Low-surface-brightness (LSB) structures play a crucial role in understanding galaxy evolution by providing significant insights into galaxy interactions, the histories of mass assembly, and the distribution of dark matter. Nevertheless, their inherently faint nature, coupled with observational difficulties such as stray light interference and variations in the sky background, has significantly impeded comprehensive studies of LSB features. The KASI Deep Rolling Imaging Fast Telescope (K-DRIFT) project aims to address these observational challenges by developing off-axis freeform three-mirror telescopes and observational strategies specifically designed for LSB imaging surveys. The first generation of K-DRIFT (G1) has been successfully completed, and the forthcoming survey, scheduled to commence shortly, is expected to yield novel insights into the LSB universe. This paper outlines the scientific motivations of the project, discusses the technical challenges encountered, highlights the innovative solutions devised, and describes the future trajectory of the K-DRIFT.

We have developed the KASI Deep Rolling Imaging Fast Telescope (K-DRIFT) pathfinder, which utilizes a 300-mm aperture confocal off-axis freeform three-mirror system to study extremely faint stellar structures resulting from gravitational interactions between galaxies. The adoption of an off-axis unobscured system and freeform three-mirror design in the K-DRIFT pathfinder helps to minimize stray light and blur size in the image plane while also ensuring a wide field of view with consistent point spread functions. Through on-sky test observations at the Bohyunsan Optical Astronomy Observatory and a series of optical simulations, we have identified three primary factors contributing to optical performance degradation: (1) mirror fabrication errors, (2) opto-mechanical mirror mounting errors, and (3) optical misalignment errors. Misalignments among the freeform mirrors led to significant image spread, whereas mirror fabrication and opto-mechanical mirror mounting errors resulted in specific scattering patterns. By addressing these factors, we have successfully evaluated and enhanced the on-sky performance of the K-DRIFT pathfinder. We present the observational results of the K-DRIFT pathfinder and the methods employed to improve its optical performance. In addition, we discuss the accuracy and supplementary aspects of our optical performance analysis algorithm.

We propose a spiral-phase-contrast imaging system for space optics by integrating a linear astigmatism-free three-mirror system (LAF-TMS) with a spiral phase plate (SPP) in a 4f configuration. The LAF-TMS enables unobscured, off-axis reflective imaging while completely eliminating linear astigmatism across a wide field of view, ensuring robust optical performance suitable for compact space imaging. To enhance phase contrast, we implemented an SPP with a flat central region that maintains a constant phase, generating distinct spiral interference fringes via self-referenced interferometry. These fringes were utilized to reconstruct the 3D topography of a lunar surface-inspired microscale structure, known as the "fairy castle," which models the fragile, porous features typical of the Moon. Simulations confirmed accurate 3D reconstruction with a root mean square error in the sub-micron range. A Monte Carlo-based tolerance analysis further demonstrated that the required alignment precision is achievable with standard optomechanical components, supporting the system's feasibility for in-situ implementation in future space missions.

Polarization crosstalk and linear astigmatism are two principal factors that impair the optical performance of unobscured off-axis reflective systems owing to the absence of axial symmetry. In systems comprising two conic mirrors, polarization crosstalk can be minimized when the linear astigmatism-free (LAF) condition is satisfied. This study extends the polarization analysis and theoretically demonstrates that polarization crosstalk can be concurrently minimized with the LAF condition in systems with an arbitrary number of conic mirrors. A case study indicates that a system satisfying the LAF condition exhibits optical characteristics identical to those of the on-axis counterpart from the perspectives of both geometrical and physical optics.

We present a microscopic spiral-phase-contrast (SPC) imaging system that incorporates a linear astigmatism-free three-mirror system (LAF-TMS), an unobscured reflective optical configuration designed to ensure high optical performance across a wide field of view by eliminating linear astigmatism through an off-axis optical design, based on a 4f system configuration. In this system, a spiral phase plate with a flattened central region is employed to enable spiral interferometry, which is a kind of SPC imaging. The spiral interferometry is a form of self-referencing interferometry that does not require a separate additional reference beam path. It arises from the interference between the higher-order diffracted and the undiffracted zero-order beam from the sample within a single optical arm path. The fringe pattern obtained from spiral interferometry enables quantitative three-dimensional structural analysis by deriving height variations of the observed object from the optical path difference gradient. The lunar surface microstructure known as the "fairy castle" is characterized by weak cohesive strength, which causes it to collapse during the sampling process. Therefore, in-situ observation is required, yet it has not been directly observed. As an optical system for observing and performing three-dimensional structural analysis of the "fairy castle," we designed an SPC imaging system based on the LAF-TMS optical design. We performed optical simulations to analyze its performance and assess its operational feasibility. Additionally, we carried out a tolerance analysis assuming actual manufacture and assembly to confirm the overall feasibility of the system.

