A compact system for performing laser ophthalmic surgery is disclosed. The systems and methods may be used to measure corneal thickness or other anatomy to prepare a treatment plan for any of numerous treatments, such as LASIK, PRK, intra stromal lenticular lens incisions, cornea replacement, or any other treatment. By using a reduced power femtosecond laser backscatter may be measured to calculate distances such as distances between an interior boundary and an exterior boundary of a cornea or other tissue.