High fidelity sky coverage analysis via time domain adaptive optics simulations

Abstract

We describe a high fidelity simulation method for estimating the sky coverage of multiconjugate adaptive optics systems; this method is based upon the split tomography control architecture, and employs an AO simulation postprocessing technique to evaluate system performance with hundreds of randomly generated natural guide star (NGS) asterisms. A novel technique to model the impact of quadratic wavefront aberrations upon the NGS point spread functions is described; this is used to model the variations in system performance with different asterisms, and is crucial for obtaining accurate results with the postprocessing technique. Several design and algorithm improvements help to reduce the residual wavefront error in the tip/tilt and plate scale modes that are controlled using the NGS asterism. These improvements include choosing the right wavefront sensor (WFS) pixel size, optimal pixel weights, and type II control of the plate scale modes.

Increased sky coverage with optimal correction of tilt and tilt-anisoplanatism modes in laser-guide-star multiconjugate adaptive optics

“…The 50% sky-coverage goal at the galactic pole, where the NGS density is the lowest, led to a median frame rate around 90 Hz [18]. In this case, guide stars as dim as magnitude 22 in the H-band are used to correct for TT and TA modes.…”

Simulation model based approach for long exposure atmospheric point spread function reconstruction for laser guide star multiconjugate adaptive optics

“…Regarding the servos, the LGS loop implements a simple type I servo with a gain of 50%, whereas the NGS loop uses a more sophisticated type II servo, whose parameters (sampling frequency, modal gains and lead filter cross-over frequency, and phase margin parameters) are obtained from an optimization procedure performed by a separate highfidelity sky coverage simulation tool [27,28]. Sample NGS loop theoretical error rejection curves are plotted in Fig.…”

“…The red, blue, and brown constellations (asterisms 2, 3, 4) are asymmetrical (all within a 1 ft. radius NGS patrol field) and of stars of different magnitude. Sampling frequencies are respectively 100, 80, 90, and 66 Hz (result of an optimization procedure performed by a separate high-fidelity sky coverage simulation tool [27,28]). Bottom panels display the signal-to-noise ratio (SNR) of each WFS (TTFA, TT, TT ordering), as well as the RMS noise equivalent angle in units of mas (NEA, i.e.…”

Tip/tilt point spread function reconstruction for laser guide star multi-conjugate adaptive optics

“…These 3 operations can be expressed as follows: C denotes the est ated measurement noise covariance matrix, computed from centroid weights and suba perture time averaged pixels intensities [16,17]. Note t hat the noise removal step m y not always produce a symmetric positive definite (SPD) covariance m trix, hence an eigenvalue decomposition is required to null possible negative eigenvalues.…”

“…The focus measurement from the TTFA NGS WFS is required in order to disentangle the atmospheric and mesospheric sodium layer focus errors. Regarding the servos, the LGS loop implements a simple type I servo with a gain of 50%, whereas the NGS loop uses a more sophisticated type II servo, whose parameters (sampling frequency, modal gains and lead filter cross-over frequency and phase margin parameters) are obtained from an optimization procedure performed by a separate high-fidelity sky coverage simulation tool [16,17]. Sample NGS loop theoretical error rejection curves are plotted in Figure 1.…”