Far-ultraviolet filters for the ISTP UV imager

The far ultraviolet (FUV) imager for the International Solar-Terrestrial Physics (ISTP) mission is designed to image four features of the aurora: 0 1 lines at 130.4 nm and 135.6 nm and the N2 Lyman-Birge-Hopfield (LBH) bands between 140 nm -160 nm (LBH long) and 160 nm 180 nm (LBH long). We report the design and fabrication of narrow-band and broadband filters for the ISTP FUV imager. Narrow-band filters designed and fabricated for the 0 I lines have a bandwidth of less than 5 nm and a peak transmittance of 22.3% and 29.6% at 130.4 nm and 135.6 nm, respectively. Broadband filters designed and fabricated for LBH bands have the transmittance greater than 40% for LBH short and close to 60% for LBH long. Blocking of outof-band wavelengths for all filters is better than 0.001% with the transmittance at 121.6 nm of less than 10-6%.