Far ultraviolet scatter performance from holographic gratings

We present deep scatter measurements on a sinusoidal profile, high groove density, holographically ruled, large format grating. With long exposures and a low-noise detector, we were able to measure scatter at levels of 1 * 10(-6)/Ly alpha/nm at 100 nm. The result has potential impacts in the prospect of high sensitivity measurements of very faint objects at FUV wavelengths. Any instrument sensitive to hydrogen Lyman alpha, a bright geo-coronal airglow line, must control the scatter to make astrophysically significant measurements. Measurements of very faint emission line source - particularly ones that do not have known redshifts (i.e. wavelength solution), Lyman alpha scatter may be the dominant source of error. Therefore, characterization of the actual value of scatter is crucial to the success of future missions that will use diffraction-based measures for detecting very faint targets.