Daniel Johns, Georgia State University; Douglas Hope, Georgia Tech Research Institute; Stuart Jefferies, Georgia State University; Fabien Baron, Georgia State University; Dmitriy Shcherbik, Georgia State University
Keywords: Shack-Hartmann, Tomography, Imaging, SSA, SDA
Abstract:
High-resolution, high-contrast imaging of resident space objects (RSOs) represents a cornerstone of Space Domain Awareness (SDA). Achieving this capability requires accurate estimation of the complex wavefront, which carries information about reflected light from RSOs. Turbulence in the atmosphere causes random fluctuations in both the wavefront phase and intensity, and thus accurate modeling of the complex wavefront is required.
Scene-based (or “imaging”) Shack-Hartmann Wavefront sensors (ISH-WFS) have been utilized in Solar physics on a frame-by-frame basis to drive adaptive optics. When analyzing a time-series of ISH-WFS frames in post-processing, and leveraging temporal correlations in the atmosphere, wavefront estimates with low root-mean-square error (rmse) values can be achieved. Furthermore, an ISH-WFS allows for tomographic reconstruction of the atmosphere, which has been shown to provide wavefronts with significantly lower rmse values than when only a single atmospheric layer is considered. Moreover, the ISH-WFS performs well with extended targets as well as point sources. The downside of the ISH-WFS is that it requires bright targets (mV telescope. This is significantly brighter than for other types of WFS. To gain sensitivity, rather than increase the exposure time and lose the temporal correla- tions (which preserve high spatial frequency information about the RSO) in the wavefront, we propose to extend the ISH-WFS to a broad spectral region (500 nm or more). The extension to the broadband brings with it additional effects, notably chromatic dispersion. This effect, however, means the high-altitude atmospheric layers are sampled differently according to wavelength, improving the tomographic reconstruction of the atmosphere. By opening up the spectral bandwidth available to the ISH-WFS we collect an order of magnitude more photons, but then the challenge becomes estimating the spectral dependence of the RSO intensity distribution and the time-varying atmospheric PSF. Recent work has shown that hyperspectral imaging across a broad spectral range can successfully recover the spectrum of the target by modeling the PSF and object simultaneously at narrow wavelength regions across the entire bandpass. To further improve upon the estimated wavefront we include wavefront amplitude in the reconstruction and use temporal phase diversity as a lever to further improve the wavefront estimate.
We show that by observing over a broad spectral range (500 nm or more), we not only significantly improve the sensitivity of the scene-based Shack-Hartmann WFS to target brightness, but we also improve the quality of the tomographic solution for targets observed at low Zenith angles.
Date of Conference: September 19-22, 2023
Track: SDA Systems & Instrumentation