SO Small Aperture Telescope
What are the SO Small Aperture Telescopes?
The Simons Observatory will feature a number of small-aperture telescopes (SATs), so called because their light-gathering area is only about 10% of the large-aperture telescope. However, the SATs’ compact design is consistent with their goal of recovering information about large-scale fluctuations in the microwave sky–from regions just under a degree across to several degrees.
Using the information about linear polarization of light on these scales recorded by the SO SATs, we will search for a gravitational wave signature generated in the early universe.
For instruments like the SATs, we seek to fill the focal plane formed by the optical design with as many detectors as possible. Each SAT contains around 3,000 pixels, with each pixel containing four linear polarization-sensitive detectors. That’s a total of about 12,000 detectors being read out at once!
These detectors, based as they are on recording the electrical resistance of small superconducting films, require a cold environment to function. The internal design of the SATs was developed to provide stages of cooling, from the 300 degrees Kelvin world we inhabit down to a tenth of a degree above absolute zero, where the detectors operate. Two successive shells nest inside the SAT, the outer one cooled to 40 degrees Kelvin and the inner to 4 degrees Kelvin.
Inside of this innermost shell, a system called a dilution refrigerator (DR) is used to cool the SAT optics and focal plane, containing the detectors, even further. The optics, large-format silicon lenses nearly 50 centimeters across with intricately-cut outer surfaces which optimize electromagnetic transmission, are housed in an aluminum “optics tube” that is held near 1 degree Kelvin.
All of this effort helps the SO SATs to have extreme, state-of-the-art sensitivity to the small perturbations we are searching for in the microwave sky streaming from the universe above!
The other key design goal for the SAT is to make a stable instrument, where all elements of the telescope produce minimal fluctuations over the few-minute timescale during which the SAT sweeps across the sky. The unchanging CMB signals are hidden behind the much stronger and more dynamic features of Earth’s atmosphere. Fortunately, the atmosphere is mostly unpolarized: it emits equally in the two linear polarizations. If we can “spin up”, or modulate, the incoming CMB polarization signals before the detectors record them, we can separate the unpolarized atmosphere from the polarized CMB.
This is exactly what the SO SATs do! A stack of sapphire plates forms a broad-band polarization modulator. This stack, weighing almost 25 kilograms, spins inside the cryostat at rotational speeds up to 2 Hertz.
The weight of the stack is supported by a superconducting magnetic bearing, taking advantage of the way magnets levitate above superconductors. This is another way in which the SO SATs are powered by the incredible properties of superconducting metals. Without the polarization modulator, the raw sensitivity of the SATs wouldn’t achieve much.
To keep this entire system running, a complex system of cryogenic coolers, high-performance electronics, and auxiliary sensors reporting on the state of the instrument have to be supported outdoors in Chile’s Atacama Desert. Observing from an altitude of 5190 meters (17,000 feet) in altitude above sea level, the SO SATs are now taking in a mix of photons from the thermal radiation emitted by the ground, from the water and oxygen molecules in the atmosphere, from particles in our galaxy, and from the CMB itself.
Our day-to-day goals are to maintain the interlocking systems required to get the best SAT performance, and keep the SATs observing continuously for years to come.
The journey has now begun!
