Broadband chromatic spherical aberration correction in refracting telescopes: a dual-correction strategy combining spherical aberration equalization and an aspheric surface compensation

Traditional apochromatic refracting telescopes only achieve spherical aberration correction at a single wavelength and exhibit significant wavelength-dependent spherical aberration (spherochromatism), which severely constrains their broadband imaging performance in high-precision astrometric applications and wide-field surveys. We propose a dual-correction strategy integrating chromatic aberration equalization and aspheric compensation, achieving diffraction-limited imaging significantly better than the traditional apochromat. Specifically, a 6-in. triplet refractor designed using this method achieves an on-axis Strehl ratio greater than 99% across the spectral range from the g-line to the r-line, whereas the 6-in. quadruplet refractor achieves an on-axis Strehl ratio greater than 99.95% from the ultraviolet i-line to the near-infrared t-line. Astrometric accuracy simulation shows that this design enables sub-pixel centroiding (<15 mas) for 16th-magnitude stars. The designs support the application of wide-field (with short focal length) and multiband solar observation and can also eliminate manufacturing error accumulation caused by wavelength mismatch between testing and operational bands in conventional designs.