Tagged: hydrogen line

A Hydrogen Line Telescope Made from Cereal Boxes and an RTL-SDR

SpaceAustralia.com have recently been hosting a community science project that involves encouraging teams to build backyard radio telescopes that can detect the arms of our Milky Way Galaxy by receiving the Hydrogen line frequency of 1420 MHz.

This can be achieved at home by building a horn antenna out of cardboard and aluminum foil, and a feed from a tin can. Then the Hydrogen line and galactic plane can be detected by using an RTL-SDR, LNA, and software capable of averaging an FFT spectrum over a long period of time.

While most horn antennas are typically made from four walls, one participant, Vanessa Chapman, has shown that even trash can be used to observe the galaxy. Vanessa's horn antenna is made from multiple cereal boxes lined with aluminum foil and an old tin fuel can. The boxes are held together by some string and propped up by some sticks.

With her cereal box horn antenna combined with an RTL-SDR Blog V3, and an RTL-SDR Blog Wideband LNA, Vanessa was able to use software to average the spectrum over time as the galactic plane passed overhead, revealing the Hydrogen line peak and corresponding doppler shift from the galactic plane.

Vanessa's Hydrogen Line Radio Telescope made from Cereal Boxes
Vanessa's Hydrogen Line Radio Telescope made from Cereal Boxes

If you don't know what the Hydrogen line is, we'll explain it here. Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will only very rarely emit a photon, but space and the galaxy is filled with many hydrogen atoms so the average effect is an observable RF power spike at 1420.4058 MHz. By pointing a radio telescope at the night sky and integrating/averaging the RF power over time, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot. This can be used for some interesting experiments, for example you could measure the size and shape of our galaxy. Thicker areas of the galaxy will have more hydrogen and thus a larger spike, whereas the spike will be significantly smaller when not pointing within the galactic plane. You can also measure the rotational speed of our galaxy by noting the frequency doppler shift.

Creating a 21cm Galactic Sky Map with an Airspy and 1.8m Dish

Marcus Leech from ccera.ca is a pioneer in using low cost software defined radios for observing the sky with amateur radio telescopes.  In the past he's shown us how to receive things like the hydrogen line,  detect meteors and observe solar transits using an RTL-SDR. He's also given a good overview and introduction to amateur radio astronomy in this slide show.

His recent project has managed to create a full Hydrogen sky map of the northern Canadian sky. In his project memo PDF document Marcus explains what a sky map shows:

A [sky map] shows the brightness distribution over the sky for a given set of observing wavelengths. In the case of the 21cm hydrogen line wavelength, maps show the distribution of hydrogen over the sky. For amateur observers, such maps generally show the distribution within our own galaxy, since extra-galactic hydrogen is considerably more faint, and significantly red/blue shifted relative to the rest frequency of 1420.40575 MHz, due to relative motion between the observer and the target extra-galactic hydrogen.

He was able to make this observation using his radio telescope made from a 1.8m dish antenna, a NooElec 1420 MHz SAWBird LNA + Filter, a 15dB line amplifier, another filter and two Airspy R2 software defined radios locked to an external GPSDO. The system runs his custom odroid_ra software on an Odroid XU4 single board computer, which provides spectral data to an x86 host PC over an Ethernet connection. 

Over 5 months of observations have resulted in the Hydrogen sky map shown at the end of this post. Be sure to check out his project memo PDF file for more information on the project and how the image was produced. Marcus' blog post over on ccera.ca also notes that more data and different maps will be produced soon too.

Hydrogen Sky Map
Hydrogen Sky Map

Updates on the PICTOR Low Cost Open Source Radio Telescope Based on RTL-SDR

Back in July we posted about PICTOR, an open source and RTL-SDR based radio telescope project. The owner of the project recently wrote in and wanted to share some updates. His text is below:

A few months ago, PICTOR was launched. PICTOR is a free to use open source radio telescope that allows anyone to observe the sky in the 1300~1700 MHz range at any time via the easy-to-use online platform.

The goal of this effort is to introduce students, educators, astronomers and others to the majesty of the radio sky, promoting radio astronomy education, without the need of building a large and expensive radio telescope. 

