function Xstd = ConvStd(X,t0,t1,F);
% function Xstd = ConvStd(X,t0,t1,F);
% convolution standard deviation
% is the running standard deviation
% from relative times t0 to t1, e.g. -12 to 12
% if less than F (fraction of 1) of the values in an 
% average are available then the std is set to nan.
if nargin < 4,
   F = 2/3;
end
M = ConvMean(X,t0,t1,F);
var = ConvMean(X.^2,t0,t1,F);
Xstd = sqrt(var-M.^2);