Observing the 21cm Hydrogen Line with Linrad and an RTL-SDR

Over on YouTube user S53RM has uploaded a video showing his and S53MM’s observation of the 1420 MHz galactic hydrogen line with an RTL-SDR. Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will rarely emit a photon, but since space and the galaxy is filled with many hydrogen atoms the average effect is an observable RF power spike at 1420.4058 MHz. By pointing a radio telescope at the night sky, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot.

In the video they rotate their 3.6m parabolic mesh antenna dish along the Milky Way. As the dish rotates doppler shifted hydrogen line peaks can be observed on Linrad, each peak representing a different arm of the galaxy. The galaxy consists of several spinning arms, some spinning faster than others which causes the hydrogen line peaks produced by the arms to be doppler shifted by different amounts.

They used Linrad to plot the RF spectrum as they were able to use it together with a pulse generator to calibrate the RTL-SDR for a flatter frequency response.

More information about their project can be found at http://lea.hamradio.si/~s53rm/Radio%20Astronomy.htm.

Linrad showing Galactic Arm Hydrogen Line Peaks
Linrad showing Galactic Arm Hydrogen Line Peaks
Hydrogen 21cm lines with DVB-T dongle

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How do you convert radio into actual images of what the radio is seeing?
What software can be used?

Shayne O'Neill

Linrad. Though most decent SDR software would suffice. Remember, this stuff doesn’t give you a “photo”, its giving you the RF output of what telescope is looking at. To get anything like a photograph type image you’d need a much more directional antenae, some sort of very accurate motorized mount for it, and a lot of spare time. And possibly a significantly stronger signal (There are signal amps for 21cm emmission line astronomy, some quite affordable)