The DKIST low order wavefront sensor

The Low Order Wavefront Sensor (LOWFS) is a key component of the Active Optics System of the Daniel K Inouye Solar Telescope. It is designed to measure low order wavefront aberrations in the optical beam arising from gravitational and thermal flexure in the telescope as it moves through the sky during solar observations. These quasi-static aberrations are detrimental to the telescope image quality during seeing-limited observations. The LOWFS measures these quasistatic perturbations by averaging over the atmospheric turbulence. It sends its measurements to the Active Optics System, which computes a solution using the primary (M1) and secondary (M2) mirrors, and sends offsets to the M1 and M2 mirror control systems. The LOWFS is implemented using a 1k x 1k pixel Shack-Hartmann wavefront sensor coupled with a real-time cross correlating image processing engine running at 30 Hz. The real-time engine is implemented in C++ using the Armadillo linear algebra library, enabling equation-style programming with arrays and vectors, achieving essentially the same speed as hand coded loops over the same data structures. The cross correlation is implemented in the frequency domain leveraging the speed of the FFTW Fast Fourier Transform library. The entire realtime engine is embedded inside a DKIST Common Services Framework Controller, allowing for simple command and control of the wavefront sensor computations using the high-level Wavefront Correction Control System software. A Python-based script engine is used to implement various calibration tasks, allowing full access to the SciPy software stack for non-real-time scientific computations. This paper describes the design and implementation of the LOWFS and presents initial results from testing in the DKIST Wavefront Correction System Laboratory.