GroundBIRD is an experiment to observe the cosmic microwave background at 145 and 220GHz. It aims to observe the B-mode polarisation signal from inflation in the early universe. It is located at Teide Observatory, on the island of Tenerife in the Canary Islands.

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GroundBIRD

GroundBIRD in 2019
Part of	Teide Observatory
Location(s)	Tenerife, Atlantic Ocean
Coordinates	28°18′02″N 16°30′37″W
Altitude	2,400 m (7,900 ft)
Wavelength	145, 220 GHz (2.07, 1.36 mm)
Telescope style	cosmic microwave background experiment radio telescope
Diameter	30 cm (1 ft 0 in)
Angular resolution	0.5 degree, 0.3 degree
Location of GroundBIRD
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Scientific goals

The telescope was constructed to measure the B-mode signal in the polarisation of the Cosmic Microwave Background (CMB),[1] in order to look for evidence of cosmic inflation in the early universe. It aims to observe the reionization bump at ${\displaystyle l<20}$ and the recombination peak around ${\displaystyle l=200}$.[2] The name 'GroundBIRD' indicates that the telescope is ground-based, while BIRD stands for B-mode Imaging Radiation Detector.[2] It is related to the future, similarly-named, LiteBIRD CMB satellite.

Telescope

The telescope consists of two mirrors in a Mizuguchi-Dragone configuration, with a diameter of 30 cm (12 in). The telescope is inside the cryostat, which is mounted on a rotation table, with a rotary joint that provides helium gas and electricity to the cryostat.[2][3] The mirrors are cooled to 4 K (−269.15 °C) using a Pulse tube refrigerator to reduce the thermal noise from the mirror surfaces.[2]

The experiment uses microwave kinetic inductance detectors (MKIDs),[4] which are cooled to 250mK by a sorption cooler within the cryostat, which uses helium-3, and was manufactured by Chase Research Cryogenics Ltd.[2] The signals from the detector are multiplexed, and around 100 detectors can be measured in both phase and amplitude with a single digital read-out system with a bandwidth of 200MHz, recording 1,000 samples per second. The digital system uses 12-bit ADCs and a Kintex-7 FPGA from Xilinx initially,[4] and now uses Kintex ultrascale FPGAs.^{[citation needed]} Raspberry Pis are used to monitor and control the telescope.[5]

The cryostat rotates at 20 rpm (120° per second, 1 rotation every 3 seconds) to minimize 1/f noise.[2][6][7] It observes at zenith angles up to 20°, mapping around 40% of the sky. The field of view is 10°, with an angular resolution of 0.5° FWHM at 145GHz, and 0.3° at 220GHz.[2] It will measure the CMB at ${\displaystyle 6<l<300}$[6]

The telescope was constructed at KEK in Japan.[2] Test observations started in Japan in 2014. While it was originally intended that it would observe from the Atacama Desert in Chile,[6] an agreement to install it at Teide Observatory was reached in 2016,[8][1][9] at an altitude of 2,400 metres (7,900 ft).[2][10] It was shipped to Tenerife in January 2019.[7] In February 2020, the experiment was visited by Kenji Hiramatsu, the Japanese Ambassador to Spain.[11]

Collaboration

The collaboration includes scientists from:

• Delft University of Technology, Netherlands[2]
• The Graduate University for Advanced Studies, Japan[4]
• Korea Astronomy and Space Science Institute, Korea[2][12]
• Instituto de Astrofísica de Canarias, Canary Islands, Spain[2]
• KEK, Japan[2]
• Korea University, Korea[2]
• Kyoto University, Japan[2]
• National Astronomical Observatory of Japan, Japan[2]
• RIKEN, Japan[2]
• Saitama University, Japan[2]
• Tohoku University, Japan[2]
• University of Tokyo, Japan[2]

Funding

The project is funded by:

• Ministry of Education, Culture, Sports, Science and Technology, Japan[4][6]
• The Graduate University for Advanced Studies, Japan[6]
• National Astronomical Observatory of Japan, Japan[6]
• National Research Foundation of Korea, Korea[6]

with additional support from:

• OpenIt, Japan[4][13]

References