function S = getpsf
% Square point spread matrix for blurring: positive entries between 0 and 1, odd
% matrix dimension, sum of matrix entries = 1
S = 0.1*ones(7,7);
S(2:end-1,2:end-1) = 0.2;
S(3:end-2,3:end-2) = 0.3;
S(4,4) = 2;
S = S/sum(sum(S));  