Since the initial launch, PICTOR has gotten lots of updates and improvements, particularly in the software backend, providing more data to the users, using advanced techniques to increase the signal-to-noise ratio by calibrating spectra and mitigating radio frequency interference (RFI) (if present).

Here is an example observation with PICTOR, clearly showing the detection of 3 hydrogen-dense regions corresponding to 3 unique spiral arms in the Milky Way!

Graphs from the PICTOR RTL-SDR Radio Telescope showing the 3 unique spiral arms in the Milky Way.
Graphs from the PICTOR RTL-SDR Radio Telescope showing the 3 unique spiral arms in the Milky Way.

If you’re new to radio-astronomy, the developer of PICTOR has provided a PDF including some introductory radio astronomy information and instructions on how to observe the radio sky with PICTOR: https://www.pictortelescope.com/Observing_the_radio_sky_with_PICTOR.pdf

Building An Open Source SDR Based Hydrogen Line Radio Telescope

Over on Reddit we've seen a post by u/ArtichokeHeartAttack who has been working on a hydrogen line radio telescope, based on an RTL-SDR dongle and horn antenna designs by the DSPIRA program, and the Open Source Radio Telescopes website (site appears to be down, linked to the archive.org copy). [u/ArtichokeHeartAttack] has documented their radio telescope building journey, providing a comprehensive top-level document that is able to point interested people in the right direction towards understanding and building their own Hydrogen line radio telescope.

Briefly, their build consists of a horn antenna and reflector designed for the 1,420.4 MHz Hydrogen line frequency. The horn is built out of a few pieces of lumbar, metallic house wall insulation sheets and aluminum tape. The feed is made from a tin can and piece of wire. In terms of radio hardware, they used an Airspy SDR, GPIO labs Hydrogen Line Filter + LNA, and 2x Uputronics Wide band preamps, and a Minicircuits VBF-1445+ filter. For software processing, they used a GNU Radio flowgraph to integrate and record the spectrum.

The results show that they were able to achieve a good hydrogen line peak detection, and they were able to measure the galactic rotation curve doppler shift, and tangent points which prove that we do in fact live in a spiral galaxy.

The Finished Hydrogen Line SDR Based Horn Radio Telescope Antenna
The Finished Hydrogen Line SDR Based Horn Radio Telescope Antenna

PICTOR: An Open Source Low Cost Radio Telescope based on RTL-SDR

PICTOR is an open source and open hardware radio telescope that aims to promote radio astronomy on a budget. It consists of a 1.5 meter parabolic dish antenna, 1420 MHz feedhorn, a two stage low noise amplifier (LNA), high pass filter, and from what we gather, an RTL-SDR. Future designs may also use higher bandwidth SDRs. Currently there doesn't seem to be much information about the build and exact components used in their design, but we're hoping that those details will come in time.

The radio telescope allows a user to measure hydrogen line emissions from our galaxy. Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). The emissions themselves are very rare, but since our galaxy is full of hydrogen atoms the aggregate effect is that a radio telescope can detect a power spike at 21cm. If the telescope points to within the plane of our galaxy (the milky way), the spike becomes significantly more powerful since our galaxy contains more hydrogen than the space between galaxies. Radio astronomers are able to use this information to determine the shape and rotational speed of our own galaxy.

PICTOR also has a very interesting web based interface which can be used to let users from anywhere in the world access the telescope and log an observation. The first PICTOR telescope is currently online and observations can be created simply by going to their website, and clicking on the "Observe" link. Users can then enter the frequency and other parameters for their observation, and the resulting graph will be emailed to you after the observation. The software source is available on their GitHub page, and is based on a GNU Radio flowgraph and Python plot script.

For more information about PICTOR, logging an observation, and radio astronomy in general, we recommend checking out their PDF guide. We test ran a short observation at the hydrogen line frequency, and we received a graph with the hydrogen line peak clearly visible (spliced in to the photo below). We note that the wavy shape is due the to shape of the filters they used.

PICTOR Radio Telescope
PICTOR Radio Telescope

Radio Astronomers listen to the Early Universe at 78 MHz with a Dipole and Custom SDR

Radio astronomers from Arizona State University and MIT have recently observed a predicted radio phenomenon that originates from the very first stars formed in the Universe.

