I think this may be causing some confusion. Below I have attempted to draw the same clock seen either in the same frame as the clock or moving past it at velocity -v. I don't find it easy to draw diagrams - it isn't perfect so please forgive the imperfections. Please Register or Log in to view the hidden image! In the A frame photon Alice has reached the detector. In the B frame (after the same length of time ct) Alice has reached the point marked Colin .. so Colin is the Alice photon. If v=-0.5 c then I think we are all agreed that Alice is only 0.866 of the way to the detector - hence in the B (moving) frame the same clock ticks at 0.866 times the rate at which the clock ticks in frame A.