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The ellipse parameter calibration assumes all the POI found on a single diffraction ring can be fit fitted to an ellipse (hence the tilts must not be extreme and enough of the ring must be on the image to get accurate ellipse parameters). The method used to calibrate the image from the ellipse parameters can be found in this article: Complete elliptical ring geometry provides energy and instrument calibration for synchrotron-based two-dimensional X-ray diffraction Michael L. Hart , Michael Drakopoulos , Christina Reinhard , Thomas Connolley Journal of Applied Crystallography (Vol: 46, Pages: 1249 - 1260) DOI: 10.1107/S0021889813022437
The Ellipse Parameter Calibration works best using multiple images at a very well defined separation and requires no indication of the energy or distance the measurement was take taken at. The beam centre and detector tilts are explicitly calculated during this calibration procedure, and so cannot be fixed.
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The Point Parameter Calibration is a more basic approach than the Ellipse Parameter Calibration and simply relies on the fact that each POI found on a standard ring corresponds to a specific q value, and that doing an initial "by-eye" alignment gives a close energy/wavelength and experimental geometry.
From these model initial energy and geometry parameters, q values for each point of interest can be calculated. During the calibration, the model energy, distance, beam centre and detector tilts are adjusted until the model POI q values best match the expected standard q values.
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