Baldr: a Zernike wavefront sensor for VLTI/Asgard

The simultaneous push to unprecedented contrast, spectral and spatial resolution at the Very Large Telescope Interferometer (VLTI), as promised by the Asgard instrument suite, requires unprecedented high Strehl ratios and phase-front stability. This will be the job of Baldr: a new J or H band Zernike wavefront sensor (ZWFS) used as a second stage adaptive optics (AO) system for the VLTI/Asgard instrument suite. ZWFS was selected for Baldr due to its optimal photon sensitivity and the need for Baldr to be extremely low latency to reduce the residuals from the first stage AO systems. Here we briefly present the design of Baldr which uses a novel architecture where all four Baldr beams for adaptive optics from each telescope are sensed with a low-latency C-RED One detector, permitting AO frame rates up to 3.5kHz. With this architecture simulations show that the Baldr design can deliver J-band Strehl ratios for the 1.8m Auxiliary Telescopes (AT's) of over 80%, which is a significant improvement on the Strehl ratios otherwise delivered to Asgard, which would limit the scientific objectives of the instrument. Finally we present preliminary test-bed results and give an overview of progress and future work.