The invention relates to a method of determining the distortions in the projection system of a TEM, and a method of correcting for these aberrations. This is attained by providing a communication device in a virtual private network having a VPN accommodation function for connecting an intra-organization network or inter-organization network through the Internet. 0where R4 represents the radius of curvature of the surface of the second lens facing the image, and R6 represents the radius of curvature of the surface of the third lens facing the object. The method also includes receiving, from a second secure entity in the secure network, at least one security key formed based on the global challenge and second authentication information for uniquely challenging the mobile unit.