Type IV aberration corrected flat field and imaging gratings are designed to focus a spectrum onto a plane surface, making them ideal for use with linear or 2-D array detectors. These gratings are produced with grooves that are neither equispaced nor parallel, and are computer optimized to form near-perfect images of the entrance slit on the detector plane.
Owing to their large optical numerical aperture and correction from aberrations, these Type IV aberration corrected flat field & imaging
gratings provide much better light collection efficiency and signal to noise ratio than traditional Type I Rowland circle concave gratings.
When an area detector such as a CCD is utilized, it is often possible to focus multiple sources onto the entrance slit and independently evaluate the spectrum from each source. These “Imaging Gratings” are nearly free from astigmatism, and therefore only one fixed optical element is required to construct an imaging spectrograph.