A 30-cm objective grating for far-UV astronomy: theoretical study and laboratory tests

We have performed theoretical determination and experimental calibrations of an objective grating designed for high resolution spectroscopy of astronomical faint sources in the EUV and far-UV wavelength ranges (500-1400 A). First through theoretical calculations we show the feasibility of the concept with an aspheric shape for the grating blank and determine its geometrical parameters. A grating of this large size has been manufactured and tested, associated with a photon counting detector, in a vacuum environment. Finally we demonstrate that a resolving power of 3 x 10(4), a total equivalent effective area of ~5-10 cm(2) can be achieved, together with a very low scattered light level (10(-4)-10(-5) of the peak value).