The Simons Observatory: Frequency-resolved beam calibration for the SO SATs with POLOCALC

Nadia Dachlythra; Gabriele Coppi; Ludovico Bizzarri; Federico Nati; Alessandro Novelli; Elle C. Shaw; Samuel Day-Weiss; Federico Astori; Susanna Azzoni; Dillon J. Bass; Artem Basyrov; Federico Cacciotti; Rolando Dünner Planella; Nicholas Galitzki; Remington Gerras; John C. Groh; Jon E. Gudmundsson; Saianeesh K. Haridas; Kathleen Harrington; Carlos Hervías-Caimapo; Bradley R. Johnson; Brian Keating; Adrian T. Lee; Anto I. Lonappan; Noemi Mezzanzanica; Lyman Page; Giulia Rancati Cattaneo; Sara M. Simon; Thierry Souverin; Yoshinori Sueno; Edward J. Wollack; Kyohei Yamada; Mario Zannoni

arXiv · 2610.15913

The Simons Observatory: Frequency-resolved beam calibration for the SO SATs with POLOCALC

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Abstract

We present a far-field beam calibration of the Simons Observatory (SO) Small Aperture Telescopes (SATs) using scanned single-frequency measurements of a drone-mounted source emitting in the 90 GHz band, where the SATs have an angular resolution of $\sim$30$'$. We then compare the results from two campaigns in April and December 2025 to the band-averaged beams from planet observations by the SATs. Measuring the telescope beam response at different frequencies across the observing bands enables modeling the system's frequency-dependent behavior. This helps track optical non-idealities. The comparison with planet beams informs the frequency resolution necessary to accurately reconstruct the band-averaged beams in this way; it uses only the average drone and planet products, as it is intended to demonstrate the feasibility of the frequency-resolved calibration scheme rather than to set the precise beam covariance and instrument requirements. We find that the beams of the two SATs near 90 GHz follow the frequency scaling expected for diffraction-limited optics, with only modest deviations observed in the nearest lobes of one telescope. We establish consistency between the real band-averaged planet beams and the synthesized drone band-averaged beams for the center wafers of both telescopes, with an average deviation smaller than 4% for multipoles $\ell < 500$.

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Nadia Dachlythra, Gabriele Coppi, Ludovico Bizzarri, Federico Nati, Alessandro Novelli, Elle C. Shaw, Samuel Day-Weiss, Federico Astori, Susanna Azzoni, Dillon J. Bass, Artem Basyrov, Federico Cacciotti, Rolando Dünner Planella, Nicholas Galitzki, Remington Gerras, John C. Groh, Jon E. Gudmundsson, Saianeesh K. Haridas, Kathleen Harrington, Carlos Hervías-Caimapo, Bradley R. Johnson, Brian Keating, Adrian T. Lee, Anto I. Lonappan, Noemi Mezzanzanica, Lyman Page, Giulia Rancati Cattaneo, Sara M. Simon, Thierry Souverin, Yoshinori Sueno, Edward J. Wollack, Kyohei Yamada, Mario Zannoni. 2026-10-11. The Simons Observatory: Frequency-resolved beam calibration for the SO SATs with POLOCALC. https://arxiv.org/abs/2610.15913

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