The on- and off-axis light injection module for Asgard/BIFROST at VLTI

BIFROST is a new Y+J and H band beam combiner for the VLTI, and part of the Asgard suite of visitor instruments. BIFROST will unlock a new parameter space at the VLTI by including the astronomical J band and high spectral resolution (up to R ≈ 25,000). It will also have the ability to simultaneously observe on- and off-axis targets. BIFROST's beam combiner will be an integrated optics chip, fed by single mode optical fibers. BIFROST therefore requires a light injection module to couple the starlight from free space in the VLTI laboratory into the single mode fibers. The light injection module of BIFROST is also responsible for redirecting the starlight towards the fiber couplers; removing the optical path difference between the beams; co-phasing BIFROST with the rest of the Asgard suite; splitting the light off for the off-axis field; selecting the pointing of the off-axis field; optimizing the injection into the fibers; co-phasing the on- and off-axis light; supporting the passage of the full two arcsecond diameter field of view and providing sufficient space for additional BIFROST pre-injection optics. In this contribution we detail the novel design of the BIFROST light injection module, highlighting how it achieves this functionality using as few optics as possible. We also present Zemax Opticstudio tolerancing analysis, demonstrating the feasibility of building this design in the laboratory.