9055-DX is a motorized double 1/4m monochromator that can be switched between additive and subtractive configurations. Double monochromators have the advantage of extremely low stray light levels by virtue of using two gratings, plus an intermediate slit between the two units, used to define the wavelengths that pass to the second unit, that also masks off light scattered within the body of the first unit. In double additive operation the 9055DX effective focal length (1/2m), dispersion and resolution (1.7nm/mm and .09nm with a 1200 l/mm grating) are doubled compared to a single unit, making this mode useful where high resolution is also important. In double subtractive operation there is no dispersion in the output beam and only a single slit image appears at the output instead of a broad spread of spectral images. This allows the system to operate as a tuneable bandpass or notch filter. In the 9040DX the individual monochromator drives are computer controlled so that the two units track to a degree not possible with mechanically linked double monochromators, allowing a much narrower intermediate slit.