A monochromator apparatus for an optical spectrum analyzer may include a diffraction grating, a rotatable oblique prism reflector element with a non-right-angle apex angle, and a mirror. An input optical beam received from an input component may be diffracted by the grating element and reflected by a reflector element, where the reflector element may include a rotatable oblique prism with an apex angle that is different from a right angle. A mirror may reflect the reflected diffracted optical beam back to the reflector element and the grating element. An output optical beam from the grating element may be provided via an output element to a detection element for high resolution optical measurement. The oblique prism reflector element may reduce or eliminate a Littrow ghost effect or secondary ghost effects caused by the grating element.