Asgard/NOTT: water vapor and CO₂ atmospheric dispersion compensation system

The direct detection of exoplanets and circumstellar disks is currently limited by a combination of high contrast and small angular separation. At the scale of single telescopes, these limitations are fought with coronagraphs, which remove the diffracted light from the central source. To obtain similar benefits with interferometry, one must employ specialized beam-combiners called interferometric nullers. Nullers discard the on-axis light and part of the astrophysical information to optimize the recording of light present in the dark fringe of the central source, which may contain light from circumstellar sources of interest. Asgard/NOTT will deploy an advanced beam-combination scheme offering favorable instrumental noise characteristics when the inputs are phased appropriately, although this tuning will require a specific strategy to overcome the resulting degeneracy. Furthermore, this must bring the phase of the incoming light to a good accuracy across the usable spectrum. Since the fringe-tracker operates at different wavelengths, it can only sense part of the offending errors, and we discuss the measurement of these errors with the science detector. NOTT operates in the L band and suffers from various effects such as water vapor, which has already been experienced with N-band nullers (Keck Interferometer Nuller, Large Binocular Telescope Interferometer). This effect can be corrected with prisms forming a variable thickness of glass and an adjustment of air optical path. Moreover, observations in the L band suffer from an additional and important chromatic effect due to longitudinal atmospheric dispersion coming from a resonance of carbon dioxide at 4.3µm that is impractical to correct with glass plates because of its non-linear wavelength dependency. To compensate for this effect efficiently, a novel type of compensation device will be deployed leveraging a gas cell of variable length at ambient pressure. After reviewing the impact of water vapor and CO₂, we present the design of this atmospheric dispersion compensation device for Asgard/NOTT and describe a strategy to maintain this tuning on-sky.