function theta2 = rot_elps(a,b,theta1)

r1=sqrt((a^2*b^2)/((b*cos(theta1))^2+(a*sin(theta1))^2));
r2=a+b-r1;

costheta2=sqrt(((a^2*b^2)/(r2^2)-a^2)/(b^2-a^2))*sign(sin(theta1));
sintheta2=sqrt(((a^2*b^2)/(r2^2)-b^2)/(a^2-b^2))*sign(cos(theta1));

theta2=atan2(sintheta2,costheta2);