p u2032 and as a consequence moving boundary locus 60 determines an end position of a ray bundle in contrast in prior art hartmann shack wavefront sensor 200 lenslet array 251 is disposed at plane p u2032 and as a consequence ccd camera 252 determines an end position of a ray bundle because of this difference inventive wavefront measurement apparatus 100 has advantages over prior art hartmann shack wavefront sensor 200 that are best understood in conjunction with fig4 a 5e fig4 a illustrates a wavefront measurement made with prior art hartmann shack sensor 200 for flat wavefront 291 as shown in fig4 a a ray bundle that passes through each lenslet element of lenslet array 251 is focused at one spot on ccd 252 and the separation between focal spots is equal to the spacing between the lenslet elements fig4 b illustrates a wavefront measurement made with prior art hartmann shack sensor 200 for converging wavefront 294 as shown in fig4 b in this case the separation between focal spots is smaller than the spacing between lenslet elements and as a result the dynamic range of tilt measurement of prior art sensor 200 is limited by overlapping of focal spots fig4 c illustrates a wavefront measurement made with prior art hartmann shack sensor 200 for diverging wavefront 296 as shown in fig4 c in this case the separation between focal spots is larger than the spacing between lenslet elements and as a result the dynamic range of tilt measurement of prior art sensor 200 is limited by a size of ccd camera 252 lastly fig4 d illustrates a wavefront measurement made with prior art.