Hydrogen tends to emit radio signals in the 21cm (1.4 GHz) region of the frequency spectrum. An emission from a single Hydrogen atom is very rare, but since there is so much Hydrogen in space a bump at 1.4 GHz can be observed on the frequency spectrum if a sensitive radio is used with a directional antenna pointing up at the sky. This is a moderate difficulty experiment that can be performed by amateur radio astronomers today with cheap RTL-SDRs or other SDRs together with some LNAs. 

The astronomers in this experiment focus on a distortion in the 21cm line signal that is expected to have been created when the first stars formed. The their paper they write:

After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz.

The results show a successful detection of the expected phenomena at 78 MHz, confirming the age at when the first stars have been predicted to have begun forming. The phenomena is detected at 78 MHz instead of 1.4 GHz because the wavelength of a Hydrogren line signal gets stretched the further the source is from us, due to the redshift doppler effect from the expansion of the Universe. This detection is from some of the furthest (and thus oldest) stars in the Universe, so a big stretch is expected.

The experiment consisted of a broadband blade dipole which was set up in the Australian outback. Since the cosmic signal is expected to be detected right in the middle of the broadcast FM band, a dedicated radio-quiet location is required to stand any chance of detection. The receiving SDR hardware consists of an LNA, line amp, filtering and a 14-bit ADC that is connected to a PC.

It seems possible that this experiment could be repeated by amateur radio astronomers with commercial SDR hardware, but the biggest challenge would probably be finding a very radio-quiet location without broadcast FM radio signals.

The 78 MHz Cosmic Signal SDR Detection Setup
The 78 MHz Cosmic Signal SDR Detection Setup
Dipole antenna with 30mx30m ground plane
Dipole antenna with 30mx30m ground plane

Building a Hydrogen Line Front End on a Budget with RTL-SDR and 2x LNA4ALL

Adam 9A4QV is the manufacturer of the LNA4ALL, a high quality low noise amplifier popular with RTL-SDR users. He also sells filters, one of which is useful for hydrogen line detection. Recently he’s uploaded a tutorial document showing how to use 2x LNA4ALL, with a filter and RTL-SDR for Hydrogen Line detection (pdf warning). 

Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will only very 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 and integrating the RF power over time, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot. This can be used for some interesting experiments, for example you could measure the size and shape of our galaxy. Thicker areas of the galaxy will have more hydrogen and thus a larger spike.

In his tutorial Adam discusses important technical points such as noise figure and filtering. Essentially, when trying to receive the hydrogen line you need a system with a low noise figure and good filtering. The RTL-SDR has a fairly poor noise figure of about 6dB at 1420MHz. But it turns out that the first amplifier element in the receive chain is the one that dominates the noise figure value. So by placing an LNA with a low noise figure right by the antenna, the system noise figure can be brought down to about 1dB, and losses in coax and filters become negligible as well. At the end of the tutorial he also discusses some supplementary points such as ESD protection, bias tees and IP3.

One note from us is that Adam writes that the RTL-SDR V3 bias tee can only provide 50mA, but it can actually provide up to 200mA continuously assuming the host can provide it (keep the dongle in a cool shaded area though). Most modern USB 2.0 and USB3.0 ports on PCs should have no problem providing up to 1A or more. We’ve also tested the LP5907 based Airspy bias tee at up to 150mA without trouble, so the 50mA rating is probably quite conservative. So these bias tee options should be okay for powering 2xLNA4ALL.

Finally Adam writes that in the future he will write a paper discussing homebrew hydrogen line antennas which should complete the tutorial allowing anyone to build a cheap hydrogen line radio telescope.

One configuration with 2xLNA4ALL, 1x interstage filter, and 1x recceiver side filter with bias tee.
One configuration with 2xLNA4ALL, 1x interstage filter, and 1x recceiver side filter with bias tee.

Helping to Raise Funds for the Canadian Centre for Experimental Radio Astronomy (CCERA)

Patchvonbraun (aka Marcus Leech) is one of the pioneers in using low cost SDR dongles for amateur radio astronomy experiments. In the past he’s shown us how to receive things like the hydrogen line,  detect meteors and observe solar transits using an RTL-SDR. He’s also given a good overview and introduction to amateur radio astronomy in this slide show.

