J. Eur. Opt. Society-Rapid Publ. 22, 35( 2026) 351
Figure
2. Comparisons of FoV, angular resolution, and PSF properties.( a) Subaru / SC image of the galaxy cluster Abell 2744 with the JWST / NIRCam [ 20 ] and Kim TMS Type-I FoV footprints overlaid. Insets show the same region observed with Subaru / SC and JWST / NIRCam, as well as in a“ denoised” JWST image reconstructed using an efficient-Transformer network [ 21 ], approximating a deeper observation.( b) PSFs for Subaru / SC and JWST / NIRCam at the same angular scale and in logarithmic intensity.( c) Whisker plot of PSF ellipticity e and position angle h across a JWST / NIRCam mosaic. Whiskers derived from the“ denoised” image, shifted by a few pixels for clarity, retain the same pattern as the original, indicating that the field-dependent PSF variations remain even after image denoising.
E �1
A XFoV XPSF; k
RW; k ¼
: ð2Þ
1m 2 1 deg 2 1 arcsec 2
Here, X PSF, k isexpressedinarcsec 2, as commonly used in the literature for reporting PSF or beam sizes in arcseconds.
The RWE becomes especially important for space telescopes capable of achieving diffraction-limited performance over large FoVs. Figure 1 presents both standard and resolution-weighted étendues for two distinct comparison groups: optical / near-IR ground- and space-based facilities( filled symbols) and far-IR space-based facilities( open symbols). Under the conventional étendue definition, wide-field ground-based survey telescopes occupy the large-étendue regime, whereas space telescopes— and the next generation of extremely large-aperture telescopes( e. g., the European Extremely Large Telescope; E-ELT [ 15, 16 ]) as a notable exception that achieves near diffraction-limited performance from the ground despite its limited FoV— cluster at values of order unity or below. When diffraction-limited PSFs are taken into account through the RWE, this comparison is reframed, placing these high-resolution facilities within— and in some cases extending beyond— the region of design and optimization parameter space occupied by wide-field ground-based telescopes. The Kim Three-Mirror System( TMS) Type-I space telescope [ 17 ] and the ground-based E-ELT provide a representative example of this shift. Its large aperture and diffraction-limited PSF compensate for the relatively smaller FoV, yielding RWE values comparable to those of ground-based wide-field survey facilities such as Rubin / LSST.
In Figure 1, X PSF, k values for each facility are based on the delivered FWHM at representative spectral bands as defined above. For JWST, we utilize data from NIRCam F070W( 0.70 lm; [ 5 ]) and MIRI F2550W( 25.5 lm; [ 18 ]) to align with the nominal wavelengths of the Kim TMS Type-I and Type-II configurations discussed in Section 3, respectively. For Kim TMS Type-I and Type-II, the PSF FWHM is evaluated at the detector focal plane based on the reported nominal optical performance [ 17, 19 ]. In the absence of measured on-orbit jitter data, these values serve as the best-available estimate for the delivered PSF, providing a rigorous yet realistic benchmark for the RWE comparison. Minor variations in detector sampling or other systemlevel disturbances are treated as independent factors and do not affect the qualitative conclusions of this comparison.
The very large RWE of Kim TMS Type-I / II and E-ELT translates into increased survey depth, while providing a higher degree of resolved detail per pointing. This combination enables more efficient source detection within limited observing time and supports uniform sky coverage. Realizing these benefits, however, requires stringent control of temporal and spatial variations of the PSF across the