This plot show the optiocal distortion coefficient X_1_0 and Y_0_1 as a function of the IFS bench temperature. 
The polynomial to correct the shifts in x direction is:
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Delta X = X_0_0 + X_1_0 * x + X_0_1 * y + X_1_1 * x * y + ...
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and for the shifts in y-direction:
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Delta Y = Y_0_0 + Y_1_0 * x + Y_0_1 * y + Y_1_1 * x * y + ...
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Hence X_1_0 is the first order linear scale (the plate scale) in x-direction and 
Y_0_1 is the first order scale in y-direction.