J. Eur. Opt. Society-Rapid Publ. 2026, 22, 35 Ó The Author( s), published by EDP Sciences, 2026 https:// doi. org / 10.1051 / jeos / 2026035 Available online at: https:// jeos. edpsciences. org
EOSAM 2025 Guest editors: Omar El Gawhary, Stefan Witte, Ignacio Moreno
Journal of the European Optical Society-Rapid Publications
RESEARCH ARTICLE
Large resolution-weighted étendue space telescope designs categorized by tertiary mirror location diversity
Hyejeon Cho 1, 2, Hyosun Park 1, M. James Jee 1, 3, and Daewook Kim 4, 5, 6,*
1 Department of Astronomy, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea 2 Center for Galaxy Evolution Research, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea 3 Department of Physics and Astronomy, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA 4 Wyant College of Optical Sciences, University of Arizona, 1630 E. University Blvd., Tucson, AZ 85721, USA 5 Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA 6 Large Binocular Telescope Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
Received 21 January 2026 / Accepted 31 March 2026
Abstract. Modern observational astronomy makes extensive use of deep, wide-field imaging and large spectroscopic surveys, defining telescope design spaces in which angular resolution, achievable depth, and survey efficiency are considered jointly rather than independently. Conventional étendue alone does not fully capture the efficiency with which resolved and deblended information can be acquired. We introduce a resolution-weighted étendue metric that provides a quantitative basis for comparing information throughput across different facilities. Applying this metric, we illustrate how accounting for angular resolution reshapes the relative placement of existing telescopes in design parameter space, with a representative space telescope design serving as an example of this shift. We consider two distinct reflective space telescope designs utilizing a variation of the three-mirror anastigmat configuration. The strategic placement of the tertiary mirror significantly influences the telescope’ s form factor, enabling compact architectures for very large aperture systems. These designs achieve large resolution-weighted étendue and imaging throughput, making them well-suited for precision cosmological and astrophysical measurements.
Keywords: Resolution-weighted étendue, Space telescope design, Tertiary mirror location, Astronomical image processing.
1 Introduction
Recent decadal surveys and community planning documents in astronomy and astrophysics show that current and future observational studies make extensive use of deep and wide-field imaging and large spectroscopic surveys across a broad range of wavelengths( e. g., [ 1, 2 ]). These approaches underpin precision cosmology, statistically robust studies of star and galaxy formation and evolution, time-domain astronomy, exoplanet detection and characterization, and related topics in astrophysics and fundamental physics. Achieving these science goals requires observations that deliver statistically powerful source samples together with precise, stable, and well-calibrated measurements, thereby constraining the parameter space for telescope design and optimization.
Ground-based wide-field facilities, such as the Subaru Telescope Hyper Suprime-Cam( HSC) [ 3 ] and the Vera C.
* Corresponding author: dkim @ optics. arizona. edu
Rubin Observatory Legacy Survey of Space and Time Camera( Rubin / LSST) [ 4 ], are optimized for large étendue— a combination of wide Field of View( FoV) and large photon collecting area— to enable efficient surveys over very large angular scales. Using a uniform, simplified definition of effective collecting area, their étendue values are of order tens to hundreds of m 2 deg 2. Space-based facilities occupy complementary regions of observational parameter space by providing stable, diffraction-limited performance. The James Webb Space Telescope( JWST) [ 5, 6 ], for example, prioritizes sensitivity and angular resolution over a relatively small FoV, yielding étendue values 1 m 2 deg 2, whereas survey-oriented missions such as Euclid [ 7 ] and the Roman Space Telescope [ 8 ] target wide-area coverage at intermediate étendue.
Advances in launch-vehicle and spacecraft technologies have reduced space access costs and development barriers for small satellite missions [ 9 ]. Science cases at far-infrared wavelengths further illustrate how space telescope architectures can be driven toward large apertures, deployable structures, and active control [ 10 ]. These developments
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