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 have developed a way to use radio-holography to characterize a fully integrated cryogenic telescope instrument prior to deployment,” said University of Chicago research team member Grace Chesmore. “In the lab, it’s much easier to detect 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 the optical performance of a telescope, it is difficult to make adjustments once everything is in place. However, a full analysis usually cannot be performed before installation because laboratory techniques are designed for analysis at room temperature while telescope components are held at cryogenic temperatures to improve sensitivity.
In the journal Applied Optics from the Optica Publishing Group, 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 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 before the deployment of a new receiver.
“The Simons Observatory will create unprecedented maps of the Big Bang afterglow, providing insight into the early moments and inner workings of our universe,” said Chesmore, the paper’s first author. “The observatory will help make these ultra-sensitive microwave cosmic background maps possible.”
Look back in time
The cosmic microwave background maps that will be produced by the Simons Observatory will provide a window into our universe at a time so early in its history that tiny quantum gravity signals could be detectable, Chesmore says. However, probing space with such sensitivity requires a better understanding of how electromagnetic radiation propagates through the telescope’s optical system and eliminating as much scattering as possible.
In the new work, the researchers used a technique known as near-field radio holography, 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 capable of mapping a very bright coherent source while operating at the extremely cold temperature of 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 worked as expected.
“All objects, including lenses, shrink and exhibit optical property changes as they cool,” Chesmore explained. “Using the 4 Kelvin holographic detector allowed us to measure the optics in the shapes it will have when observing in Chile.”
From the laboratory to space observations
Once these measurements were completed, the researchers developed software to predict how the telescope would perform with photons from space rather than the near-field source used in the lab.
“The software uses the near-field maps we measured 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 the phase of the microwaves, and there is a known relationship between the properties in the near and far field.”
Using their new approach, the researchers found that the telescope’s optics matched predictions. They were also able to identify and mitigate a scatter source before the telescope was deployed.
The large aperture telescope optical system 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 for precise and detailed observations of the cosmic microwave background radiation. University of Chicago researchers will continue to characterize components of the Simons Observatory telescopes and say they look forward to using 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. Additionally, collaborating institutions around the world are working to make the Simons Observatory a success.
Paper: Chesmore GE, Harrington K, Sierra CE, Gallardo PA, Sutariya S, Alford T, Adler AE, Bhandarkar T, Coppi G, Dachlythra N, Golec J, Gudmundsson J, Harida SK, BR Johnson, Kofman AM, Iuliano J, McMahon J, Niemack MD, Orlowski-Scherer J, Perez Sarmiento K, Puddu R, Silva-Feaver M, Simon SM, Robe J, Wollack EJ, Xu Zu, “Simons Observatory: Characterization of the Large Aperture Telescope Receiver with Radio Holography », Applied Optics, 61, 34, 10309–10319 (2022). DOI: doi.org/10.1364/AO.470138
About applied optics
Applied Optics publishes in-depth, peer-reviewed content on applications-focused research in optics. These articles cover research in optical technology, photonics, lasers, information processing, sensing and environmental optics. Applied Optics is published three times monthly by Optica Publishing Group and overseen by Managing Editor Gisele Bennett, MEPSS LLC. For more information, go to Applied Optics.
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