[Paper Review] Asgard/NOTT: L-band nulling interferometry at the VLTI I. Simulating the expected high-contrast performance

Romain Laugier, Denis Defrère+14 more|arXiv (Cornell University)|Nov 17, 2022

Adaptive optics and wavefront sensing4 citations

TL;DR

This paper presents SCIFYsim, an end-to-end simulator for L-band nulling interferometry at the VLTI, modeling instrumental and background noise to predict high-contrast performance. It demonstrates that with current wavefront correction, instrumental errors dominate down to magnitude 6–7, enabling contrast limits up to 10⁵ at 5–80 mas separations around bright stars.

ABSTRACT

Context: NOTT (formerly Hi-5) is a new high-contrast L' band (3.5-4.0 extmu m) beam combiner for the VLTI with the ambitious goal to be sensitive to young giant exoplanets down to 5 mas separation around nearby stars. The performance of nulling interferometers in these wavelengths is affected both by fundamental noise from the background and by the contributions of instrumental noises. This motivates the development of end-to-end simulations to optimize these instruments. Aims: To enable the performance evaluation and inform the design of such instruments on the current and future infrastructures, taking into account the different sources of noise, and their correlation. Methods: SCIFYsim is an end-to-end simulator for single mode filtered beam combiners, with an emphasis on nulling interferometers. It is used to compute a covariance matrix of the errors. Statistical detection tests based on likelihood ratios are then used to compute compound detection limits for the instrument. Results: With the current assumptions on the performance of the wavefront correction systems, the errors are dominated by correlated instrumental errors down to stars of magnitude 6-7 in the L band, beyond which thermal background from the telescopes and relay system becomes dominant. Conclusions: SCIFYsim is suited to anticipate some of the challenges of design, tuning, operation and signal processing for integrated optics beam combiners. The detection limits found for this early version of NOTT simulation with the unit telescopes are compatible with detections at contrasts up to $10^5$ in the L band at separations of 5 to 80 mas around bright stars.

Motivation & Objective

• To evaluate the high-contrast performance of L-band nulling interferometry at the VLTI under realistic noise conditions.
• To identify the dominant noise sources affecting null depth and detection sensitivity in integrated optics beam combiners.
• To develop a simulation framework capable of modeling correlated instrumental errors and their impact on detection limits.
• To inform the design and operation of the NOTT instrument by forecasting performance under various observing scenarios.
• To establish a baseline for future instrument optimization and atmospheric correction integration.

Proposed method

• SCIFYsim is an end-to-end simulator for single-mode filtered beam combiners, focusing on nulling interferometers in long-baseline configurations.
• It computes a covariance matrix of errors by modeling fringe-tracking residuals, chromatic amplitude and phase aberrations, and thermal background contributions.
• Statistical detection tests based on likelihood ratios are applied to determine compound detection limits under Gaussian assumptions for differential nulled outputs.
• The simulator accounts for Earth’s rotation, spectral channel combination, and instrumental error correlations across all baselines and wavelengths.
• It uses forward modeling to compensate for static dispersive effects via air and ZnSe path delays, excluding dynamic atmospheric effects for now.
• The framework supports future extension to kernel-nulling and other advanced beam combiner architectures.

Experimental results

Research questions

• RQ1 What are the dominant noise sources limiting high-contrast detection in L-band nulling interferometry at the VLTI?
• RQ2 How do correlated instrumental errors affect null depth and detection sensitivity in the NOTT instrument?
• RQ3 To what extent can current wavefront correction systems suppress noise to enable contrast levels of 10⁵ at 5–80 mas separations?
• RQ4 How does thermal background from telescopes and relay systems impact performance beyond stars of magnitude 6–7?
• RQ5 Can the likelihood ratio framework accurately predict detection limits when errors are non-Gaussian and temporally correlated?

Key findings

• Instrumental errors are the dominant noise source in L-band nulling interferometry down to stars of magnitude 6–7, limiting performance before thermal background becomes significant.
• Thermal background from telescopes and relay systems becomes the dominant noise source beyond magnitude 6–7 in the L band.
• The simulated performance of NOTT with unit telescopes achieves contrast limits up to 10⁵ at angular separations of 5–80 mas around bright stars.
• The likelihood ratio detection test framework provides reliable sensitivity bounds when assuming Gaussian error distributions, though deviations from normality are measurable with 100+ Monte Carlo samples.
• SCIFYsim enables realistic performance prediction by modeling correlated errors, Earth’s rotation, and spectral channel combination, offering a first-of-its-kind approach for nulling interferometry.
• The simulator is extensible and adaptable for future instruments like GRAVITY, GLINT, or space-based missions such as LIFE, supporting kernel-nulling and advanced signal processing.