Internal checkup illumination sources for METIS coronagraph on solar orbiter

METIS is one of the remote sensing instrument on the Solar Orbiter mission [1]. It will acquire coronal images from distances from the Sun as close as 0.28 AU. The mission innovations rely not only in the spacecraft orbit [2]; METIS introduces many technical breakthroughs in the optical layout and in many other areas, mainly the inverted external occulter and the visible light (VL) polarimeter[3,4]. In order to check the proper functioning of each optical subsystem, an “internal checkup system” has been conceived and proposed. It is based on the acquisition of the visible light emitted from three LEDs positioned inside the telescope tube that illuminate the rear part of the entrance door of the instrument. The door itself acts as a diffuser that illuminates the optics and finally the detector. The monitoring of the intensity variations on the acquired images gives a simple and reliable operating feedback on the uniformity response of the whole optical chain as a function of time. The optical geometry of the system is presented. In particular, a ray-tracing model for the estimation of the signal at the focal plane has been developed. The results of the simulations are presented. The main requirements for the LED have been identified. A commercially available space-qualified LED model, suitable for the requests, has been identified. The power budget – the signal at the focal plane compared to the electrical power – is detailed. Furthermore, functionality tests on the identified LED have been started. The quantities that have been monitored are the optical power curve, the emitted spectrum, the emission angular distribution, the efficiency. All the parameters have been monitored at different temperatures, in the range, 20- 80 ˚C. The results of this characterization are presented.