Status of commissioning stabilized infrared Fizeau interferometry with LBTI

TL;DR

This paper reports on the progress and challenges in commissioning the Fizeau interferometry mode of the LBTI, aiming to enhance high-resolution infrared imaging capabilities.

Contribution

It details the development, testing, and lessons learned in stabilizing the Fizeau interferometry mode for the LBTI, including active phase control and NCPA correction strategies.

Findings

Successful on-sky engineering tests conducted in 2018 and 2019

Development of a correction loop to remove non-common-path aberrations

Enhanced control methods to improve fringe contrast and imaging quality

Abstract

The Large Binocular Telescope Interferometer (LBTI) has the longest baseline in the world, 22.7 m, for performing astronomical interferometry in Fizeau mode, which involves beam combination in a focal plane and preserves a wide field-of-view. LBTI can operate in this mode at wavelengths of 1.2 to 5 and 8 to 12 {\mu}m, making it a unique platform for carrying out high-resolution imaging of circumstellar disks, evolved stars, solar system objects, and possibly searches for planets, in the thermal infrared. Over the past five years, LBTI has carried out a considerable number of interferometric observations by combining the beams near a pupil plane to carry out nulling interferometry. This mode is useful for measuring small luminosity level offsets, such as those of exozodiacal dust disks. The Fizeau mode, by contrast, is more useful for generating an image of the target because it has more (u, v) (Fourier) plane coverage. However, the Fizeau mode is still in an ongoing process of commissioning. Sensitive Fizeau observations require active phase control, increased automation, and the removal of non-common-path aberrations (NCPA) between the science and phase beams. This increased level of control will increase the fringe contrast, enable longer integrations, and reduce time overheads. We are in the process of writing a correction loop to remove NCPA, and have carried out tests on old and synthetic data. We have also carried out on-sky Fizeau engineering tests in fall 2018 and spring 2019. In this article, we share lessons learned and strategies developed as a result of these tests.