The first optical bench is based on a crossed Czerny-Turner (CZ) configuration which consists of two concave mirrors and one plano diffraction grating. Mirror 1 is used to collimate the light emitted from the entrance slit and mirror 2 is used to focus the dispersed light from the grating onto the detector plane. The Czerny-Turner configuration offers a compact and flexible spectrograph design. For a diffraction grating with given angular dispersion value, the focal length of the two mirrors can be designed to provide various linear dispersion values, which in turn determines the spectral coverage for a given detector sensing length and resolution of the system. By optimizing the geometry of the configuration, the Czerny-Turner spectrograph may provide a flattened spectral field and good coma correction. When aspheric mirrors (such as toroidal mirrors) are used instead of spherical mirrors, the Czerny-Turner configuration can also provide certain degree of correction to spherical aberration and astigmatism.

However, due to its off-axis geometry, the Czerny-Turner optical bench exhibits a large image aberration, which may broaden the image width of the entrance slit by a few tens of microns. Thus the Czerny-Turner optical bench is mainly used for low to medium resolution spectrometers. To minimize image aberrations, the Czerny-Turner optical bench is generally designed with an f-number* of >3, which places a limit on its throughput.

*Note: The f-number of an optical system expresses the diameter of the entrance pupil in terms of its effective focal length. The f-number is defined as f/# = f/D, where f is the focal length of the collection optic and D is the diameter of the element. The f-number is used to characterize the light gathering power of the optical system. The relation of f-number with another important optical concept, Numerical Aperture (NA), is that: f/# = 1/(2×NA), where the numerical aperture of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light.