added documentation that describes alg. re #10689

Code/Mantid/docs/source/algorithms/TOFSANSResolutionByPixel-v1.rst

@@ -9,6 +9,24 @@
 Description
 -----------
 
-Calculates the Q-resolution per pixel according to Mildner and Carpenter.
+Calculates the Q-resolution per pixel according to Mildner and Carpenter equation
+
+.. math:: (\sigma_Q )^2 = \frac{4\pi^2}{12\lambda^2} [ 3(\frac{R_1}{L_1})^2 + 3(\frac{R_2}{L_3})^2 + (\frac{\Delta R}{L_2})^2 ] + Q^2(\frac{\sigma_{\lambda}}{\lambda})^2
+
+where :math:`L1` and :math:`L2` are the primary and secondary flight-paths respectively and 
+
+.. math:: \frac{1}{L_3} = \frac{1}{L_1} + \frac{1}{L_2}
+
+:math:`\sigma_Q` is returned as the y-values of the InputWorkspace, and the 
+remaining variables in the main equation above are related to parameters of this
+algorithm as follows:
+
+* :math:`R_1` equals SourceApertureRadius
+* :math:`R_2` equals SampleApertureRadius
+* :math:`\Delta R` equals SigmaModerator
+* :math:`\sigma_{\lambda}` equals SigmaModerator although note that the former is in units of AA and the later microseconds 
+
+:math:`\lambda` in equation is the midtpoint of wavelength 
+histogram bin values.
 
 .. categories::