I am working in NSC mode and observing a top hat like beam on a detector. I want to know the D4s beam diameter of the beam profile. From the documentation it looks like the RMS spot radius in the “Beam Info” tab on the detector viewer is the second moment – but it’s unclear to me.

For example when I calculate the D4s beam diameter from the beam profile using Matlab I get a value that is about 1.41 times larger than the 2x the RMS Spot Radius.

Can you please provide details on what the RMS spot diameter is calculating?  

Thanks!