We present a concept to perform low-order wavefront sensing in multi-laser guide star adaptive optics systems
operating using a large format NIR detector with windowing capability with near diffraction limited or partially
corrected NGS tip-tilt stars with time varying Strehls. Most contemporary adaptive optics systems in development
for large telescopes, viz., the next VLT adaptive optics facility that serves as a pathfinder to the European
ELT, Gemini MCAO, W. M. Keck observatory's Next Generation Adaptive Optics (NGAO) System, The Large
Binocular Telescope and the Thirty Meter Telescope's NFIRAOS are multi-laser guide star systems that provide
AO correction over a large field. In such systems even faint tip-tilt (TT) stars image are characterized by either
a well corrected (MOAO case) or at least a partially corrected (MCAO or GLAO case) diffraction limited core
due to high order sharpening by the LGS WFS. In such a regime of low-order sensing one could envisage using
pixels as field stops and choosing a appropriate plate scale to minimize the sky background.
Simulations are used to predict the performance of such a sensor when guiding on point sources and on
extended objects of varying brightness and for different levels of high order correction.
The parameter space explored includes tip-tilt and tip-tilt, focus and astigmatism (TTFA) sensor performance
for various plate scales, TT sensor performance vs. level of high order correction (TT star Strehl) and TT sensor
performance vs. TT object size for a given detector noise, gain and a simple centroiding algorithm. Due to small
sky noise contribution at plate-scales le 100 mas/pixel, the optimum low-order wavefront sensor plate scale is
found to be 80-100 mas/pixel (3×-4× λ/d in J- and H- bands) for the Keck NGAO system.