Dual-OPD Interferometric Imaging for MLT Airglow Temperature Retrieval Under Instrumental Visibility Uncertainty

Accurate temperature measurements in the mesosphere and lower thermosphere (MLT) are important for investigating atmospheric dynamics and vertical coupling. Temperature retrieval from the OI 557.7 nm airglow interferometry requires an appropriate optical path difference (OPD), while the measured fringe visibility is also affected by variations in instrumental visibility. To address the coupling between these two effects, this study develops a dual-OPD interferometric method in which the line-of-sight effective temperature and common instrumental-visibility variation are jointly considered in the retrieval. The OPD pair is selected according to both temperature-dependent coherence sensitivity and the expected instrumental-visibility uncertainty. Under a baseline uncertainty of 1%, a 12.0/13.5 cm configuration provides temperature retrieval precision comparable to that of an optimized single-OPD measurement under the same total photon budget. Under the adopted forward-model assumptions, end-to-end Monte Carlo simulations with noise introduced directly into four phase-stepped interferograms show close agreement with the analytical uncertainty model and yield a maximum simulated temperature RMSE of approximately 3 K over 140–210 K at an interferogram SNR of 150. More importantly, as instrumental visibility becomes less well-constrained, the preferred dual-OPD configuration changes from two closely spaced temperature-sensitive OPDs to a widely separated pair. The shorter OPD then provides a reference-like constraint on the common instrumental response, while the longer OPD retains the main temperature sensitivity. Consequently, when instrumental-visibility variations are predominantly common to the two channels, the re-optimized dual-OPD design degrades more slowly than the optimized single-OPD design as instrumental-visibility uncertainty increases. Additional sensitivity analyses show that channel-dependent instrumental-visibility variations progressively weaken this benefit, while residual background-subtraction errors can introduce systematic temperature bias. The results show that dual-OPD sampling can retain comparable photon-limited precision to an optimized single OPD under well-constrained instrumental visibility while providing greater robustness to predominantly common-mode instrumental-visibility uncertainty. These results provide design-level guidance for selecting fixed OPDs according to the expected atmospheric temperature range and instrumental stability.