Now Marcus and others are starting up a new project called the “Canadian Centre for Experimental Radio Astronomy (CCERA)”. They write that this will be an amateur radio astronomy research facility that will produce open source software and hardware designs for small scale amateur radio astronomers. Currently they also already have a hydrogen line telescope set up, which is producing live graphs and data. From their recent posts it also looks like they’re working on building antennas for pulsar detection. They also have a GitHub available for any software they produce at https://github.com/ccera-astro.

Currently CCERA is looking for donations over at gofundme, and they are hoping to eventually raise $25k. They write:

About CCERA:

Radio astronomy is one of the most important ways to observe the cosmos. It is how we learned about the existence of the afterglow of the big bang (the cosmic microwave background), it is how we observe huge swaths of the universe that are otherwise obscured by dust. Most of what’s going on out there can’t be seen with visible light.

Astronomy has traditionally been one of the areas in science where dedicated non-professionals have continued to make an enormous contribution to the field. Optical astronomy requires little more than a telescope and knowledge.

Radio astronomy has, up until recently, required a lot more skill and resources. However, technology has advanced enough that small groups could be making serious contributions to radio astronomy. With the right sorts of software and information, many dedicated non-professionals could be doing good work in the area, and CCERA intends to help make that a reality.

CCERA will be producing open source software and hardware designs to help non-professional and professional radio astronomers alike, documenting them, and helping people get up to speed so that they can use these powerful tools themselves. Our GitHub repository is: https://github.com/ccera-astro

CCERA will also be operating its own radio astronomy facilities, initially in Ontario, Canada. These will serve as a test-bed for our own designs, as a place for us to train interested people in the operation of low cost radio astronomy equipment, and will also be used for real radio astronomy work. All our data will be publically-available.

About us:

Roughly 10 years ago, I and a number of others started a project to restore a large, historic, satellite earth station antenna at Shirleys Bay in Ottawa. Our goal was to bring the dish back on-line for use in amateur radio astronomy, research, and importantly, educational outreach about science, and radio astronomy.

The project came to a sudden end back in 2013/14 when the owner of the dish (The Canadian Space Agency) needed to dismantle it to make way for other occupants of the site.

However, during that period, we became fascinated with the possibilities that opening up radio astronomy to skilled non-professionals could bring.

Since then, our group has been working on another far lower cost project to build our own a specialized radio telescope somewhere in the Rideau Valley area. Many of our group live in the area, and Marcus lives in Smiths Falls. With good attention to the usability of our designs and open publication of our tools under appropriate open source licenses, our work should be replicable by others. We thus hope to kick off a new era in non-professional radio astronomy.

What we need the money for:

We’ve secured a small office in the Gallipeau Center outside of Smiths Falls, and will be able to erect our specialized antenna arrays over the coming year.

While we have a lot of the equipment we’ll need, we’ll have more equipment to buy, and on-going expenses to cover, including rent, insurance, miscellaneous mechanical construction materials (lumber, metal, etc). We also need to cover expenses relating to incorporation as a not-for-profit.

Our goal is to provide a test facility for small-scale radio astronomy research, and to develop techniques that allow small organizations and educational institutions to run their own small-scale radio astronomy observing programs.

If we are successful, in addition to making our designs and software available under open source licenses, we’ll be holding regular public lectures, host training seminars, host school groups, etc. We will also produce videos of our work for those who cannot visit us directly in Ottawa. We want to make some of the techniques of “big science” accessible and understandable.

We can’t do it without the help of the public, who, we hope, will become our students, collaborators, and ongoing supporters.

We will also make all of our data available to the public without fee or restrictions. We believe in openness in scientific endeavours, even small ones such as ours.

Marcus Leech
(tentative) Director
Canadian Centre for Experimental Radio Astronomy

If you have even a passing interest in radio astronomy please consider donating, as CCERA’s work may open up exciting new possibilities for amateur radio astronomers with low cost SDR dongles.

The pulsar antenna being built at CCERA.
The pulsar antenna being built at CCERA.