THE EVALUATION OF A DEFORMABLE DIFFRACTION GRATING FOR A STIGMATIC EUV SPECTROHELIOMETER Progress Report for NASA Grant NAGW-540 for the period

A high-efficiency, extreme ultraviolet (EUV) imaging spectrometer has been constructed and tested. The spectrometer employs a concave toroidal grating illuminated at normal incidence in a Rowland circle mounting and has only one reflecting surface. The toroidal grating has been fabricated by a new technique employing an elastically-deformable sub-master grating which is replicated in a spherical form and then mechanically distorted to produce the desired aspect ratio of the toroidal surface for stigmatic imaging over the selected wavelength range. The fixed toroidal grating used in the spectrometer is then replicated from this surface. Photographic tests and initial photoelectric tests with a twodimensional, pulse-counting detector system have verified the image quality of the toroidal grating at wavelengths near 600 A. The results of these initial tests are described in detail and the basic designs of two instruments which could employ the imaging spectrometer for astrophysical investigations in space are briefly described; namely, a high-resolution EUV spectroheliometer for studies of the solar chromosphere, transition region, and corona; and an E W spectroscopic telescope for studies of non-solar objects. The extreme ultraviolet (EUV) spectral region at wavelengths MOW 1200 A is of critical importance for the diagnostics of conditions in high-temperature plasmas since this region contains emission lines and continua characteristic of temperatures in the range from about 104 OK to greater than 106 OK. This spectral region is thus particularly useful for studies of dynamic phenomena in the outer atmosphere of the sun and other stars. In addition, observations at these wavelengths can provide unique astrophysical data on dynamic phenomena in planetary atmospheres and magnetospheres, on the exchange of matter and energy between stars and the interstellar medium, on the variations of light element abundances in the galaxy and on the dynamics of the hot interstellar gas in galactic haloes. Because of the lack of window materials and of high-reflectivity optical surfaces at these wavelengths, practical systems for imaging and spectroscopy must employ openstructure detector systems and a minimum of reflecting surfaces. The recent development of high-resolution imaging detector systems which can operate with high efficiency at E W wavelengths has stimulated the design of optical systems which can provide a stigmatic spectroscopic capability. These imaging spectrometer systems can provide the simultaneous spectral, spatial and temporal resolutions required for many scientific observations that have not been previously possible. In this paper we describe the fabrication and the results of initial tests of a toroidal diffraction grating for use in an imaging E W spectrometer at wavelengths between 300 and 1200 A. L SPECIRO METER DESIGN The design of the spectrometer was stimulated initially by the need for high-timeresolution observations of the solar chromosphere, transition region, and corona with a high spectral resolution (h/A,h 2 x 104) and a spatial resolution of 1 arc sec or better.l The basic configuration of the spectrometer mounting is shown in Figure la. In the conventional Rowland circle mounting, the vertical and horizontal foci from a spherical diffraction grating are not coincident. However, Habe? has pointed out that it is possible to obtain exact stigmatic focusing at two points in the spectrum by selecting the