Single-iteration multigrid (SIMG) wavefront reconstruction schemes were implemented and validated on the adaptive optics system at the Hale 5.1 m telescope at the Palomar Observatory. Results indicate that even the simplest such method produces a performance indistinguishable from that of the standard leastsquares reconstructor for both bright and dim guide stars. SIMG provides a dramatic reduction in computational cost when compared to vector-matrix multiplication and can be implemented in parallel, making it the obvious choice for reconstruction in future large-scale adaptive optics systems. [6]. In this Letter, we detail the experimental validation of this work on a real SCAO system.Two computationally efficient methods that have previously been tested on-sky are the FD method [7] and a sparse method [4]. These methods are O͑n log n͒. The SIMG is O͑n͒ and shows no performance degradation when compared to the leastsquares reconstructor.A good model for the wavefront sensor (WFS) is [6,8] where x is the wavefront phase, y are the measurements, v is white noise, and G is a sparse influence matrix. The least-squares reconstruction matrix is found by taking the pseudoinverse. In practice, we can compute it by evaluating K = ͑G T G + ⑀I͒ −1 G T for a small ⑀. This ensures that unobservable modes such as piston and waffle are zeroed out. The SIMG method [6] uses a single multigrid sweep with an initial guess of 0 to obtain an approximate solution to the equationIf the measurements are taken in open-loop, x is the wavefront phase. In this case, x 0 = 0 is a bad guess, so multiple iterations are required to achieve acceptable convergence [6]. However, when we operate in closedloop, x is the change in wavefront phase between successive time steps, and only one iteration is required. Our tests were performed on the Palomar Adaptive Optics (PALAO) system on the Hale 5.1 m telescope [9]. The PALAO system has a deformable mirror (DM) with 241 active actuators and a ShackHartmann WFS array with 256 subapertures, producing a total of 512 measurements. The DM and WFS are aligned in a Fried geometry.The AO system collects measurements y at up to 2 kHz. Tip and tilt are removed from the wavefront using a fast-steering mirror (FSM) and proportionalintegral (PI) controller. The rest of the wavefront offset x is reconstructed by VMM: x = Ky. This estimate is fed back through a second PI loop to the DM. The closed-loop corrected wavefront is split using a dichroic mirror. The near-infrared portion is sent to the Palomar High Angle Resolution Observer (PHARO) [10]. See Fig. 1.To implement the SIMG on the PALAO system, virtual actuators were added to fill the 17ϫ 17 grid containing the circular actuator arrangement, and the influence matrix G was augmented with zeros appropriately, as in [3]. The new system is y = Ḡ x, where Ḡ is 512ϫ 289 instead of 512ϫ 241. This new system is September 15, 2008 / Vol. 33, No. 18 / OPTICS LETTERS 2047 0146-9592/08/182047-3/$15.00