crossdispersion

Crossdispersion is an optical arrangement used in spectroscopy to separate spectra produced by a primary disperser that would otherwise overlap when recorded with a single detector. In a cross-dispersed spectrograph, light first encounters a high-dispersion element, typically an echelle grating, which spreads light into many high-order spectra along one axis. A second disperser—the cross-disperser—is positioned nearly perpendicular to the first and disperses the light along the orthogonal axis. Common cross-dispersers are prisms or low-dispersion gratings. The result is a two-dimensional spectrum projected onto a detector, usually a charge-coupled device, wherein each order appears as a distinct stripe and different wavelengths occupy different positions along both axes. This configuration enables simultaneous access to a broad wavelength range while preserving high spectral resolution.

Cross-dispersion is especially associated with echelle spectrographs used in astronomy and laboratory spectroscopy. By separating orders, it minimizes overlap and allows high resolving power (R typically 10,000–100,000 in many instruments) across a wide band, from the near-UV to near-infrared, depending on design. The method also requires careful calibration and data reduction because the wavelength solution varies with position and order; two-dimensional order tracing, flat-fielding, and order-merging steps are employed to extract one-dimensional spectra.

Trade-offs include throughput losses at crossing surfaces, complexity of alignment, and the need for precise calibration. Cross-dispersion remains a standard technique in modern high-resolution spectroscopy, facilitating detailed composition measurements of astronomical objects, laboratory species, and planetary atmospheres.