The optical design for the Ultraviolet Explorer (UVEX) mission: a next-generation wide-field UV telescope

The Ultraviolet Explore (UXEX) mission is a wide-field two band imager with a long multi-width slit spectrometer to study the dynamic ultraviolet universe. The UVEX mission will provide an all-sky survey in the ultraviolet to complement Rubin, Euclid, and Roman operating in the optical/infrared. The telescope will be capable of deep imaging and spectroscopic follow-up within hours of an event for study of targets of opportunity. The telescope optical design provides a continuous 5.0°×3.5° Field of View (FoV) that achieves an rms spot radius of <10 μm. The optical design is a compact unobstructed freeform f/2.7 Three Mirror Anastigmat (TMA) with a 75 cm virtual entrance pupil. The all-reflective design is driven to support imaging into the far UV down to 115 nm. The telescope provides light to three instrument channels: a NUV imager (200 - 260 nm), a FUV imager (130 - 190 nm), and a Long-Slit Spectrometer (LSS) operating over the entire wavelength range (115 – 265 nm). The spectrometer is an all-reflective re-imager based on an Offner-Chrisp design where the secondary convex mirror is the dispersing element. The spectrometer has a 2° long-slit length with three discrete slit widths of 2′′, 4′′, and 8′′ wide. With a 2′′ slit width, the spectrometer has a spectral resolving power of 1400 - 3200 from 115 - 265 nm wavelength range.