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PICTOR OPEN-SOURCE RADIO TELESCOPE DETECTS THE 21CM HYDROGEN LINE OF THE MILKY WAY

A backyard dish that maps the spiral arm of the Milky Way by listening to cold hydrogen gas at 1420 MHz.

by Apostolos Spanakis-Misirlis (0xCoto)

FULL CAD BOM FIRMWARE DOCS

ScienceOpen-hardware

difficulty
●●●●○
time
a weekend-plus
license
GPL-3.0
repo
repo ACTIVE280 stars

WHAT YOU’LL NEED

  • Electronic parts — full list with part numbers in the repo BOM
  • Dev board / microcontroller — runs the project firmware

Partner

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COMPAREE VERDICT

PICTOR is the software half of a hydrogen-line radio telescope — the part that takes raw SDR samples and turns them into a map of the Milky Way's neutral hydrogen. It is mature, well-documented, and genuinely capable of detecting the 21 cm line from a backyard. The repository includes everything from GNU Radio signal chains to Doppler correction and RFI blanking. What it does not include is a hardware build guide, because the original PICTOR is a 1.5-metre dish on a university roof in Greece, not a design you replicate. For hardware, you will need a companion project: HLine3D is a 3D-printed horn that works with an RTL-SDR and a satellite LNB, or you can adapt a surplus satellite dish with a commercial hydrogen-line feed. The difficult part is not the code — it is getting clean 1420 MHz reception without local interference, aligning your dish or horn to within a degree or two, and understanding enough radio astronomy to interpret the waterfall plots. If you have never touched GNU Radio or calibrated an SDR, expect a steep weekend learning curve before you see a hydrogen spike. The most common mistake is underestimating RFI: urban environments are brutal at 1420 MHz, and no amount of software will fix a hardware setup that is picking up your neighbour's Wi-Fi booster. Rural skies win here.

NOT IN THE REPO

  • —PICTOR is GPL-3.0 software for radio telescope signal processing and observation scheduling, not a hardware build guide.
  • —Hardware comes from companion projects like HLine3D (3D-printed horn) or community satellite dish setups — no single canonical bill of materials.
  • —The repository includes Python scripts, GNU Radio flowgraphs, RFI mitigation tools, and a full observation pipeline.
  • —Documentation covers installation, calibration, and data reduction — assumes you already have RF hardware and know how to point it.
  • —No PCB files or mechanical drawings in this repo; the telescope at github.com/0xCoto/PICTOR is a live instrument in Athens, not a DIY kit.
  • —Licence allows commercial use.

Can I build this?

Printnothing required for PICTOR itself; companion hardware builds like HLine3D have 3D-printed horn and mount files
Buysoftware-defined radio (RTL-SDR or better), low-noise amplifier for 1420 MHz, satellite dish or hydrogen-line horn, coax and connectors, Linux machine for processing
ToolsGNU Radio installed, Python 3 environment, basic RF test equipment helpful but not mandatory, ability to aim a dish or mount a horn outdoors
Skillsintermediate Python and command-line work, basic radio astronomy concepts (Doppler shifts, drift scans, RFI), patience with signal processing — not a first SDR project
Timea weekend to get PICTOR running and integrate it with your hardware, then evenings of observation and tuning before you see a clean hydrogen line
Cost$$ — RTL-SDR and LNB setup under $100, better SDR and dish or commercial feed push it to low hundreds; the software itself is free
SafetyNone beyond ordinary electronics care — this is receive-only, no transmit, no licence required.

Build at your own risk. Projects involve tools, electronics and sometimes mains voltage — follow the creator’s safety notes.

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Start here

Navigation into the creator’s own docs — we don’t rewrite the guide, we route you to the source.

  1. 1.Read the PICTOR README and installation guide (Covers dependencies, GNU Radio setup, and the observation pipeline.)
  2. 2.Choose your hardware path: 3D-printed horn (HLine3D) or adapted satellite dish (HLine3D is the most documented beginner-friendly hardware companion.)
  3. 3.Install GNU Radio and test your SDR at 1420 MHz with a dummy load(Verify you can see the noise floor before you add an antenna.)
  4. 4.Run the PICTOR calibration and RFI survey scripts(Map your local interference before attempting a hydrogen-line observation.)

Resources

Documentation, files and community threads for this build — we link straight to the original sources and never rehost the creator’s files.

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KNOWN ISSUES

  • Buying an RTL-SDR without checking if it covers 1420 MHz cleanly — some have a factory notch or poor sensitivity at that exact frequency.
  • Urban RFI will drown the hydrogen line unless you have a very quiet site or excellent filtering — the 21 cm emission is faint and the software cannot fix a bad signal.
  • PICTOR assumes you bring your own hardware and know how to point it — there is no step-by-step dish assembly guide in this repository.
  • Expecting instant results: a single drift scan takes hours, and you will need multiple passes to build up signal-to-noise for a clean Milky Way profile.
  • Underestimating the learning curve for GNU Radio and radio astronomy data reduction — this is not a plug-and-play SDR app.
  • Not accounting for Doppler shifts and Earth's rotation in your observation plan — the scripts handle it, but you need to understand what they are correcting.

Do I need a large dish to detect the hydrogen line?

No — HLine3D proves a 3D-printed horn and an RTL-SDR can see it. A larger dish gives better signal-to-noise and narrower beam width, but the line is detectable with small apertures if your site is quiet.

Is this receive-only, or do I need a radio licence?

Receive-only. No transmit, no licence required in any jurisdiction.

Can I use PICTOR with a different SDR, or is it RTL-SDR only?

PICTOR works with any SDR supported by GNU Radio — HackRF, Airspy, LimeSDR, USRP. The scripts are SDR-agnostic; you just change the source block.

What is the actual PICTOR telescope, and can I build a copy of it?

PICTOR is a 1.5-metre offset dish with a custom feed and mount, operated remotely in Athens. The repository is the software that runs it, not a hardware design. For a DIY build, use a companion project like HLine3D or adapt a surplus satellite dish.

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Discussion1

FROM THE COMPAREE TEAM

267 stars and a live telescope that has been mapping the Milky Way since 2020 — but you still need to bring your own dish or horn. What hardware path would you take: 3D-printed horn, or a salvaged satellite dish?

CompareeTEAM28d agoedited

Practical notes from our verification: PICTOR is the software that processes hydrogen-line data, not a hardware build you replicate — the actual telescope in the repository photos is a 1.5-metre dish on a university roof in Athens, and there are no CAD files or assembly instructions for it. For hardware, the most documented companion is HLine3D, a 3D-printed horn that pairs with an RTL-SDR. The repository itself is actively maintained (last push August 2024), the documentation is thorough, and the GNU Radio flowgraphs are production code that has logged thousands of hours of real sky. The single biggest challenge is not the software — it is getting a clean 1420 MHz signal in the first place, because urban RFI is relentless at that frequency.

Apostolos Spanakis-Misirlis (0xCoto)

Apostolos built PICTOR as an open-source radio telescope to make hydrogen-line astronomy accessible beyond university observatories. The software has been running observations in Athens since 2020 and is now used by community builders worldwide to process their own 21 cm data.

GitHub

Star the project on GitHub

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  • Information is provided without warranty and may become outdated as projects evolve. Prices are indicative bands only — always check the creator’s parts list for current costs.
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