Physical optics modeling of sky coverage for TMT NFIRAOS with advanced LQG controller

Abstract

We have implemented the linear quadratic Gaussian (LQG) controller in our physical optics sky coverage simulator (MAOS) for the Thirty Meter Telescope (TMT) Narrow Field InFrared Adaptive Optics System (NFIRAOS) aimed for improved correction of tip/tilt and plate scale modes. The LQG controller has a built-in capability to correct narrow frequency vibrations that are above the closed loop bandwidth of the system and is a very desirable solution for this application. The LQG controller is tuned with the combined power spectral density (PSD) of turbulence, wind shake, and vibration computed from the telemetry. We will show how LQG performs for various telescope/instrument vibration spectral (such as broadband or drifting peaks). We will also show the performance and sky coverage of LQG in comparison with single or double integrator controllers for correcting low order atmospheric turbulence with a set of up to three tip/tilt(/focus) natural guide star wavefront sensors. We found that the LQG controller reduces the median sky coverage wavefront error by 25 nm in quadrature.

Kalman filter design for atmospheric tip/tilt, tip/tilt anisoplanatism and focus filtering on extremely large telescopes

“…For TMT's first light AO system, NFIRAOS, performance gains up to 40nm RMS are predicted for the tip/tilt, tilt anisoplanatism and focus modes for the AHST control architecture under median turbulence conditions. Detailed end-to-end Monte Carlo sky coverage simulations are presented in a companion paper [19]. Kalman filtering applied to the more optimal minimum variance split tomography (MVST) architecture [20] will be evaluated in a follow up publication.…”

Advanced control laws for the new generation of AO systems

“…[41][42][43] A continuous-time model is provided in 44 which is quite convenient for LGS-based tomographic systems where the low-order modes are corrected separately and potentially at lower frame rates to allow for improved sky-coverage. 19,20 In practice the vibration model is concatenated with that in Eq. ( 18) in order to obtain a notch at the vibration central frequencies.…”

“…[16][17][18] For low-order modes however there is a general consensus on the need for more advanced controllers capable of improved performance. [19][20][21] A brief presentation is provided next. The residual error ε is a joint contribution od the phase ψ and noise η.…”

“…[16][17][18] For low-order modes however there is a general consensus on the need for more advanced controllers capable of improved performance. [19][20][21] A brief presentation is provided next. In this paper we focus on the choice of the controller, assuming the rest of the loop to be represented by pure delays.…”

The Infrared Imaging Spectrograph (IRIS) for TMT: instrument overview

“…Figure 2 plots the median performance (i.e., 50% sky coverage) of NFIRAOS over the full sky with median seeing at the TMT site. The recent addition of a LQG (Linear Quadratic Gaussian) controller we expect will improve the rejection of vibration and hence improve the sky coverage presented in Figure 2 (Wang et al 8 ).…”