place an actor relative to camera


i have a stereo tracking camera, with the transformations i am getting from that camera i am applying the transform on vtk camera through SetUserViewTransform or SetModelTransformMatrix. i am getting the same result from both of them and the camera is moving as i am moving the tracking camera in the real world.

my problem is i need to place an actor at 0, 0, 50 point relative to camera view, as in the actor should be at the center of the current view at a distance of 50 on z axis, how can i achieve this?

any help will be greatly appreciated, Thanks