  

  function J = spring_function_2params(params,obs,t)

  C = params(1);
  K = params(2);
  factor = sqrt(K - C^2/4);
  denom = sqrt(4*K - C^2);
  f = 2*exp(-C*t/2).*cos(factor*t);
  J = (f-obs)*((f-obs)');
