We report on a test bed to compare the performance of three different wavefront sensors, the Shack-Hartmann Wavefront Sensor (SHWFS), the Pyramid Wavefront Sensor (PWFS), and the non-linear Curvature Wavefront Sensor (nlCWFS). No single wavefront sensor easily allows for sensing all aspects of atmospheric turbulence. For instance the SHWFS has a large dynamic range and a linear response to input phase aberrations but is not sensitive to low order modes. The PWFS uses the full spatial resolution of the pupil which gives it increased sensitivity to low order modes, however it still treads the line between achieving high dynamic range and high sensitivity. The nlCWFS is the only wavefront sensor designed to sense low and high, spatial frequencies, however this leads to a complex algorithm. We discuss the reconstruction algorithm for each WFS along with simulated comparisons, we present the optical design for the WFS comparison tes tbed, and outline the adaptive optics controls system.
Simulations of satellite downlinks have previously shown adaptive optics (AO) can enable daytime satellite QKD by allowing strong spatial filtering of sky noise while preserving the quantum signal. In the downlink scenario, the light from the satellite beacon samples the same atmospheric path as the QKD photons. In an uplink scenario however, the satellite changes position after launching the beacon light and the QKD photons are directed upward through a different atmospheric path in order to intercept the satellite at its new location. As a consequence, the quantum channel and beacon channel deviate from one another by more than 50 urad in angle for a LEO engagement. This angular separation exceeds the isoplanatic angle by several multiples. Yet, this simulation suggests that AO in this scenario will still provide sufficient wavefront compensation to justify its inclusion in a QKD uplink.
Past research in adaptive optics (AO) has demonstrated the link between apparent beacon extent and wavefront gradient estimation sensitivity, or optical gain, of a classical Shack-Hartmann (SH) subaperture when using quad-cell detector regions. Pixel diffusion and residual wavefront error broaden the effective subaperture point spread functions as the atmospheric seeing varies in time. Although the AO community has generally shifted toward resolved subapertures to combat these interlinked issues, the quad-cell subaperture design offers efficient light usage for dim beacons, integrating less pixel noise while also reducing sensor readout latency. Particularly for telescopes in poor seeing conditions, in order to reduce beacon magnitude requirements, a quad-cell SH design, coupled with the proposed algorithm, can be an enabling solution. We present research conducted at the Starfire Optical Range over the past 8 years in implementing a robust approach that measures the real-time sensitivity on the site’s natural guidestar and laser beacon AO systems at the 3.5- and 1.5-m telescopes. Emphasis is given toward the practical aspects that must be considered beyond the pure theory, which has been presented in several prior works. A high-signal-to-noise strategy has been implemented that estimates the aperture-averaged subaperture sensitivity (related to beacon size) by exploiting the null space of the least-squares wavefront reconstructor. Careful consideration has gone into the implementation of this estimation method to avoid unintended effects, particularly at low-light levels. Unfortunately, this solution does not in itself address aperture-variant effects, such as sodium beacon elongation for extremely large telescopes.
A new technique for phase retrieval in non-linear Curvature Wavefront Sensors is presented. Unlike the iterative Gerchberg-Saxton technique, this technique retrieves pupil phase in a single step. It starts by calculating the optical transfer function from several images each with its own known aberration. It then solves for the pupil phase by inverting the process of autocorrelation, which, in effect, produced the optical transfer functions.
The gain in Hartmann sensors varies if the shape of the guide star varies. For Natural Guide Star (NGS) operation, this
can be an extended object which changes, e.g. a Comet. For a Sodium Laser Guide Star (LGS) system the spots change
if the Sodium Layer changes or r0 changes. New techniques are presented for estimating and correcting gain tables in
real time AO closed loop operation. One involves looking for reference in the null space (i.e. slope discrepancy space) of
the reconstruction matrix, and the other involves a new WFS output, the cross difference. These techniques are applied
to NGS and LGS operation.
A 941 channel, 1500 Hertz frame rate adaptive optical (AO) system has been installed and tested in the coude path of the 3.5m telescope at the USAF Research Laboratory Starfire Optical Range. This paper describes the design and measured performance of the principal components comprising this system and present sample results from the first closed-loop test of the system on stars and an artificial source simulator.
A new wavefront sensors design is presented which allows reconstruction of phase without the insensitivity to waffle mode which has previously plagued the Fried Geometry. This new system senses and reconstructs waffle, with less total computational burden, and little change in noise propagation, than previous systems.
At the Starfire Optical Range many different Adaptive Optics systems are configured and tried out, with many pieces of equipment supplied, programmed and supported by different people. To accommodate this dispersion of responsibility, while allowing centralized configuration, and operation, and ease of use and programming, a new software technology based on AppleEvents is utilized.
To function properly, the three key elements of an adaptive optics system must be properly registered. At the Starfire Optical Range (SOR) of the Air Force Phillips Lab we have developed a set techniques which align the Lenslet array to the WFS camera, and register the DM to the Lenslet array. The former uses the spot centroids to factor misalignment into X,Y translation, magnification, and rotation. The latter process, using an insight from Jim Spinheme of RTS, takes advantage of the mirror mode called "waffle." It can generate the same misalignment factors for deformable mirror (DM) misregistration. These two techniques are implemented in the wavefront sensor (WFS) user interface, as a convenient, easy to use tool. Both theory and implementation are presented.
The adaptive optics system for the 1.5-m telescope at the Starfire Optical Range, Kirtland AFB, New Mexico has recently been upgraded. Two of the key components in the new system are improved Generation III Shack-Hartmann Wavefront Sensors (WFSs) built by Adaptive Optics Associates (AOA). The performance of the new WFSs has been measured. Measurements indicate a factor of two improvement in noise performance and less inter- subaperture pixel crosstalk resulting in improved closed loop stability. System design and performance measurements are presented.
We present the results of a Horizontal Propagation Experiment (HoPE) that was performed at the Phillips Laboratory Starfire Optical Range. In this experiment a laser beam was phase corrected using an adaptive optics system located at the transmitting site and focused toward a target located two miles away. Irradiance patterns of the corrected and uncorrected beam were recorded at the target site. Weather and atmospheric turbulence characteristics along the optical path were recorded at the same time. Strehl ratios calculated from the recorded images show that phase-only correction of a horizontally propagated laser beam can significantly improve the energy collected on-axis even under strongly scintillated conditions. Time- averaged strehl ratios were improved by as much as a factor of 5. Improvements in strehl for varying turbulence conditions and the effect of hardware limitations on the results are discussed.
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