Spectroscopy at Home: Splitting Starlight with a Simple Grating

When you look at a star through your telescope, you see a point of light. Beautiful, yes, but that single dot is hiding an extraordinary amount of information. Split that light into its component wavelengths and suddenly you can determine the star's temperature, chemical composition, radial velocity, and even whether it has a companion. This is spectroscopy, and it is one of the most powerful tools in all of astronomy.

The best part? You can do it from your backyard with surprisingly modest equipment.

What Is a Spectrum?

When light passes through a prism or a diffraction grating, it separates into its component wavelengths, the familiar rainbow of colors from violet through red. But stellar spectra are not smooth rainbows. They are crossed by dark lines called absorption lines, each one marking a specific chemical element in the star's atmosphere that has absorbed light at that exact wavelength.

Equipment You Need

Getting started with spectroscopy is simpler than you might expect:

• A diffraction grating: The most accessible option is a Star Analyser 100 or similar blazed transmission grating. It threads into your telescope's eyepiece holder like a standard 1.25-inch filter. Cost: around $50-180 depending on quality.
• Your telescope: Any telescope works, but a telescope with a tracking mount makes life much easier since you need to keep the star centered while imaging.
• A camera: A planetary camera (like a ZWO ASI series) or even a DSLR at the telescope's focus. You need to capture the spectrum as an image for analysis.
• Software: Free tools like RSpec (trial), BASS (free), or VSpec (free) can extract a calibrated spectrum from your image.

The Simplest Setup

Place the diffraction grating in front of your camera sensor (in the filter wheel or threaded into the nosepiece). When you point at a bright star and take an exposure, you will see the star as a point source plus a streak of rainbow color extending to one side. That streak is the star's spectrum, and it contains all the information you need.

What You Can Actually See

Even with a basic grating, you can observe remarkable things:

Stellar Classification

Stars are classified into spectral types, O, B, A, F, G, K, M, based on their spectra. Each type has distinctive absorption line patterns:

Emission Nebulae

Point your grating at a bright nebula like the Orion Nebula (M42) and something dramatically different happens. Instead of a continuous rainbow with dark absorption lines, you see bright emission lines against a mostly dark background. The strongest lines come from hydrogen (H-alpha at 656nm, H-beta at 486nm) and doubly-ionized oxygen (OIII at 496nm and 501nm). You are directly seeing the chemical elements that make up the nebula.

Going Further: Calibration and Measurement

Once you can capture spectra, the next step is wavelength calibration, assigning precise nanometer values to each position along the spectrum. Software like RSpec or BASS does this semi-automatically: you identify two known lines (such as hydrogen-alpha and hydrogen-beta), and the software calculates the wavelength scale for the rest of the spectrum.

With a calibrated spectrum, you can:

• Classify stars by their spectral type
• Measure radial velocities by detecting Doppler shifts in line positions
• Identify chemical elements in nebulae and unusual stars
• Monitor variable stars as their spectra change over time
• Contribute to science, amateur spectroscopy is one of the few areas where backyard observers still make valuable contributions to professional research

If the idea of analyzing starlight appeals to you, spectroscopy pairs well with understanding star colors and temperatures. And for the camera setup, our ZWO camera guide covers the sensors that work best for this type of work.