Building a Window into the Galaxy: Designing a Home Radio Telescope for Detecting 21 cm Hydrogen Emission

This paper by Phelps explores the design and application of a home-built radio telescope that can detect the 21 cm hydrogen line, a specific radio frequency emitted by neutral hydrogen atoms. The author’s setup is designed to be affordable and accessible to amateurs, providing a unique way to observe galactic structure and hydrogen movement within our galaxy. By studying the Doppler shifts in the 21 cm emission, Phelps aims to uncover information about the distribution and velocity of hydrogen clouds across the Milky Way, particularly within the galaxy's spiral arms.

Understanding the 21 cm Hydrogen Line

The 21 cm line, also known as the hydrogen line, is a crucial spectral marker in radio astronomy. Emitted by neutral hydrogen atoms, this frequency results from a process called "hyperfine splitting." This occurs when the electron in a hydrogen atom transitions from a parallel to an anti-parallel spin state relative to the proton, releasing energy at a frequency of 1420.405 MHz. Since hydrogen is the most abundant element in the universe, observing this line allows scientists to map out hydrogen-rich regions within galaxies. Unlike visible light, which can be blocked by dust, the 21 cm emission passes through galactic material, providing a clear view of hydrogen clouds and enabling researchers to analyze the structure of the Milky Way.

Designing the Telescope

The telescope setup includes a one-meter parabolic dish typically used for satellite data, paired with a low-noise amplifier (LNA) tuned to the hydrogen frequency, and a software-defined radio (SDR) device. This SDR converts radio waves into digital data that can be analyzed on a Raspberry Pi computer. By using software and minimizing hardware needs, this design is cost-effective, allowing flexibility to detect hydrogen emission from a simple at-home observatory.

Signal Processing and Calibration

For data processing, Phelps utilizes a Fast Fourier Transform (FFT) to convert the radio signal from the time domain to the frequency domain, which highlights the 21 cm hydrogen line in the frequency spectrum. Signal quality is improved by methods like vector medians, which help reduce background noise, and calibration techniques using “hot” and “cold” observations to ensure accurate temperature measurements. These calibrations convert raw signal counts into meaningful brightness temperatures, helping to refine observations and improve the signal-to-noise ratio of hydrogen emissions.

Observations and Data Collection

Observing the Milky Way’s structure, Phelps uses drift scans, which involve varying the dish elevation to capture data from different regions of the sky. Observations revealed multiple peaks in the velocity profiles, which correspond to the motion of hydrogen gas in different parts of the galaxy. These profiles are essential for understanding the dynamics of galactic arms, as gas clouds in arms closer to the galactic center move faster than those in outer regions. Observations consistently matched known features in the Milky Way, such as the hydrogen-rich Sagittarius and Perseus arms, supporting the telescope’s ability to detect and differentiate galactic structures.

Reducing Noise and Interference

Radio Frequency Interference (RFI) from urban environments presents a significant challenge. The study used shielding, including tinfoil, around the electronics to block unwanted noise, which is especially important in the suburban environment of Los Angeles. This shielding, combined with specific RFI mitigation strategies in the data processing software, helped to maintain data integrity, allowing clear identification of the hydrogen line even amidst interference.

Insights on Galactic Rotation and Structure

Through velocity measurements, Phelps identifies variations in the speed and distribution of hydrogen gas, highlighting how gas in spiral arms moves at different speeds depending on its distance from the galactic center. This information aligns with models of galactic rotation, providing indirect evidence for dark matter, which is thought to influence the motion of outer gas clouds. These observations contribute to a broader understanding of how matter is distributed and moves within the galaxy.

Conclusion

Phelps’ radio telescope offers a glimpse into the vast structure and dynamics of our galaxy, made possible through a relatively simple and affordable setup. This approach opens doors for amateur astronomers to engage in meaningful scientific observation, enabling them to map hydrogen clouds and trace the rotation of the Milky Way.

Source: Phelps