Comparative study of optical designs of ultraviolet spectrometers onboard spaceborne telescopes

We present a comparative study of optical designs of ultraviolet (UV) spectrometers onboard spaceborne telescopes. Such spectrometers provide unique findings for the space sciences that cannot be obtained via ground-based observation due to the atmospheric opacity in this wavelength region. Instruments for observing in the UV wavelength range should have as few reflective surfaces as possible to maintain a high throughput. For instance, the Hisaki spectrometer uses simple optics, consisting of only one toroidal grating. However, in this case, the spot qualities produced by optics onto detectors have room for improvement. In this study, we present several optical designs with improved spot qualities by adding one or a few reflective surfaces, and compare them. We also discuss optical designs that is potentially useful for the proposed Life-environmentology, Astronomy, and Planetary Ultraviolet Telescope Assembly (LAPYUTA) mission.