and a second signal to the controller that is a function of the intensity of light registered by second detector region 114 detector 108 is oriented with respect to latch needle 104 so that dividing line 122 is substantially perpendicular to the plane the same as the plane of fig7 a 7c of the latch needle slot not shown in which latch needle 104 is held and perpendicular to the direction of the back and forth motion of latch needle 104 indicated by doubled headed arrow 124 fiducial 102 is illuminated by light from fiducial illuminator 101 and reflects some of the light indicated by dotted line 128 onto lens 106 fiducial 102 preferably reflects light from fiducial illuminator 101 diffusely in a cone not shown of half energy angle on the order of 10 u00b0 15 u00b0 fiducial illuminator 101 and fiducial imager 100 are located with respect to each other so that for any position of latch needle 104 in the operating range of motion of latch needle 104 fiducial 102 reflects light from fiducial illuminator 101 into fiducial imager 100 lens 106 forms an image 130 of fiducial 102 on light sensitive surface 110 from the light reflected by fiducial 102 a first image portion 132 of image 130 falls on first detector region 112 and a second image portion 134 of image 130 falls on second detector region 114 views 116 118 and 120 first detector region 112 registers an intensity of light on its surface that is a function of the size of first image portion 130 and second detector region 114 registers an intensity of light that is a function of the size of second image portion 134 detector 108 therefore sends a first signal to the controller that is as.