Tutorial Radio astronomy

Tutorial: Radio astronomy (the hydrogen line)

What it is. Neutral hydrogen in our galaxy radiates at 1420.4058 MHz (the 21 cm line). With a small dish or horn, a low-noise amplifier and an SDR you can see the Milky Way's hydrogen as one or a few humps a few hundred kHz wide, a few percent above the noise, and read the velocity of the gas along the line of sight from their Doppler shift. OnAir integrates the spectrum for seconds to an hour, takes out the shape of the band with a reference or a baseline fit, and shows frequency or radial velocity.

What you need

• A radio that tunes 1420 MHz with a low noise figure. The mode asks for 2.5 Msps and works down to 250 ksps; the table covers 10 to 6000 MHz. Check that your radio reaches 1.42 GHz (see its radio page); the radio's own noise matters, so a front-end LNA is needed. See Supported radios.
• An antenna: a horn, a small dish, or a simple "can" feed (a tin about 15 cm across with a quarter-wave probe), as the in-app help describes.
• A low-noise amplifier (noise figure under 1 dB) right at the antenna, and a band-pass filter for 1420 MHz after it. A bias-tee or the radio's antenna power can feed the LNA.
• The frequency database lists the line: the 21 cm hydrogen line at 1420.4058 MHz, plus the OH lines (1612.231, 1665.402, 1667.359 and 1720.530 MHz), deuterium (327.384 MHz) and formaldehyde (4829.66 MHz). The Line drop-down in the tuner picks the rest frequency and tunes the radio there; there is a custom entry too.

Before you start

• The line sits very near the middle of the spectrum, where the radio's DC spike is. Keep Remove DC ticked: it bridges the spike.
• Raise the gain until the noise floor rises clearly when the LNA is powered, without overload. Powering the LNA on and off is a good test: the floor should move.
• Interference is the enemy: radios, Wi-Fi, switching supplies and GNSS signals. 1420 MHz is a protected radio-astronomy band, but nearby transmitters on adjacent channels still leak in; the filter helps.
• Point at the galactic plane (Cygnus, Cassiopeia, Aquila, Sagittarius). Pointing at the zenith also works when the plane passes overhead: a drift scan.

Step by step

1. In the mode list open Weather & science and choose Radio astronomy (second line Hydrogen line, 1420 MHz). The tab is called Astronomy; the ? button there opens a short how-to.
2. In Line choose the hydrogen line.
3. Choose FFT (1024 to 16384 bins; more bins give finer velocity resolution) and Integrate (1 s, 5 s, 10 s, 30 s, 1 min, 2 min, 5 min, 10 min, 30 min or 1 h). Press Start.
4. A strong plane shows in about 10 s; weaker parts need a minute or more. Restart begins the integration again.
5. Choose the Reference: none (the spectrum over its median), reference (off source), or frequency switching. For a reference: point away from the plane, press Capture off, then point back at the source. For frequency switching the radio alternates every second between its frequency and one shifted by the kHz offset (a radio is needed, not a recording).
6. Fit a baseline: set the polynomial degree (2 to 4) and the Window (the velocity range of the line in km/s, excluded from the fit; right-drag on the chart to mark it). Smooth averages neighbouring bins.
7. Choose the horizontal axis (frequency or velocity) and the vertical one (relative dB, temperature K with a K Tsys value, or raw dBFS). Save CSV writes frequency, velocity, power, the processed spectrum and the baseline, one row per bin.

What you should see

• A noisy band shape in the raw spectrum. After the reference or baseline, a flat floor with one or several humps near 1420.4 MHz, a few hundred kHz wide (tens of km/s).
• Below the spectrum a drift-scan waterfall: each finished integration is a row, so the sky going by shows up over time.
• A results table of finished integrations: time, peak velocity (km/s), excess (%) and SNR. The text beside the chart gives the resolution (Hz and km/s per bin), the span, the reference used, and the peak: frequency, velocity, percent over the reference and SNR with the noise in percent rms.

Reading the plot

• Velocity uses the radio convention v = c (f0 - f) / f0, topocentric: no correction to the local standard of rest. The Earth's orbit and rotation and the Sun's motion shift it by up to about +-30 km/s depending on date, time and direction.
• The peak marker shows the strongest point in the line window.
• The excess is relative to the reference: (on - off) / off; with K Tsys it becomes an antenna temperature.

Options and settings

• Fold (in frequency switching): puts the negative image of the line back onto it (the average of the two).
• Custom line frequency in MHz.
• The test signal: noise with a sloped band and a weak hydrogen line at -42 km/s, 20 dB under the noise; Off source (no line) gives the empty case. Use it to learn the controls.

Troubleshooting

• No hump at all. Check LNA power, filter and aiming; integrate longer; make sure Remove DC is on and that the reference is off source.
• Humps appear as flat tops or have spikes. Interference or overload; filter and reduce gain.
• The baseline wobbles. Increase the polynomial degree to 3 or 4 and set the line Window so the fit does not eat the line.
• Velocities do not match a catalogue. They are topocentric, not LSR-corrected.
• Frequency switching says "needs a radio". It works only with a live radio.

Tips

• Take the reference at the same gain and sample rate as the observation; a changed FFT size warns "other FFT size".
• Long integrations beat bigger antennas for a first result.

Legal and privacy notes

Receiving is passive and legal. 1420 to 1427 MHz is a protected radio-astronomy band; please keep any transmitters of your own off it.

Under the hood

The receiver computes averaged power spectra (FFT 1024 to 16384) over the chosen integration time, optionally subtracts a captured off-source reference or the other phase of frequency switching, fits a polynomial baseline outside the line window, and converts frequency to Doppler velocity relative to the rest frequency.

Related

Mode page: Radio astronomy | Spectrum explorer | Spectrum analyser | Supported radios | Troubleshooting