Superresolution color images from the sparse data cubes of the Hyperscout-H hyperspectral imager aboard the Hera mission

On 26 September 2022, NASA's Double Asteroid Redirection Test (DART) mission impacted Dimorphos, the moonlet of near-earth asteroid (65803) Didymos, performing the world's first planetary defence test. ESA's Hera mission will be launched in October 2024 and rendezvous with the Didymos system end of 2026 or beginning of 2027. It will closely investigate the system and in particular the consequences of the DART impact.   Hera carries the hyperspectral imager Hyperscout-H. Its sensor consists of 2048 x 1088 pixels which are arranged in macro pixel blocks of 5 x 5 pixels. The 25 pixels of each block are covered with filters in 25 different wavelengths where the center response ranges from 657 to 949 nm. Therefore, each of the 2048 x 1088 pixels provides only the brightness information for one wavelength and hence the theoretical 2048 x 1088 x 25 data cube is only sparsely populated.  A simple straight forward approach to replenish the sparse data cube would be to move a 5 x 5 pixel window with one pixel steps horizotally and vertically over the whole frame and assign the obtained 25 wavelength spectrum to the center pixel of the window. Besides reducing the image resolution to the quite coarse macro pixels, the accuracy of this method is limited by pixel to pixel variations of the spectra and even more by varying albedo and shading effects caused by varying surface inclination. This makes the resultant spectra very noisy.  In order to retrieve more accurate spectra with higher spatial resolution, we separate the spectrum at each micro pixel into a normalized spectrum and a brightness scaling factor. We assume the normalized spectra to be spatially smooth, but not necessarily the scaled spectra. Ratios of measured values are used to iteratively compute the normalized spectral value from adjacent pixels. After convergence, the spectra are brightness scaled to reproduce the measured values. This approach allows to replenish the complete data cube with full micro pixel resolution. The application to test images shows that spectra are recovered much more accurately than with the direct approach and that only very little spatial detail is lost.  Having replenished the complete data cube allows us to construct color images at full micro pixel resolution. The three colors are sufficient to capture most of the spatial variation of the spectra of asteroid surfaces and hence the constructed color images provide a concise visualization of the respective full data cubes.