Compact prism spectrographs based on aplanatic principles

We describe a new form of prism spectrograph system based on aplanatic principles. The basic system is simple, comprising a prism with two spherical refracting surfaces, both operating near their aplanatic conjugates, and a spherical mirror operating near its center of curvature. This form provides a flat, accessible focal surface suitable for use with modern array detectors. Good image quality can be maintained over large wavelength intervals at fast focal ratios, making this form particularly useful for moderate resolution spectrography. Its simplicity, compactness, and tolerance of misalignment make it attractive for space and cryogenic instruments. We present three examples of operating instruments that have been constructed using this new form.