Stellar Spectroscopy Using Diffraction Grating, CMOS Monochrome Sensor, and Reflecting Telescopes

TL;DR

We address the challenge of making stellar spectroscopy accessible for undergraduate and outreach contexts by developing a compact, diffraction-grating-based spectrometer that couples a CMOS sensor to reflecting telescopes. The instrument uses a 600 lines/mm grating, a 3D-printed mount, and a Python pipeline for wavelength calibration and spectral stacking, with Vega serving as the spectrophotometric standard. Calibrated spectra of five stars spanning spectral types A–M demonstrate alignment with space-based references and capture key features (Balmer lines, metallic lines, and TiO bands) despite modest resolution. The work validates a practical framework for teaching and student-led instrumentation, illustrating feasible, high-impact astronomy experiments with low-cost equipment.

Abstract

We present the design and testing of a compact, low-cost stellar spectrometer developed for undergraduate and outreach applications. The instrument employs a 600 lines/mm diffraction grating, a CMOS monochrome sensor, and a 3D-printed mount integrated with reflecting telescopes. Calibration was performed using helium emission sources in the laboratory and Vega as a spectrophotometric standard, supported by a custom Python-based image-processing pipeline for wavelength calibration and spectral stacking. The spectrometer successfully recorded usable spectra of bright stars including Vega, Sirius, Procyon, Capella, and Betelgeuse, covering spectral types A through M. The results demonstrate that meaningful stellar spectroscopy can be achieved with accessible, low-cost equipment, providing a practical framework for student-led astronomical instrumentation projects.

Figures (8)

• Figure 1: Left: Skyris 236M CMOS monochrome sensor used for spectral imaging. Right: 11-inch Celestron CPC Deluxe 1100 HD telescope with computerized tracking, used for primary stellar observations.
• Figure 2: Left: Ideal geometric alignment showing the diffraction grating, sensor position, and angular geometry for first-order diffraction. The distance $l$ between grating and sensor was optimized to capture the full visible spectrum. Right: 3D model of the custom 3D-printed spectrometer mount designed in SolidWorks. The mount securely holds the diffraction grating and CMOS sensor while interfacing with standard telescope eyepieces.
• Figure 3: Wavelength calibration using helium emission lines. The quadratic fit relates pixel position to wavelength, with residuals $<$1 nm across the visible range.
• Figure 4: Recorded Vega spectrum with the blue error bands representing standard deviations from stacked images, demonstrating the robustness of our data reduction pipeline.
• Figure 5: Identification of H-$\alpha$ and H-$\beta$ spectral markers in Vega observations. The algorithm identifies absorption line minima across multiple frames, enabling precise spectral alignment via $\chi^2$ minimization before stacking.