Drift Scan

The DSES 60-foot radio telescope silhouetted against a twilight sky at the Haswell, Colorado site, with the operations building lit beside it.
The 60-foot dish at Haswell, Colorado at dusk — pointed near the zenith, recording around the clock.

For six days in August 2026, our 60-foot dish at Haswell, Colorado sat motionless, pointed near the zenith, while the Earth’s rotation swept the sky through its beam. A small computer at the dish recorded a spectrum of the 21 cm hydrogen line every 25 seconds — day and night, unattended.

What came back was the structure of our own galaxy.

Waterfall plot of five days of 21 cm hydrogen-line spectra showing two bright bands where the Milky Way’s plane crosses the telescope beam.
Five days of hydrogen spectra, stacked by sidereal time. The two bright bands appear each time the plane of the Milky Way drifts through the beam — once in Cygnus, once toward the galactic anticentre — and they repeat, identically, every sidereal day.

The run, by the numbers

  • 16,205 spectra over 4.8 days — not one missing
  • Receiver stability better than 0.1 dB day to day
  • Hydrogen detected on every crossing of the galactic plane, with gas components resolved out to −100 km/s
  • The radio galaxy Cygnus A detected every single day, its transit time repeating to one second over five days
  • Telescope beam measured at 0.69° — using the sky itself as the ruler

Watching the spiral arms

Hydrogen atoms throughout the galaxy broadcast at exactly 1420.4058 MHz. Because the gas orbits the galactic centre, the line reaches us Doppler-shifted — and the shift tells us how fast each cloud moves and, through the geometry of galactic rotation, roughly where it is. When the beam crossed the plane in Cygnus, the spectrum resolved into separate components: local gas near zero velocity, and gas in the outer spiral arms rushing toward us at 45 and 70 km/s.

Two hydrogen-line velocity profiles showing multiple gas components at the Cygnus and anticentre galactic-plane crossings.
Averaged hydrogen profiles at the two galactic-plane crossings. In the Cygnus direction (left) the line splits into local gas and two outer-arm components — spiral structure, measured from the Colorado plains.

A radio galaxy, every day at the same second

Averaging each spectrum away from the hydrogen line turns the same data into a continuum survey — a map of total radio power around the sky. One object dominates: Cygnus A, one of the strongest radio sources in the sky, powered by a supermassive black hole 600 million light-years away. It swept through our beam every sidereal day, at the same second, at the same strength.

24-hour radio continuum plot showing a strong spike from Cygnus A and broader emission from the galactic plane.
The whole sky at our declination, folded over five days of sidereal time. The spike at 20 hours is Cygnus A; the smaller bumps beside it are the Gamma Cygni supernova remnant and the Cygnus X star-forming region. The broad rise at 3–5 hours is the plane of the Milky Way itself.
Five overlaid transit curves of Cygnus A from consecutive days, showing near-perfect repeatability.
Five consecutive daily transits of Cygnus A, overlaid. The five curves are nearly indistinguishable — transit timing repeatable to one second of right ascension, with the dish never moving.

The sky calibrates the telescope

Cygnus A is effectively a point source, so its transit measures the telescope itself: the width of the curve gives our beam size (0.69°), its strength gives our sensitivity, and its timing checks our clocks to the second. The measurements also revealed something useful — the dish is pointing a fraction of a degree away from where its position sensors claim. That is not a flaw in the data; it is a free calibration, and it sets up our next campaign: stepping the dish night by night across Cygnus A to map the beam and pin down the true pointing.

What’s next

The scan is running right now — the receiver was just upgraded so that its own internal artifacts land outside the measurement band, and the dish keeps recording a spectrum every 25 seconds while you read this. Coming up: the Cygnus A pointing campaign, longer multi-week hydrogen surveys, and — with the same dish and the same receiver — pulsar observations.

How drift scanning works

A drift scan is radio astronomy at its simplest and most robust: park the telescope, let the Earth’s rotation do the scanning, and record continuously. Every object at the beam’s declination passes through once per sidereal day, so a fixed dish surveys a full ring of sky every 23 h 56 m — with perfectly repeatable geometry and no tracking errors. It is the oldest survey technique in radio astronomy, and it remains one of the best ways to characterize a telescope and its site.

Reports

Observations and analysis by DSES members using the DSES Spectrum Analyzer, our in-house software, running on a Raspberry Pi at the Haswell site. Page updated August 2026.