How professionals convert raw telescope voltages into intensity maps of hydrogen in different parts of the Milky Way
Great question, Andrew — this is right at the heart of how the Milky Way has been mapped in neutral hydrogen (HI) at 1420.405 MHz.
Professional radio astronomers follow a fairly standard pipeline to convert raw telescope voltages into intensity maps of hydrogen in different parts of the Milky Way. The main steps are:
1. Collecting the raw signal
- A radio telescope measures the electric field from the sky, usually with a feed/receiver.
- This gets downconverted to an intermediate frequency (IF), digitised by an ADC, and stored as time-domain voltages.
2. Spectral analysis
- The time-domain data is Fourier-transformed (using an FFT or autocorrelator) to get power as a function of frequency.
- Since the HI line is very narrow, they need high spectral resolution (e.g. a few kHz).
3. Calibration
Two kinds are essential:
- Flux calibration: Compare with a known source (e.g. Cas A, Tau A) or inject a noise diode to convert arbitrary units into Kelvin (brightness temperature) or Jansky (flux density).
- Bandpass calibration: Remove the frequency-dependent response of the receiver so that features in the spectrum are astrophysical, not instrumental.
4. Baseline subtraction
- Fit and subtract smooth polynomial “baselines” (due to system noise and receiver gain curve).
- Leaves only the hydrogen line emission above the noise floor.
5. Doppler correction
- Because Earth is rotating and orbiting the Sun, observed frequencies are shifted.
- Correct spectra into the Local Standard of Rest (LSR) so velocities match Galactic kinematics.
6. Velocity mapping
- Convert frequency offset (Δf from 1420.405 MHz) into radial velocity using the Doppler relation.v=c⋅Δffrestv = c \cdot \frac{\Delta f}{f_{rest}}
- Each frequency bin now corresponds to hydrogen moving at a specific velocity relative to us.
7. Position + velocity cube
- For each telescope pointing, you get intensity vs. velocity (a spectrum).
- Combine spectra across different pointings → build a data cube:TB(l,b,v)T_B(l, b, v)where l = Galactic longitude, b = Galactic latitude, v = radial velocity.
8. From velocity to distance
- Using a Galactic rotation model, astronomers map line-of-sight velocities into approximate distances (inside the Milky Way disk).
- This lets them reconstruct a 3D map of HI gas distribution.
9. Final products
- HI spectra at each direction (line profile).
- Channel maps (images at a fixed velocity slice).
- Integrated intensity maps (sum over velocity ranges).
- 3D HI cubes, which show the hydrogen structure of the Milky Way.
👉 For amateur/pro-am work (like with Radio JOVE or small 21 cm setups), you can usually get to steps 2–6 fairly easily. Steps 7–9 need systematic sky coverage and more sophisticated calibration, but even a modest setup can show hydrogen spectra and Galactic rotation signatures.