Optomechanical design of a broadband cryogenic optical system for ground testing of the Ariel payload

Abstract

ESA’s Ariel space telescope is due to launch in 2029 and aims to measure broadband (0.5 – 7.8 µm) spectra from ∼1000 known transiting exoplanets. For optimum stability the payload uses an all aluminium telescope with an extremely narrow (30 ′′) field of view. Prior to integration with the spacecraft, the Ariel payload will undergo a TVAC campaign to check that the telescope and instruments are capable of achieving the mission’s scientific goals. During these tests the payload will be illuminated by a quarter aperture test beam generated by a highly stable and adaptable cryogenic optical system – the payload optical ground support equipment (OGSE). This OGSE is required to output a 275 mm x 183 mm elliptical collimated beam with an RMS wavefront error (WFE) of < 214 nm to enable diffraction limited imaging in Ariel’s instruments. Furthermore this must be achieved after the collimation and beam expansion optics are cooled to an operating temperature of 70 K. In this proceeding, the optical design, specifications and simulations of the OGSE cryogenic optical system are presented. This system is designed to be athermal between room temperature and 70 K while also producing a low WFE output by using a combination of glass and aluminium mirrors. To prevent stressing and deforming the optics during cooldown, a selection of low distortion and minimally constraining mounts have been designed and simulated. The alignment and verification philosophy of the optical system is also presented. Emphasis is placed on precision mechanical alignment with only a minimal number of adjustable degrees of freedom required to focus and optimise the system. Following alignment, the end-to-end optical quality of the OGSE is verified in a single-pass configuration with a Shack-Hartmann based metrology system.