Beam Maps of the Canadian Hydrogen Intensity Mapping Experiment (CHIME) Measured with a Drone

TL;DR

This work tackles the challenge of precisely calibrating the CHIME telescope beams for 21 cm HI intensity mapping by deploying a pulsed radio source on a drone as an on-site calibrator. It develops a synchronized, near-field beam-mapping workflow that uses pulsar gating and a reduced data path to recover the full $N^{2}$ visibilities for a CHIME cylinder, with analysis based on the top eigenmode of the drone signal. The study cross-validates drone-derived beam parameters against solar transit and holography measurements, finding consistent evolution of beam width and centroid with frequency and position, and demonstrates a practical path toward near-field to far-field transformations for large cylindrical arrays. The results enable improved beam models and polarization characterization, with implications for precise cosmological measurements and future drone-based calibration strategies.

Abstract

We present beam measurements of the CHIME telescope using a radio calibration source deployed on a drone payload. During test flights, the pulsing calibration source and the telescope were synchronized to GPS time, enabling in-situ background subtraction for the full $N^{2}$ visibility matrix for one CHIME cylindrical reflector. We use the autocorrelation products to estimate the primary beam width and centroid location, and compare these quantities to solar transit measurements and holographic measurements where they overlap on the sky. We find that the drone, solar, and holography data have similar beam parameter evolution across frequency and both spatial coordinates. This paper presents the first drone-based beam measurement of a large cylindrical radio interferometer. Furthermore, the unique analysis and instrumentation described in this paper lays the foundation for near-field measurements of experiments like CHIME.

Figures (12)

• Figure 1: Photograph of the Matrice 600 Pro drone with payload, in flight at the DRAO during measurements.
• Figure 2: Optical path of calibration signal from drone-based transmitter as seen by CHIME feeds. The incoming radio waves are focused by the cylindrical reflector onto the feeds, which each observe the drone at a different elevation. In the reference frame of the southernmost feed (Feed 0, black) the drone appears to be higher in the sky than for feeds in the middle (Feed 127, purple) or at the North (Feed 256, yellow) end of the cylinder. The drone altitude in this figure is substantially lower than the flight altitude to exaggerate the feed-dependent parallax. The reference frame of the local Cartesian coordinate system is the survey location of the center of the CHIME array (yellow star). See text for more details.
• Figure 3: The upper half of this figure shows coordinate conventions for the data from a drone flight at 35 degrees of zenith angle. One transit (upper left) across the beam of Cylinder C is shown true to scale in a local Cartesian coordinate system centered on the CHIME telescope. This flight trajectory is represented in three per-feed coordinate systems: Cartesian (upper right), Celestial (middle right) referenced to Local Meridian, and Orthographic (TelX/TelY)(lower right) using the color scale from Figure \ref{['CHIMEray']}. The spherical coordinate system (bottom) with axes and angle definitions given to show the transformation from a spherical coordinate system to TelX (projection onto X-axis) and TelY (projection onto Y-axis).
• Figure 4: Beam characterization measurements have been calculated from three unique data sets acquired in different portions of CHIME's field of view. Coordinate axes are TelX and TelY, explained in more detail in Figure \ref{['CHIMEcoordinates']}, which correspond to the projection to East and North, respectively. Solar beam measurements are presented in a continuous semicircular slice. Holographic measurements of galactic radio sources appear as pale yellow arcs. Measurements from the drone trajectory shown in Figure \ref{['CHIMEcoordinates']} are indicated by the square patch with solid yellow outline. Some additional drone flights (not analyzed here) are indicated by patches with dashed yellow outlines. The color scale of the solar and drone datasets is proportional to received power to illustrate the alignment of the main beam, but has not been appropriately calibrated for direct comparison. Comparison of the FWHM and Centroid measurements obtained from these data sets are shown in Figures \ref{['CHIMEFWHM']} & \ref{['CHIMEcent']}.
• Figure 5: CHIME data (dotted lines) acquired during the drone flight. The power measured by feed 190 (corresponding to TelY coordinate -0.59 and declination 13.15$^{\circ}$) is shown at three frequencies in both polarizations (XX top, YY bottom). The power has been normalized by the amplitude of the best-fit 1d Gaussian from Equation \ref{['eqn:1dGauss']} (solid line). The beam narrows with increasing frequency, as expected, and frequency-dependent sidelobe structure beyond the main beam shape is also apparent in the data.