Removes parabolic (top to bottom) additive error from Voyager 800 x 800 frames.

Calculate the DN offset values for each line to correct additional dark current buildup on the vidicon detectors on Voyager 1 during a 15 hour passage through the Io plasma torus. The additive error increased from top to bottom of each frame and is directly related to the spacecraft event time. Three two-dimensional [DN offset/Spacecraft event] plots (ICARUS 73,385-426(1988) Global Color and Albedo Variations on IO, by Alfred S. McEwen) reveal the DN offset values based on hour by hour event times, one for top of the frame, middle and bottom. Full 16 hour points have been identified of each plot and loaded into three arrays. The event time is given to this routine in GMT hour, minutes and seconds. The arrays are referenced by the whole integer hour passed. Two points are identified by x1 = hour and y1 = array(hour); x2 = hour + 1 and y2 = array(hour + 1); where x = event time and y = * DN value. The hour, minute and seconds of the event time are also converted to a real value and then all is passed on to a routine which will perform a linear interpolation between the two points and return the associated DN value for the real event time. This program is meant to be run after voycal has been run on the image, as it provides additional corrections for a specific time period.