The modified SX-700 monochromator with a spherical focusing mirror: a configuration for -1 diffraction order

In this letter we will discuss the modified SX-700 monochromator with a spherical focusing mirror [Rev. Sci. Instr. 60 (1989) 1608] but working in the configuration of −1 diffraction order. The coma aberration can be kept very small with a much shorter exit arm than in the configuration of +1 diffraction order. The differences between the two cases are given.