Correction of etaloning effects in ground-based hyperspectral image cubes of Jupiter

Abstract

The New Mexico State University Acousto-optic Imaging Camera (NAIC) at the Apache Point Observatory 3.5-m telescope is collecting narrowband hyperspectral image cubes of Jupiter from 470–950 nm during the perijove passes of the Juno spacecraft. For operations prior to 2018, the focal plane used for NAIC was a 1024×1024 pixel2 backside illuminated, high quantum efficiency CCD. However, the narrowband images show evidence of “fringing”, due to “optical etaloning.” For much of our collected data, a flat-field correction successfully removes the fringing from the science images. However, for some absorption features, especially in Jupiter's prominent CH4 bands at ∼890 nm, differences in the illumination spectrum of the flat-field source and Jupiter leave residual fringing in the images. Observation of the fringe pattern in the flat-field images as a function of wavelength suggested we could assume a thickness function with a single layer involving a single reflective surface. The thickness of the sensor has a “dish-shaped” variation as a function of pixel position in addition to finely spaced surface polishing marks. Using a mathematical interference (fringe) model, we were able to solve for the 2-D physical thickness function of the CCD at each pixel by minimizing the mean square error between the fringe model and the pixel spectral data. Synthetic fringe frames were created using the 2-D thickness function and scaled with best-correction contrast values. The flat-field and Jupiter images were corrected separately for fringing with division by the synthetic fringe frames. Fringe-corrected Jupiter frames are divided by fringe-corrected flat field frames to yield the final Jupiter science images, which show little evidence of etaloning.