The Astronomical Telescope of the University of Stuttgart (ATUS): Development, Optimization, and Lessons Learned

TL;DR

The paper documents the ATUS telescope, a remote 0.6 m Ritchey-Chrétien instrument originally developed as a SOFIA testbed and subsequently optimized for high-cadence, time-domain astronomy. It details iterative mechanical redesigns (OTA and mount), wavefront sensing collaboration (SHIFT), stray-light mitigation, and a custom off-axis guider that together yield diffraction-limited imaging and sub-arcminute pointing accuracy. The work demonstrates sub-microsecond time-referenced, high-cadence photometry enabling stellar occultations, exoplanet transits, and space-surveillance tasks, while highlighting the importance of robust software, reliable hardware, and precise calibration in field conditions. The lessons span OTA design, system architecture, and collimation procedures, offering actionable guidance for planning and deploying similar time-domain telescopes at new sites or for future remote platforms.

Abstract

ATUS, the Astronomical Telescope of the University of Stuttgart, is a fully remote-controlled 0.6 m f/8.17 Ritchey-Chrétien telescope optimized for high-cadence, high-fidelity photometry of transient sources. Observations are time-referenced with very high accuracy and precision, making it an ideal platform for time-domain astronomy and space situational awareness. Initially conceived to support instrument developments and operations of SOFIA, the Stratospheric Observatory for Infrared Astronomy, it evolved into a scientific instrument for various use cases in instrument development, astronomical research, and teaching. This paper presents an overview of its development and optimization to achieve diffraction-limited images and highly accurate pointing and tracking, even at high speeds. The findings and lessons learned are universally applicable to other telescopes that are currently at the planning stage, or where similar issues might be encountered.

Figures (29)

• Figure 1: ATUS during initial commissioning (left) and in its final configuration (right) with a completely redesigned telescope, a shorter and much stiffer counterweight assembly, an optimized weight distribution to lower the setup's moment of inertia, and a custom off-axis guider (see discussions in Section \ref{['sec:optimization']}). The Wide Field Imager (WFI) is mounted on the west side of the telescope tube and thus hidden; it is pictured in Figure \ref{['fig:WFI+PDU']}.
• Figure 2: Modified polar fork assembly of the 3600GTOPE mount. Stiffness increased significantly by using thicker, solid side plates, an additional cross brace, and a thicker, solid base.
• Figure 3: Evolution of the ATUS telescope: "Mark I" (Mk I, top, installed at the end of September 2013) and "Mark II" (Mk II, bottom, installed in May 2015). The finite-element analysis (FEA) illustrates the flexure of the telescope structure under its own weight while pointing at the horizon, exaggerated many times over. Lateral misalignment and tilting of the mirrors relative to each other were minimized. FEA plots were included with kind permission of Officina Stellare.
• Figure 4: Pointing offsets of the Mark-I and Mark-II OTA due to gravity-induced flexure, measured after mirror cell modifications in the respective OTA design (revision "b"). Images were acquired along the meridian in 2 steps in declination; once in 2014 for the Mk I.b, and three times in 2022 and 2024 for the Mk II.b. The shaded areas represent the confidence interval (CI) and the prediction interval (PI) of the respective function fits. See text for more information.
• Figure 5: Average FWHM values derived from Gaussian-profile PSF fits of images taken at various elevations during pointing model runs in February 2014 (Mk I.b, n=103, $\geq 30\degr$ el., $t_\mathrm{exp} = 15~\mathrm{s}$), August 2015 (Mk II.a, n=166, $\geq 20\degr$ el., $t_\mathrm{exp} = 12~\mathrm{s}$), and September 2024 (Mk II.b, n=265, $\geq 20\degr$ el., $t_\mathrm{exp} = 12~\mathrm{s}$). The histograms on the right quantify the site seeing monitor readings during the time of data acquisition, indicating very good to near-ideal conditions. See text for details.