Some of the largest and most sophisticated telescopes ever built are under construction at the Simons Observatory in northern Chile. They are designed to measure the cosmic microwave background, the electromagnetic radiation left over from the formation of the universe, with unprecedented sensitivity. In a new study, researchers detail an analysis method that could improve these telescopes by evaluating their performance before installation.
“We developed a way to use radio-holography to characterize a fully integrated cryogenic telescope instrument prior to deployment,” said Grace Chesmore, a member of the University of Chicago research team. “In the lab, it’s much easier to spot problems before they become problematic and to manipulate the components inside the telescope to optimize performance.”
Although it is common to wait until after installation to characterize a telescope’s optical performance, it is difficult to make adjustments once everything is in place. However, full analysis cannot normally be performed prior to installation because laboratory techniques are designed for analysis at room temperature, while telescope components are kept at cryogenic temperatures to improve sensitivity.
In Optica Publishing Group magazine Applied Optics, researchers led by Jeff McMahon of the University of Chicago describe how they applied their new measurement approach to the receiver optics of the Simons Observatory’s Large Aperture Telescope, which includes lenses, filters, deflectors, and other components. This is the first time that such parameters have been confirmed in the laboratory prior to the deployment of a new receiver.
“The Simons Observatory will create unprecedented maps of the afterglow of the Big Bang, providing an understanding of the earliest moments and inner workings of our universe,” said Chesmore, the paper’s first author. “The observatory will help make these ultrasensitive cosmic microwave background maps possible.”
looking back in time
The cosmic microwave background maps that the Simons Observatory will produce will provide a window into our universe at a time so early in its history that tiny signals from quantum gravity could be detectable, Chesmore says. However, probing space with such sensitivity requires a better understanding of how electromagnetic radiation travels through the telescope’s optical system and the removal of as much scatter as possible.
In the new work, the researchers used a technique known as near-field radioholography, which can be used to reconstruct how electromagnetic radiation travels through a system such as a telescope. To do this at cryogenic temperatures, they installed a detector that can map a very bright coherent source while operating at the extremely cold 4 Kelvin. This allowed them to create maps with a very high signal-to-noise ratio, which they used to ensure that the Large Aperture Telescope’s receiver optics performed as expected.
“All objects, including lenses, shrink and show changes in optical properties when they cool,” Chesmore explained. “Operating the holography detector at 4 Kelvin allowed us to measure the optics in the shapes they will have when observing in Chile.”
From the laboratory to space observations
Once these measurements were complete, the researchers developed software to predict how the telescope would perform with photons coming from space instead of the near-field source used in the lab.
“The near-field maps we measured are used by the software to determine the behavior of a far-field microwave source,” Chesmore said. “This is only possible using radio-holography because it measures both the amplitude and phase of microwaves, and there is a known relationship between the properties in the near and far fields.”
Using their new approach, the researchers found that the telescope’s optics matched the predictions. They were also able to identify and mitigate a scatter source before deploying the telescope.
The optical system of the Large Aperture Telescope that they characterized is now on its way to Chile for installation. The Simons Observatory will include the Large Aperture Telescope, as well as three Small Aperture Telescopes, which will be used together to make precise and detailed observations of the cosmic microwave background. Researchers at the University of Chicago will continue to characterize the components of the Simons Observatory telescopes and say they hope to use these telescopes to better understand our Universe.
Other members of the University of Chicago team include postdoctoral researchers Katie Harrington and Patricio Gallardo, as well as graduate students Carlos Sierra, Shreya Sutariya, and Tommy Alford. Also, c.Collaborating institutions from around the world are working to make the Simons Observatory a success.
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