A two-band approach to nλ phase error corrections with LBTI's PHASECam

Abstract

PHASECam is the Large Binocular Telescope Interferometer's (LBTI) phase sensor, a near-infrared camera which is used to measure tip/tilt and phase variations between the two AO-corrected apertures of the Large Binocular Telescope (LBT). Tip/tilt and phase sensing are currently performed in the H (1.65 µm) and K (2.2 µm) bands at 1 kHz, and the K band phase telemetry is used to send tip/tilt and Optical Path Difference (OPD) corrections to the system. However, phase variations outside the range [-π, π] are not sensed, and thus are not fully corrected during closed-loop operation. PHASECam's phase unwrapping algorithm, which attempts to mitigate this issue, still occasionally fails in the case of fast, large phase variations. This can cause a fringe jump, in which case the unwrapped phase will be incorrect by a wavelength or more. This can currently be manually corrected by the observer, but this is inefficient. A more reliable and automated solution is desired, especially as the LBTI begins to commission further modes which require robust, active phase control, including controlled multi-axial (Fizeau) interferometry and dual-aperture non-redundant aperture masking interferometry. We present a multi-wavelength method of fringe jump capture and correction which involves direct comparison between the K band and currently unused H band phase telemetry.

Towards controlled Fizeau observations with the Large Binocular Telescope

“…We also hope to decrease the incidence of Phasecam's phase jumps by supplementing the K S -band pupils with H-band. (See [33] in these proceedings.) Reduction of already-captured Fizeau datasets will allow us to investigate achievable contrasts [38,39] and make further improvements for making this observing mode routine.…”

“…It can only partially remove the 5-10 µm contributions of telescope resonances at 12-18 Hz and instrument (particularly UBC) vibrations at 100-150 Hz. In addition, Phasecam experiences occasional "phase jumps" due to the strong 2π phase degeneracy between fringes (see Table 2 and [33] in these proceedings). Until now, those jumps have had to be counteracted manually in real-time, resulting in some loss in efficiency.…”

“…The second will be upgraded later on. Work is progressing to upgrade the third to supplement the K S -band correction with H-band (see [33] in these proceedings). The Fizeau calibration loop does not yet exist, and the general methodology is explained in the following section.…”

Implementing multiwavelength fringe tracking for the Large Binocular Telescope Interferometer’s phase sensor, PHASECam

“…This manuscript is an updated version of work presented by Erin Maier at the 2018 SPIE Astronomical Telescopes and Instrumentation conference in Austin, Texas. 47 Portions of this work were supported by the Arizona Board of Regents Technology Research Initiative Fund (TRIF). This work also made use of various Python packages for computing and plotting, including Matplotlib, 48 NumPy, 49 IPython, 50 Jupyter notebooks, 51 pandas, 52,53 and plotly.…”

Status of commissioning stabilized infrared Fizeau interferometry with LBTI

“…6 shows examples of the MTF with and without phase control. Though there are no immediate plans for modifying the PID loop itself, we are supplementing the PID loop with software which uses the PhaseCam H-band illumination to automatically correct Ks-band phase "jumps" [24], which occur when an atmospherically-induced phase shift happens quickly enough that the PID loop latches on to the wrong fringe. (Until now, corrections have required manual intervention.…”