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The National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) has made the transition from construction to science operations. It is currently operating with a “classical” single-conjugate Adaptive Optics (AO) system, which will be upgraded to a multi-conjugate AO (MCAO) system within the next few years. One of the key challenges for the DKIST MCAO system will be procuring a suitable Deformable Mirror (DM) to replace the current M7 flat, which is conjugate to 11.2 km.
The DM must be large in size with an elliptical clear aperture 884 by 625 mm. It must also have a high actuator density, with actuator spacings smaller than 11 mm. Additionally, it must have an actuator rise time of 100 µs and an update rate greater than 2 kHz. We have identified the surface-parallel actuated silicon carbide DMs made by Northrop Grumman’s AOA Xinetics (AOX) as a likely candidate to meet our requirements. However, there are some challenges that come with using this technology for the DKIST MCAO system. We must design our controller to avoid exciting resonant modes in the mirror. We also must minimize actuator saturation or it will become the dominant error term in our wavefront fitting error.
We discuss the advantages and disadvantages of this deformable mirror technology for astronomical imaging through the turbulent atmosphere. We use NSO’s Blur adaptive optics simulation software and the KAOS Evo 2 control software to simulate the performance of a large aperture silicon carbide DM. We also present simulation results that model the temporal error incurred by reducing the control bandwidth of the mirror’s resonant modes.
The DKIST wavefront correction system will provide active alignment control and jitter compensation for all six of the DKIST science instruments. Five of the instruments will also be fed by a conventional adaptive optics (AO) system, which corrects for high frequency jitter and atmospheric wavefront disturbances. The AO system is built around an extended-source correlating Shack-Hartmann wavefront sensor, a Physik Instrumente fast tip-tilt mirror (FTTM) and a Xinetics 1600-actuator deformable mirror (DM), which are controlled by an FPGA-based real-time system running at 1975 Hz. It is designed to achieve on-axis Strehl of 0.3 at 500 nm in median seeing (r0 = 7 cm) and Strehl of 0.6 at 630 nm in excellent seeing (r0 = 20 cm).
The DKIST wavefront correction team has completed the design phase and is well into the fabrication phase. The FTTM and DM have both been delivered to the DKIST laboratory in Boulder, CO. The real-time controller has been completed and is able to read out the camera and deliver commands to the DM with a total latency of approximately 750 μs. All optics and optomechanics, including many high-precision custom optics, mounts, and stages, are completed or nearing the end of the fabrication process and will soon undergo rigorous acceptance testing.
Before installing the wavefront correction system at the telescope, it will be assembled as a testbed in the laboratory. In the lab, performance tests beginning with component-level testing and continuing to full system testing will ensure that the wavefront correction system meets all performance requirements. Further work in the lab will focus on fine-tuning our alignment and calibration procedures so that installation and alignment on the summit will proceed as efficiently as possible.
OPERA, an automatic PSF reconstruction software for Shack-Hartmann AO systems: application to Altair
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