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The ISIS blue and red cameras are of a folded Schmidt design, with a focal length of 500 mm giving a scale of 14.9 arcsec/mm along the slit. The reduction factor along the spectrum is dependent upon the grating angle, and is presented graphically for the ISIS grating set in Appendix D. The air-glass surfaces of the refracting elements of the two cameras are coated with anti-reflection coatings optimised for the respective wavelength ranges, and the reflecting elements are coated with silver plus a reflective stack for the red arm, and aluminium for the blue arm.