Flat field spectrograph using convex holographic diffraction grating and concave mirror

In this paper we have discussed the aberration properties and the design procedure of a spectrograph which uses a convex holographic grating as the dispersing element and a concave mirror as the focusing element. Both the concave mirror and the convex grating have common axis and they are concentric. The grating is constructed by recording the interference fringes on the convex surface which are formed by two convergent light beams incident from opposite sides of the axis. The illuminating source of the spectrograph is located on a plane perpendicular to the axis and passing through the common centre. It has been found that under certain conditions both positive and negative order spectra are perfectly focused on this plane. Aberration properties of the zero order image as well as the positive and the negative order spectral images have been studied by actual ray tracing. Using this system, design parameters of a medium-sized spectrograph having moderate dispersion and good resolution throughout theuv-visible region of the spectrum have been specified. The performance of the spectrograph has been evaluated by plotting spot diagram. The authors felicitate Prof. D S Kothari on his eightieth birthday and dedicate this paper to him on this occasion.