Optical Tolerance analyses for the Far-ultraviolet Imaging Spectrograph onboard KAISTSAT-4 Kwang-Il Seon 1 , Kwang-Sun Ryu 2 , Eric Korpela 3 , In-Soo Yuk 1 , Uk-Won Nam 1 , Wonyong Han 1 , Jong-Ho Seon 2 , Kyoung-Wook Min 2 , and Jerry Edelstein 3 1 Korea Astronomy Observatory, 61-1 Whaam-dong Yusong-gu Taejon, 305-348, Korea 2 Satellite Technology Reseach Center, KAIST , 373-1 Kusong-dong Yusong-gu Taejon, 305-701, Korea 3 Space Sciences Lab., Univ. of California, Berkeley, California 94720-7450, U.S.A. The Far-ultraviolet IMaging Spectrograph (FIMS) is a spectrograph optimized for the observations of diffuse emissions in far-ultraviolet wavebands. FIMS is the main payload of first Korean scientific satellite, KAISTSAT-4, which will be launched in 2002. Extensive tolerance study of FIMS optical system has been performed, including decentering and tilt of the optical elements and manufacturing figure errors of parabolic-cylinder collecting mirror and ellipsoidal grating. We describe the tolerance analyses and its results. 1. Introduction Figure 1. FIMS schematic diagram FIMS is a dual (short wavelength band: 900– 1150Å, long wavelength band: 1335–1750Å) imaging spectrograph, optimized for faint diffuse radiation by employing an off-axis parabolic cylinder mirror in front of a slit that guides lights to a diffraction grating. The reflective grating is an ellipse of rotation providing angular resolution. The FIMS design is derived from the two flight-proven EURD instruments 1) . The cylindrical-source method provides twice the grasp and field of standard spectrographs, even with fast (f/2.2) optics. The imaging performance allows for a large field with imaging resolution (arc minute scales) similar to other important interstellar all-sky surveys. The schematic diagram of the FIMS optics and the optical specifications are shown in figure 1 and table 1, respectively. Table 1. FIMS Optical Specifications Parameters Short Wave Band Long Wave band Band Pass 900–1150Å 1335–1750Å FOV 4°×5′ 8°×5′ Spectral Resolution 1.4Å @ 1035Å 2.2Å @ 1550Å Spatial Resolution 5′–10′ 5′–10′ Grasp 0.6×10 -4 cm 2 sr 1.25×10 -4 cm 2 sr Mirror Figure Off-axis Parabolic Cylinder Mirror Focal Length 125 mm (f/2.2) Slit to Grating 177.4 mm Grating to Detector 170.0 mm Grating Figure Ellipse of Rotation (z/A) 2 +(x/A) 2 +(y/C) 2 =1 Ellipse Axis A 180.0 mm Ellipse Axis C 242.6 mm Ruling Constant G 2250/mm 3000/mm Diffraction Order -2 -1 Mirror Coating B 4 C MgF 2 Grating Coating B 4 C MgF 2 Photocathode KBr CsI + Grid Fixed Filter MgF 2 CaF 2