Some of the largest and most sophisticated telescopes ever made are under construction at the Simons Observatory in Northern Chile. They are designed to measure cosmic microwave background 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 research team member Grace Chesmore from the University of Chicago. “In the lab, it’s much easier to spot issues before they become problematic and 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 hard to make adjust­ments once everything is in place. However, a full analysis typically can’t be done prior to installation because lab-based techniques are designed for room temperature analysis while telescope components are kept at cryogenic tempera­tures to improve sensi­tivity. The researchers led by Jeff McMahon applied their new measurement approach to the Simons Obser­vatory Large Aperture Telescope receiver optics, which includes lenses, filters, baffles and other components.

This is the first time such parameters have been confirmed in the lab 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 first moments and inner workings of our universe,” said Chesmore. “The obser­vatory will help make these ultra-sensitive cosmic microwave background maps possible.”

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 signals from quantum gravity could be detectable, says Chesmore. However, probing space with such sensi­tivity requires a better under­standing of how electro­magnetic radiation travels through the telescope’s optical system and the elimination of as much scattering as possible.

The researchers used 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 that can map 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 make sure the Large Aperture Telescope receiver optics performed as expected.

“All objects, including lenses, shrink and exhibit changes in optical properties when they cool down,” explained Chesmore. “Operating the holography detector at 4 Kelvin allowed us to measure the optics in the shapes they will be when observing in Chile.” After these measure­ments were complete, the researchers developed software to predict how the telescope would work with photons coming from space rather than the near-field source used in the laboratory.

“The software uses the near-field maps we measured to deter­mine the behavior of a far-field microwave source,” said Chesmore. “This is only possible using radio-holo­graphy because it measures both the amplitude and 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 source of scattering 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. The University of Chicago researchers will continue to charac­terize components for the Simons Obser­vatory tele­scopes and say that they look forward to using these telescopes to better understand our Universe. (Source: Optica)

Reference: G. E. Chesmore et al.: Simons Observatory: characterizing the Large Aperture Telescope Receiver with radio holography, Appl. Opt. 61, 10309 (2022); DOI: 10.1364/AO.470138

Link: Cosmology, Dept. of Physics, University of Chicago, Chicago, USA