Far Sidelobes from Baffles and Telescope Support Structures in the Atacama Cosmology Telescope

Abstract

The Atacama Cosmology Telescope (ACT) is a 6 m telescope located in the Atacama Desert, designed to measure the cosmic microwave background (CMB) with arcminute resolution. ACT, with its third generation polarization sensitive array, Advanced ACTPol, is being used to measure the anisotropies of the CMB in five frequency bands in large areas of the sky ($\sim 15,000$ $\mathrm{de}{\mathrm{g}}^2$). These measurements are designed to characterize the large scale structure of the universe, test cosmological models and constrain the sum of the neutrino masses. As the sensitivity of these wide surveys increases, the control and validation of the far sidelobe response becomes increasingly important and is particularly challenging as multiple reflections, spillover, diffraction and scattering become difficult to model and characterize at the required levels. In this work, we present a ray trace model of the ACT upper structure which is used to describe much of the observed far sidelobe pattern. This model combines secondary mirror spillover measurements with a 3D CAD model based on photogrammetry measurements to simulate the beam of the camera and the comoving ground shield. This simulation shows qualitative agreement with physical optics tools and features observed in far sidelobe measurements. We present this method as an efficient first-order calculation that, although it does not capture all diffraction effects, informs interactions between the structural components of the telescope and the optical path, which can then be combined with more computationally intensive physical optics calculations. This method can be used to predict sidelobe patterns in the design stage of future optical systems such as the Simons Observatory, CCAT-prime, and CMB Stage IV.

arXiv commentPoster presented at SPIE Astronomical Telescopes & Instrumentation 2018

References in corpus (8)

• Advanced ACTPol Cryogenic Detector Arrays and Readout
• The Atacama Cosmology Telescope: The Receiver and Instrumentation
• The Atacama Cosmology Telescope: The polarization-sensitive ACTPol instrument
• The Atacama Cosmology Telescope: Two-Season ACTPol Spectra and Parameters
• Optical Design of the Atacama Cosmology Telescope and the Millimeter Bolometric Array Camera
• The Atacama Cosmology Telescope: CMB Polarization at $200<\ell <9000$
• Survey strategy optimization for the Atacama Cosmology Telescope
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• The Simons Observatory: Modeling Optical Systematics in the Large Aperture Telescope
• Studies of Systematic Uncertainties for Simons Observatory: Optical Effects and Sensitivity Considerations
• Cosmology Large Angular Scale Surveyor (CLASS): 90 GHz Telescope Pointing, Beam Profile, Window Function, and Polarization Performance
• Sidelobe Modeling and Mitigation for a Three Mirror Anastigmat Cosmic Microwave Background Telescope