Expand documentation and add equations

docs/source/algorithms/CombineDiffCal-v1.rst

@@ -9,18 +9,35 @@
 Description
 -----------
 
-This algorithm calibrates groups of pixels after cross correlation so that diffraction peaks can be adjusted to the
-correct positions.
+This algorithm combines pixel calibration results (:math:`prev` subscript below) from the ``PixelCalibration`` workspace with the calibration of focussed data from :ref:`PDCalibration <algm-PDCalibration-v1>` (:math:`pd` subscript below).
+The :math:`arb` subscript below denotes values found in the ``CalibrationWorkspace``.
+
+.. note::
+
+   The ``GroupedCalibration`` table is assumed to not have :math:`DIFA` or :math:`TZERO` set.
+   It is also assumed to have calibration values for **all** detector pixels.
+
+The effective calibration values are calculated using the following equations
+
+.. math::
+
+    DIFC_{eff} = \frac{DIFC_{pd}}{DIFC_{arb}} * DIFC_{prev}
+
+where :math:`DIFC_{pd}`  is from :ref:`PDCalibration <algm-PDCalibration-v1>`,  :math:`DIFC_{arb}` is found in the parameter ``CalibrationWorkspace`` that was the input workspace to ``PDCalibration``, and :math:`DIFC_{prev}` is from the previous calibration.
+The value of :math:`TZERO` is unchanged
 
 .. math::
 
-    DIFC{eff} = \frac{DIFC{pd}}{DIFC{arb}} * DIFC{prev}
+   TZERO_{eff} = TZERO_{prev}
+
+The value of :math:`DIFA` is updated by
+
+.. math::
 
-:math:`DIFC{pd}` is the DIFC produced by PDCalibration, found in the parameter "GroupedCalibration"
+    DIFA_{eff} = \left( \frac{DIFC_{pd}}{DIFC_{arb}} \right)^2 * DIFA_{prev}
 
-:math:`DIFC{arb}` is found in the parameter "CalibrationWorkspace"
 
-:math:`DIFC{prev}` is the previous DIFCs generated by CrossCorrelation, found in the parameter "PixelCalibration"
+If pixels are masked in ``MaskWorkspace``, missing from ``PixelCalibration``, or missing from the list of contributing pixels in ``CalibrationWorkspace``, then the calibration constants found in ``GroupedCalibration`` will be copied.
 
 .. categories::