The Calibrator tool handles lens distortion, lens magnification and camera magnification using a Nth order nonlinear calibration. The calibration is based on an image of  high precision grid. Using calibrator the actual number of grid points and the grid spacing are added to the Grid definition. With Calibrator the user can set the order of the calibration. Normally will 4 or 5 order be good values to use. Higher order filter may result in better model fit but does not really give a better correction. The result of model-fit are given in pixel-coordinates. A max-deviation of 0.2 pixels is a good model-fit. 

Hint: To improve the image of the grid on can use standard software like Microsoft Paint or Corel PhotoPaint. Normal corrections are:

• To remove points only partially shown
• Improve contrast by maybe binarising the image

 

Here we see the pixel size after calibration. They are slightly changed.