow orange. If your area uses LED streetlights, you'll see a broad blue peak around 450nm, plus a few sharp yellow lines from the phosphor coating that makes LEDs appear white. Old mercury vapor streetlights (still common in some industrial areas) will have sharp, bright green and blue lines. I spent an hour last month mapping every type of streetlight within a 2-mile radius of my house, and even spotted a few leftover mercury bulbs from the 1990s still in use on a side street near my local park.
2. Neon and LED signs : If you have a local diner, bar, or store with a neon sign, point your spectroscope at it to see the signature of the gas inside. A classic red neon sign will have a single sharp red line at 640nm from neon gas, plus faint blue or green lines if the sign has argon or mercury mixed in for extra color. LED billboards will have the same broad blue peak as LED streetlights, plus sharp lines from the red, green, and blue LED diodes used to make the display.
3. Propane grill flames : If you have a gas grill, hold the spectroscope up to the flame (don't touch the grill, obviously) and you'll see bright orange lines from ethyl mercaptan, the smelly additive added to propane to make leaks detectable, plus faint blue emission lines from burning carbon. It's a weirdly satisfying way to test the spectroscope, and you can even compare the spectrum of propane to natural gas if you have a gas stove to test too.

Once you're comfortable with terrestrial targets, move on to celestial objects. You don't need a dark sky preserve for these -- even from a Bortle 5 or 6 backyard, you'll get clear results for bright targets:

1. Bright stars : Start with Sirius, the brightest star in the night sky, which has a clear continuous spectrum with faint dark absorption lines from hydrogen and helium. Betelgeuse, the red supergiant in Orion's shoulder, has a deep red continuous spectrum dotted with dozens of dark absorption lines from ionized metals in its outer atmosphere, super easy to spot even with a DIY spectroscope.
2. The Moon : Don't point it at the Sun (seriously, never do that, even with the slit, it can cause permanent eye damage), but the light reflected off the Moon carries the same absorption lines as the Sun, plus faint lines from the lunar regolith. It's a super bright, easy target to test your setup before you move to fainter objects.
3. Gas giants : Jupiter and Saturn have clear, broad dark absorption bands in their spectra from methane and ammonia in their upper atmospheres, easy to spot even with a low-resolution DIY device.

Pro Tips for Clear, Useable Results

The biggest hurdle for backyard spectroscopy is stray light washing out your spectrum, so follow these tips to get the best results:

• Always work from the darkest part of your yard, and block local light sources (porch lights, nearby streetlights) with your body or a lens hood when you're pointing the spectroscope at faint celestial targets.
• Adjust the slit width if your spectrum is too blurry or too dim: a narrower slit will give you a sharper, more detailed spectrum, but it will be dimmer. A wider slit is brighter but blurrier, so find a middle ground that works for your target.
• If you're taking photos with your phone, use your camera's night mode with the longest possible exposure, and zoom in slightly on the spectrum to get a clear, sharp shot. Turn off your flash, obviously -- it will wash out the entire spectrum.
• If you want to measure the exact wavelength of the lines you're seeing, use a known reference line to calibrate your device. A cheap red laser pointer has a single sharp line at 650nm, so point it at your spectroscope first, mark where that line falls on your viewing port, and use that to estimate the wavelength of other lines you see.
• If the spectrum looks washed out or rainbow-y without distinct lines, your slit is probably too wide, or stray light is getting into the tube. Add more black tape around the edges of the slit and grating to block extra light.

A Quick Safety Note

Never, ever point your spectroscope directly at the Sun. Even the narrow 1mm slit will let through enough concentrated sunlight to cause permanent retinal damage in a fraction of a second. Stick to terrestrial light sources, the Moon, and bright stars for your sessions, and you'll be completely safe.

I've been running backyard spectroscopy sessions with my DIY spectroscope for 8 months now, and I've learned more about the light around me than I did in 10 years of casual astronomy. I've identified the gas in every neon sign within 3 miles of my house, mapped the spectral lines of 20+ bright stars, and even used it to test the LED bulbs I was considering buying for my porch. Last week, I ran a build session for the local middle school's STEM club, and 12 kids built their own spectroscopes to take home, all for less than $2 each in bulk supplies.

You don't need a fancy lab, a grant, or a $1,000 piece of equipment to do real, hands-on science. All you need is a cardboard tube, a piece of cheap film, and a willingness to look at the world a little closer. Next time you're out in your backyard at night, grab a cereal box and a scrap of DVD, build a quick spectroscope, and point it at the nearest streetlight -- you'll be surprised how much of the universe's chemical fingerprint you can pick up, just a few feet from your back door.