Fringe correction

The fringe correction should be made from regions of really empty sky. For that the sky frames should be combined such that cosmic rays and faint stars are eliminated. Hereafter, by smoothing the frame determine the average intensity level and subtract this value for the frame. After all objects have been removed from this sky frame the frame is essentially zero except for the fringe pattern. The frame is scaled to the same exposure time as the science frame and then subtracted. Because the night sky lines are variable, matching the fringe amplitude to the one in the science frame may not be as straightforward as expected, but should be possible with robust estimation.