This involves looking to detect the 21-cm (1.42 GHz) neutral atomic hydrogen (HI) line.
Mapping extragalactic hydrogen with a small backyard dish is incredibly challenging, but is possoble. It was first acheived July 2026 by Jason Burnfield.
The process breaks down into the following technical specifications:
- Dish Parameters:
- Frequency: 1420.405 MHz
- Wavelength: 21.1 cm
- F/D Ratio: Optimal range between 0.3 and 0.45 for standard prime-focus offset dishes.
- Feedhorn:
- Type: Circular waveguide feed (often a 150 mm to 160 mm diameter copper pipe cut to precise lengths) or a septum feed to handle dual polarization and eliminate feed-leg blockage.
- Electronics:
- LNA (Low Noise Amplifier): You will need a high-quality, ultra-low noise amplifier (e.g., NF < 0.3 dB such as those from Mini-Circuits or generic RTL-SDR front-ends) placed immediately at the feedhorn to minimize signal loss.
- SDR (Software Defined Radio): Units like the RTL-SDR v4, HackRF, or Airspy R2 paired with software like SDR# or GNURadio are used to process the raw RF signal into frequency spectrum data.
Signal Strength & Challenges:
Because M33 is incredibly faint in the radio spectrum, your small dish’s beamwidth (Field of View) will likely be larger than the galaxy itself (small dishes have poor angular resolution for extragalactic objects). You will primarily detect a slight “bump” in the baseline noise floor indicating neutral hydrogen presence, rather than a resolved map. To extract this faint signal, you must perform significant processing of signal data compared to baseline outside of the signal time period, or other effective method. In addition, data will need to be integrated over several nights.