The STRings for Absorption length in Water (STRAW) are the first in a series of pathfinders for the Pacific Ocean Neutrino Experiment (P-ONE), a future large-scale neutrino telescope in the north-eastern Pacific Ocean. STRAW consists of two $$150\,\mathrm {m}$$ 150 m long mooring lines instrumented with optical emitters and detectors. The pathfinder is designed to measure the attenuation length of the water and perform a long-term assessment of the optical background at the future P-ONE site. After 2 years of continuous operation, measurements from STRAW show an optical attenuation length of about 28 m at $$450\,\mathrm {nm}$$ 450 nm . Additionally, the data allow a study of the ambient undersea background. The overall optical environment reported here is comparable to other deep-water neutrino telescopes and qualifies the site for the deployment of P-ONE.

The Pacific Ocean Neutrino Explorer (P-ONE) [1] collaboration was born to build a new large-scale neutrino telescope in the Pacific Ocean, at 2600 m b.s.l. in Cascadia Basin, off Vancouver Island. The first steps aimed at the feasibility study and the characterisation of the optical properties of the site with a first pathfinder project named STRAW (STRing for Absorption length in Water) [2] [3] [4], deployed in 2018. During the last two years, a second pathfinder project has been developed: STRAW-b. The main goal of STRAW-b is to validate the attenuation length already measured by STRAW and to add new information on the background characterisation with the study of the deep sea diffused light spectrum. It consists of a 500 m mooring (electrical-optical cable communication) equipped with three Standard Modules for environmental monitoring and seven Specialised Modules for background analysis and attenuation length measurements. All the modules are hosted in spherical 13" high-pressure resistant glass housings. Its design started at the end of 2018 and after about two years it has been successfully deployed in summer 2020 in Cascadia Basin site, connected to the underwater Ocean Networks Canada infrastructure about 40 meters away from STRAW. We present all the steps from the design to the realisation of the mooring, with a special focus on the adopted technologies.

The Pacific Ocean Neutrino Experiment (P-ONE) is an initiative by a collaboration of Canadian and German universities as well as Ocean Networks Canada (ONC) to develop a new large-scale neutrino telescope 2600 m below the ocean off the coast of western Canada. While the instrumented volume needs to be at least on the order of km for the physics goals of P-ONE to be met, the density of photo sensors needs to be kept as low as possible in order to minimize construction costs. Naturally, this puts very high demands on the optical properties of water at the deployment site. Ideally, the water should exhibit minimal photon extinction and scattering to optimize the light yield and timing needed for reconstructing neutrino-induced Cherenkov light flashes. In addition, a low light background from natural undersea sources such as bioluminescence and K40 radioactive decay is necessary for achieving high sensitivity to neutrino events. In order to evaluate the proposed site for P-ONE, two pathfinder missions have been deployed successfully, one in 2018 and the other in 2020. In this presentation we will highlight the results from the first mission that was primarily aimed at evaluating the optical properties of the site in terms of absorption, scattering, and backgrounds.

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