The STIX Aspect System (SAS): The Optical Aspect System of the Spectrometer/Telescope for Imaging X-Rays (STIX) on Solar Orbiter
A. Warmuth, Hakan Önel, G. Mann, J. Rendtel, K. G. Strassmeier, C. Denker, G. J. Hurford, Säm Krucker, J. M. Anderson, Svend‐Marian Bauer, Wilbert Bittner, Frank Dionies, J. Paschke, D. Plüschke, D. P. Sablowski, F. Schüller, V. Senthamizh Pavai, M. Woche, D. Casadei, S. Kögl, Nicolas Arnold, Hans-Peter Gröbelbauer, D. Schori, H. J. Wiehl, A. Csillaghy, O. Grimm, P. Orleański, Konrad Skup, Waldemar Bujwan, K. Rutkowski, Kamil Ber
Abstract
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 $\pm 4''$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>±</mml:mo> <mml:msup> <mml:mn>4</mml:mn> <mml:mo>″</mml:mo> </mml:msup> </mml:math> , 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.