CubeSpec optical payload

CubeSpec is an ESA in-orbit-demonstration mission, based on a 12-unit CubeSat, that targets high-resolution astronomical spectroscopy. It is developed and funded in Belgium and scheduled for launch early 2026. The main parts of CubeSpec’s optical payload are an off-axis Cassegrain telescope, a high-resolution spectrograph, and a fine-guidance system. The primary mirror of the telescope has a rectangular shape, matched to the CubeSat form factor to maximise the aperture, covering approximately the surface area of two CubeSat units. The prism cross-dispersed echelle spectrograph delivers a spectral resolution of R = 55 000 and covers the wavelength range from 420 to 620 nm in a single exposure. A fine steering mirror and a fine-guidance sensor provide accurate centring of the source image on the spectrograph slit to compensate for spacecraft pointing errors and jitter. Fitting these sub-systems inside the available payload volume (slightly more than 6U or half the spacecraft volume) made their optical design a technological challenge. To accommodate the large temperature excursions endured by the spacecraft during each orbit, the opto-mechanical design of both the telescope and the spectrograph is highly athermalised. In this contribution, we present the complete optical design as well as crucial aspects of the opto-mechanical design of the CubeSpec payload.