2020, Warmuth, A., Önel, H., Mann, G., Rendtel, J., Strassmeier, K.G., Denker, C., Hurford, G.J., Krucker, S., Anderson, J., Bauer, S.-M., Bittner, W., Dionies, F., Paschke, J., Plüschke, D., Sablowski, D.P., Schuller, F., Senthamizh Pavai, V., Woche, M., Casadei, D., Kögl, S., Arnold, N.G., Gröbelbauer, H.-P., Schori, D., Wiehl, H.J., Csillaghy, A., Grimm, O., Orleanski, P., Skup, K.R., Bujwan, W., Rutkowski, K., Ber, K.

The Spectrometer/Telescope for Imaging X-rays (STIX) is a remote sensing instrument on Solar Orbiter that observes the hard X-ray bremsstrahlung emission of solar flares. This paper describes the STIX Aspect System (SAS), a subunit that measures the pointing of STIX relative to the Sun with a precision of ±4′′, which is required to accurately localize the reconstructed X-ray images on the Sun. The operating principle of the SAS is based on an optical lens that images the Sun onto a plate that is perforated by small apertures arranged in a cross-shaped configuration of four radial arms. The light passing through the apertures of each arm is detected by a photodiode. Variations of spacecraft pointing and of distance from the Sun cause the solar image to move over different apertures, leading to a modulation of the measured lightcurves. These signals are used by ground analysis to calculate the locations of the solar limb, and hence the pointing of the telescope.