Simons Observatory: characterizing the Large Aperture Telescope Receiver with radio holography

• DOI: 10.1364/ao.470138
• Publication Date: 2022-10-25T15:00:30Z
• Authors: Grace E. Chesmore; Kathleen Harrington; Carlos E. Sierra; Patricio A. Gallardo; Shreya Sutariya; Tommy Alford; Alexandre E. Adler; Tanay Bhandarkar; Gabriele Coppi; Nadia Dachlythra; Joseph Golec; Jon Gudmundsson; Saianeesh K. Haridas; Bradley R. Johnson; Anna M. Kofman; Jeffrey Iuliano; Jeff McMahon; Michael D. Niemack; John Orlowski-Sch...; Karen Perez Sarmi...; Roberto Puddu; Max Silva-Feaver; Sara M. Simon; Julia Robe; Edward J. Wollack; Zhilei Xu
• Keywords: Mechanical Engineering; FOS: Physical sciences; Optics; Optical Physics; 02 engineering and technology; 530; Atomic; 01 natural sciences; 520; optics; Engineering; Electrical engineering; Communications Engineering; Physical Sciences; 0103 physical sciences; 0202 electrical engineering, electronic engineering, information engineering; Electrical and Electronic Engineering; molecular and optical physics; Astrophysics - Instrumentation and Methods for Astrophysics; Instrumentation and Methods for Astrophysics (astro-ph.IM); Electrical Engineering

We present near-field radio holography measurements of the Simons Observatory Large Aperture Telescope Receiver optics. These measurements demonstrate that radio holography of complex millimeter-wave optical systems comprising cryogenic lenses, filters, and feed horns can provide detailed characterization of wave propagation before deployment. We used the measured amplitude and phase, at 4 K, of the receiver near-field beam pattern to predict two key performance parameters: 1) the amount of scattered light that will spill past the telescope to 300 K and 2) the beam pattern expected from the receiver when fielded on the telescope. These cryogenic measurements informed the removal of a filter, which led to improved optical efficiency and reduced sidelobes at the exit of the receiver. Holography measurements of this system suggest that the spilled power past the telescope mirrors will be less than 1%, and the main beam with its near sidelobes are consistent with the nominal telescope design. This is the first time such parameters have been confirmed in the lab prior to deployment of a new receiver. This approach is broadly applicable to millimeter and submillimeter instruments.