According to modern cosmology, most galaxies are surrounded by faint, extended halos of light called LSB structures. These subtle features are remnants of past galactic events—such as collisions, mergers, and tidal interactions—and hold important clues to galactic evolution.
However, LSB features are extremely faint, often dimmer than the night sky itself, making them difficult to capture. Traditional telescopes face challenges such as stray light, sky brightness gradients, and light scattering, which blur faint details. Deep LSB imaging therefore requires an optical design that provides a wide, unobscured field of view, fast light collection, and minimal stray light, combined with specialized observing and calibration techniques.
To overcome these challenges, a new study published in the Journal of Astronomical Telescopes, Instruments, and Systems introduced a linear-astigmatism-free three-mirror system (LAF-TMS), known as the Korea Astronomy and Space Science Institute (KASI) Deep Rolling Imaging Fast Telescope (K-DRIFT). "Unlike traditional on-axis optical designs, off-axis unobscured designs reduce light loss, stray light, and the effect of the extended wings of the point spread function. The K-DRIFT design also eliminates linear astigmatism, a major issue in typical off-axis systems, and minimizes higher-order aberrations with its three freeform mirrors," said author Gayoung Lee of KASI.
The optical design of K-DRIFT pathfinder features a 300-millimeter aperture confocal off-axis system with three freeform mirrors. Specifically, a freeform elliptical convex secondary mirror, termed M2, shares its focal point with both a freeform elliptical concave primary mirror M1 and a freeform elliptical concave tertiary mirror, M3. This setup effectively reduces stray light and scattering, producing sharper images. The tilt angles of the three mirrors eliminate linear astigmatism, and the use of three freeform mirrors minimizes higher order aberrations. The telescope uses a CMOS camera for detection.
The mirrors were made from Zerodur, a glass-ceramic material resistant to thermal deformation, and mounted on an aluminum housing with invar flexures that reduce mechanical stress. This setup minimizes mirror surface distortion and light scattering. The mirrors were aligned and integrated step-by-step using a coordinate-measuring machine. To further reduce stray light, a secondary baffle was placed in front of the detector.
For performance evaluation, the K-DRIFT pathfinder was installed at the Bohyunsan Optical Astronomy Observatory (BOAO) for on-sky testing from June 2021 to April 2022. The telescope maintained consistent imaging performance across seasonal temperature changes but initially did not meet the required resolution target—measured as the full width at half maximum (FWHM) of the point spread function (PSF).
Through a series of optical simulations, the researchers identified three main error sources: mirror fabrication errors, opto-mechanical mirror mounting errors, and optical misalignment errors. Based on this analysis, the researchers addressed the errors by replacing mirror M2 and refining the alignment process during the final assembly of the housing. As a result, K-DRIFT's performance significantly improved, with PSF FWHM decreasing from 3.8 pixels to 1.8 pixels.
"The K-DRIFT pathfinder proves that compact, freeform mirror designs can achieve the precision needed to study the faintest structures in the universe like LSB structures. In future, this project will help trace the hidden history of how galaxies formed and evolved," Lee said.
Overall, the K-DRIFT pathfinder marks a significant step forward in deep LSB imaging, paving the way for uncovering the faintest structures in the universe.
For details, see the Gold Open Access article by Lee et al., " Assessment of the on-sky performance of an off-axis freeform three-mirror telescope ," J. Astron. Telesc. Instrum. Syst. 11(4) 048002 2025, doi: 10.1117/1.JATIS.11.4.048002
