Small Aperture Telescopes

Fig.1. Cutaway views of the SAT with major components labelled

The Simons Observatory will deploy an array of three small aperture telescopes (SATs) to study the CMB at large angular scales. Each SAT has seven 6” detector wafers for a total of over 30,000 detectors in the array. Two SATs will observe at 90/150 GHz to observe the peak of the CMB signal. SO will also have one SAT observing at 220/270 GHz to allow us to understand foreground galactic dust contributions to the target CMB signal while a low frequency 27/39 GHz receiver will replace one of the 90/150 GHz receivers for a single season to allow SO to understand another type of galactic contribution to the signal called synchotron radiation.

Looking down the window in Fig.1, a continuously rotating cryogenic (40 K) half wave plate suspended on a superconducting magnetic bearing resides behind the window and filter set to modulate the incoming polarization signal which allows us to reduce a number of systematics. Each SAT contains refractive optics and a focal plane array cooled by a Bluefors dilution refrigerator to below 100mK. The dilution refrigerator and pulse tubes are tilted to allow lower telescope pointing angles.

SAT-1 backend
Fig.2. SAT-1 with the back lids removed to show the detector electronics with a single pixel tester installed on the focal plane

Each SAT is mounted on a three axis platform (SATP). The SATP allows for a full range of azimuthal pointing, a 0° to 90° elevation pointing range, and a ±90°rotation range around the optical axis which allows for additional systematics control. A conical forebaffle is attached directly to the front of the SAT to block aperture illumination from the telescope structure, ground, and sun. Additionally, a comoving shield is attached to the elevation stage and the entire SATP is surrounded by a 5 meter  high ground shield, both of which which prevent illumination of the aperture from the surrounding terrain including the peak of Cerro Toco, the mountain on which SO is located.

Fig.3. The SAT array

Vertex Antennentechnik GmbH is manufacturing the SATPs for delivery in 2020.

Fig.4. Assembly and testing of a cryogenic half wave plate

The 42 cm diameter 1 K aperture stop defines the SAT illumination and resolution on the sky. It is located at the entrance to the optics tube which contains three silicon lenses with a meta material antireflective surface cut directly into the silicon. The optics tube also contains cold baffling and additional filters. The focal plane array is cooled to 100 mK and is offset from the optics tube with a carbon fiber truss. Criotec Impianti SpA iin Chivasso Italy has manufactured the three SATs that will be part of the initial deployment of SO in 2020 and 2021.

SAT-1 is being integrated and tested at UC San Diego in #ClubHighBay.