Einstein Telescope Optical Layout

(Folding multiple interferometers into one observatory)

The Einstein Telescope consists of three detectors that are folded together into a triangle. Each detector is formed by two interferometers, one for low-frequency gravitational waves (LF) and one for high-frequency gravitational waves (HF). In total, the Einstein Telescope therefore hosts six 10km long interferometers.

General overview of the ET observatory layout, with 3 detectors forming a equilateral triangle of 10 km length. Bottom left is a sketch that shows that each detector consists of two interferometers one optimised for high frequencies (HF) and one for low frequencies (LF). The core interferometer layout is based on a Michelson interferometer with Fabry-Perot cavities in the arms and recycling.
This is a sketch of the lower left corner of the triangle, showing an example implementation of the optical layout, the vacuum system for the main optics and the corresponding cavern layout. In particular this shows the possible location of Z-shaped telescope systems for ET-LF and ET-HF detectors. In this example the telescopes have been placed to achieve an angle of incidence of 45 degrees on the ET-HF beamsplitter.

These images were published in our research paper `Feasibility study of beam-expanding telescopes in the interferometer arms for the Einstein Telescope' in 2021, which describes part of the design effort towards the ET Design Report Update 2020. Based on this work, in order to visualise the optical layout, and to be able to design the required infrastructure, the tunnels and caverns, a full 3D CAD model of the observatory has been created at Nikhef. From the model we also created rendered images and videos.