CubeSpec: optical payload

CubeSpec is an ESA in-orbit-demonstration mission, based on a 12U CubeSat, targeting high-resolution optical astronomical spectroscopy of bright targets. It is developed and funded in Belgium and scheduled for launch early 2026. The CubeSpec payload consists of an off-axis Cassegrain telescope with a rectangular aperture filling the surface area of two CubeSat units, followed by a prism cross-dispersed echelle spectrograph folded behind the primary mirror of the telescope. The complete optical payload fits in approximately 6 units (∼12 x 20 x 30 cm) of the spacecraft. CubeSpec delivers a spectral resolution of R = 55 000 and covers the wavelength range from 420 to 620 nm. The optical design is sufficiently flexible to allow tuning it with minimum hardware changes to a wide range of spectral resolution and coverage. A fine-guidance system consisting of a piezo-actuated fine steering mirror and a fine-guidance sensor provide arcsec-precise centering of the source image on the slit of the spectrograph, cancelling out pointing errors and spacecraft jitter. In this contribution, we describe the optical and optomechanical design of the CubeSpec payload, and discuss the challenged imposed by the extremely compact size and the large temperature excursions endured during each orbit.