Design of the Tx attenuating photoreceiver for the LISA stray light assessment instrumentation

This paper describes the design of a unit called Tx Attenuating Photoreceiver (TAP), dedicated to the measuring and dumping of a 2 W beam with 300 mm diameter. This unit is a part of an instrumentation called Stray Light Optical Ground Segment Equipment for the test of the Interferometric Measurement System (IMS) of the LISA mission. The geometry of the instrument is studied to reduce the size of the beam, focus it on a photodiode, dump its power and minimize scattering and reflections back to the IMS. An off-axis parabolic absorber with anti-reflection coating will stop the beam, reflecting only 0.2 % towards a pinhole and a photodiode. Most of the materials and components are commercially available, but the design and size of the parabola are specific.