The STIX Aspect System (SAS): The Optical Aspect System of the Spectrometer/Telescope for Imaging X-Rays (STIX) on Solar Orbiter
Strassmeier, K. G.; Krucker, S.; Mann, G.; Anderson, J.; Casadei, D.; Grimm, O.; Schuller, F.; Sablowski, D. P.; Csillaghy, A.; Önel, H.; Warmuth, A.; Orleanski, P.; Denker, C.; Hurford, G. J.; Rendtel, J.; Bauer, S. -M.; Bittner, W.; Dionies, F.; Paschke, J.; Plüschke, D.; Senthamizh Pavai, V.; Woche, M.; Kögl, S.; Arnold, N. G.; Gröbelbauer, H. -P.; Schori, D.; Wiehl, H. J.; Skup, K. R.; Bujwan, W.; Rutkowski, K.; Ber, K.
Germany, Switzerland, United States, Poland
Abstract
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.