Amateur Spectroscopy Guide: Analyzing Starlight with Affordable Equipment

Image Credit: NASA/ESA/Hubble – The Pillars of Creation in the Eagle Nebula. Spectroscopy revealed that these structures are composed primarily of hydrogen and dust, with young stars forming inside. Public Domain.

What Is Spectroscopy?

Spectroscopy is the science of splitting light into its component wavelengths and analyzing the resulting spectrum. It’s arguably the most powerful tool in all of astronomy — nearly everything we know about stars, galaxies, and the universe comes from analyzing light in this way. Temperature, chemical composition, velocity, magnetic fields, density, and distance can all be determined from a spectrum.

What many people don’t realize is that amateur astronomers can do real spectroscopy from their backyards — and contribute meaningful data to professional research.

How Spectra Work

When you pass starlight through a prism or diffraction grating, it spreads into a rainbow — the star’s spectrum. Three types of spectra exist:

1. Continuous Spectrum

A hot, dense object (like a star’s surface) emits light at all wavelengths, producing a smooth rainbow. The peak wavelength tells you the temperature — Wien’s Law states that hotter objects peak at shorter (bluer) wavelengths.

2. Absorption Spectrum

When a continuous spectrum passes through cooler gas, atoms absorb specific wavelengths, creating dark lines. Each element produces a unique fingerprint of absorption lines — hydrogen, helium, calcium, iron, sodium, and dozens more are identifiable. This is how we determine stellar composition.

3. Emission Spectrum

Hot, low-density gas (like a nebula) emits light only at specific wavelengths, producing bright lines on a dark background. H-alpha (656.3nm), OIII (500.7nm), and H-beta (486.1nm) are the lines astrophotographers capture with narrowband filters.

The Doppler Shift

When a star moves toward us, its spectral lines shift to shorter (bluer) wavelengths — blueshift. When it moves away, lines shift to longer (redder) wavelengths — redshift. By measuring how much the lines shift, you can calculate the object’s radial velocity with remarkable precision. This is how:

• Exoplanets are detected via the radial velocity method (the star “wobbles”)
• Binary star orbits are mapped
• Galaxy recession velocities (and thus distances) are measured
• The expansion of the universe was discovered

Amateur Spectroscopy Equipment

Entry Level ($50-200)

• Star Analyser 100/200: A blazed diffraction grating in a standard 1.25″ filter thread. Screw it into your camera’s filter wheel or nosepiece and it creates a spectrum alongside each star in your image. The Star Analyser 100 (100 lines/mm) is the single best way to start — $200 and it works with any telescope.
• DIY gratings: Inexpensive transmission gratings from educational suppliers ($10-20) can be placed in front of a camera lens for basic stellar classification.

Intermediate ($500-2,000)

• ALPY 600 (Shelyak Instruments): A complete slit spectrograph producing resolved spectra suitable for scientific work. Resolution ~600 — enough to classify stars, study emission objects, and measure velocities.
• LISA spectrograph: Similar capability to the ALPY in a different design.

Advanced ($2,000+)

• eShel spectrograph: A fiber-fed echelle spectrograph achieving resolution of ~10,000 — comparable to small professional instruments. Capable of detecting exoplanets via radial velocity.

Processing Spectra

Free software for spectral analysis:

• RSpec: Beginner-friendly commercial software ($100) with excellent tutorials.
• ISIS (Integrated Spectrographic Innovative Software): Free, powerful spectral processing — calibration, line identification, radial velocity measurement.
• BASS Project: Free software for basic spectral analysis and calibration.
• Demetra: A newer free tool optimized for the Star Analyser workflow.

What You Can Do

• Classify stars: Identify stellar types (O, B, A, F, G, K, M) from their absorption lines. Hot O-type stars show strong helium lines; cool M-type stars show molecular bands.
• Study variable stars: Track spectral changes as stars pulsate or erupt.
• Observe novae and supernovae: Capture the spectral evolution of stellar explosions — this is real science that professional astronomers need help with.
• Analyze Be stars: Monitor the emission lines from rapidly rotating stars with circumstellar disks.
• Measure redshifts: Determine recession velocities of bright galaxies and quasars.

Contributing to Science

Amateur spectroscopy is one of the most scientifically valuable activities in amateur astronomy. Organizations like the AAVSO (American Association of Variable Star Observers) and the BAA Spectroscopy Section coordinate amateur spectroscopic campaigns that complement professional observations. When a nova erupts or a Be star goes into outburst, amateur spectrographs around the world provide continuous monitoring that no single professional observatory can match.

Next in our series: Contributing to Citizen Science: How Amateur Astronomers Make Real Discoveries