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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.