The seed beam was sent to the target chamber after expanding beam size from 3 cm diameter to around 10 cm diameter, then it was focused to the smaller spot by a Parabolic mirror. We are using the 248 nm coated paroblic mirror with 20 cm focal length, so the focal spot under the diffraction limitation is about 1.2 um.
In order to measure the focal spot, we put a 40X microcope objective lens behind the spot. A CCD camera without a window to record the spot image, the CCD size is 510x492, the pixel size is 9.6 um x 7.5 um. After calculation, the focal spot is about 2.9 um x 3.8 um.