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

Area of Science:

• Astronomy and Astrophysics
• Optical Engineering
• Radio Science

Background:

• Characterizing complex optical systems at cryogenic temperatures is crucial for sensitive astronomical receivers.
• Millimeter-wave instruments require precise understanding of wave propagation to minimize signal loss and interference.

Purpose of the Study:

• To demonstrate the utility of near-field radio holography for characterizing cryogenic millimeter-wave optics.
• To predict key performance parameters of the Simons Observatory Large Aperture Telescope Receiver before deployment.

Main Methods:

• Performed near-field radio holography measurements of the receiver optics at 4 Kelvin.
• Measured amplitude and phase of the near-field beam pattern.
• Used measurements to predict scattered light and on-telescope beam pattern.

Main Results:

• Cryogenic holography provided detailed characterization of wave propagation.
• Measurements informed filter removal, improving optical efficiency and reducing sidelobes.
• Predicted spilled power below 1%; main beam and sidelobes consistent with design.

Conclusions:

• Near-field radio holography is a powerful pre-deployment validation tool for millimeter-wave instruments.
• This technique enables accurate prediction of optical performance in cryogenic environments.
• The validated performance of the Simons Observatory receiver optics is confirmed.