As stated in Part 1: The Slit, a spectrometer is an imaging system which maps a plurality of monochromatic images of the entrance slit onto the detector plane. In the past 3 sections, we discussed the three key configurable components of the spectrometer: the slit, the grating, and the detector. In this section, we will discuss how these different components work together with different optical components to form a complete system. This system is typically referred to as the spectrograph, or optical bench. While there are many different possible optical bench configurations, the three most common types are the crossed Czerny-Turner, unfolded Czerny-Turner, and concave holographic spectrographs (shown in Figures 4-1, 4-2, and 4-3 respectively).