Correcting etaloning fringes in hyperspectral image cubes of Jupiter using sensor thickness modeling with flat-field data fitting
Evidence of "fringing" due to optical etaloning was observed in narrowband hyperspectral image cubes of Jupiter collected prior to 2018 at the Apache Point Observatory 3.5-m telescope with the New Mexico State University Acousto-optic Imaging Camera. The etaloning resulted from the use of a back-illuminated, high quantum efficiency CCD. Otherwise successful flat-field correction was ineffective in removing fringes at some wavelengths associated with Jupiter's absorption regions. We describe an etaloning correction method based on a mathematical interference model that assumes a single detection layer. A two-dimensional thickness function for the sensor layer was derived and found to have an overall "dish-shaped" variation along with some finely spaced surface polishing marks. Synthetic fringe frames corresponding to the flat-field and science images were created using the thickness function. Optimized contrast values were found for the synthetic frames and defringed images of Jupiter were generated by separately correcting flat-field and science images using the synthetic fringe frames before applying the final flat-field division. Quantitative analyses of defringed flat-field images showed fringe contrast reductions by factors of 2 to 8 on average and a disk-averaged spectrum of defringed Jupiter data compared favorably with an established spectrum. This defringing approach is applicable to other detectors that can be modeled with a single detection layer and where a sequence of spectral images with adequate wavelength resolution is available.