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

Abstract

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.

Holographic phase-retrieval method of near-field antenna pattern measurement for bolometer-array-equipped millimeter-wave telescopes

“…Because the phase information cannot be directly measured by bolometric detectors, several near-field measurements of CMB telescopes using a phase-sensitive receiver that replaces a few representative pixels of the focal-plane detector array have been reported. 14,15 Because the far-field antenna patterns depend not only on the antenna optics but also on the feed-coupling to the detectors, we study a near-field measurement method using the bolometric detector array as it is by retrieving the phase of the AUT aperture field through another technique. For millimeter-wave transmitting antennas, several phase-retrieval techniques have been proposed; these can be classified into two categories: a technique that uses intensity information on two surfaces 16,17 and a holographic phase-retrieval technique.…”