The optical design and the tolerance analysis of a 400mm-aperture, f/3, three-mirror freeform telescope for the MESSIER surveyor mission is presented. Due to its linear-astigmatism- free characteristic, the telescope provides a wide field of view. Also, the tolerance analysis indicates the manufacturing and the alignment of the telescope is achievable.

Our linear astigmatism-free confocal off-axis collimator comprises two off-axis mirrors and one flat mirror with an aperture size of 73 mm and a focal length of 1200 mm. The off-axis mirrors, along with all other opto-mechanical parts, will be fabricated from the same material, such as aluminum alloy 6061-T6. This concept inherently creates an athermalized structure, meaning the entire system expands or contracts by the same amount as the thermal coefficient of expansion, ensuring that the image remains consistently in focus. We approached this collimator's design as if it were an astronomical telescope, reversing the optical path directions, and conducted tolerance analysis using Optic Studio (ZEMAX) to define the opto-mechanical design requirements. The collimator's target or telescope imaging sensor size is 4 x 4 mm, with a required imaging resolution of 13.7 cycles/mm at a wavelength of 750 nm. To achieve this, we divided the full field (4 x 4 mm) into 3 x 3 subfields, ensuring that the average Modulation Transfer Function (MTF) value exceeds 10%. We performed Monte-Carlo Simulations 5000 times to determine tolerance ranges with a 90% confidence level. Furthermore, we conducted stray light analysis for our off-axis collimator design. In comparison to typical on-axis Cassegrain designs, where baffles block some parts of the target rays and reduce intensity, our designed confocal off-axis collimator accommodates baffles without obstructing any light from the target.

We analyze the stray light characteristics of the Korea Astronomy and Space Science Institute (KASI) Deep Rolling Imaging Fast Telescope (K-DRIFT) using non-sequential raytracing, identify causes, and propose optimal baffle-and-vain design updates to improve the scientific observation performance of the K-DRIFT. (tel: +82 402.416.6191, email: jihun@ksai.re.kr ).

We are under integrating off-axis freeform mirrors for the KASI Deep Rolling Imaging Fast Telescope Generation 1 (K-DRIFT G1) using a coordinate measuring machine and assembly jig. The telescope is a confocal off-axis freeform three-mirror system designed for the detection of extremely low surface brightness structures in the sky. The optical specifications of the K-DRIFT G1 are as follows: the entrance pupil diameter is 300 mm, the focal ratio is 3.5, and the field of view is 4.43 degrees x 4.43 degrees. During the integration stage, we used a coordinate measuring machine to measure the positions of the mirrors, flexures, and bezels within a tolerance range. Following the system integration, we will measure wavefront errors at several edge fields using an interferometer at 633 nm. In this paper, we briefly present the current status of the K-DRIFT G1 and the future plans for the project.

The conventional on-axis reflective systems suffer from a diffraction effect on the Point Spread Function (PSF) due to the secondary mirror obscuration. Meanwhile, the unobscured off-axis reflective systems’ imaging performance may be impacted by linear astigmatism aberration. The Linear Astigmatism Free-Three Mirror System (LAF-TMS) is a confocal off-axis reflective system that eliminates linear astigmatism and enables a wide Field of View (FoV). We present an enhanced design of LAF-TMS, called ”wide-wide”, which has an aperture of D=40mm, an effective focal length of f=75mm, and a wide FoV of 8.25°(Horizontal) × 6.21°(V ertical) combined with a wide spectral bandwidth capability suitable for Unmanned Aerial Vehicle (UAV) applications. To evaluate the performance of this compact and fast optical system design, we use the Photon Simulator (PhoSim) to model physically accurate PSF under different conditions of the mirror surface, mechanical environment, and atmosphere. As a benchmark, we compare and analyze the PhoSim PSF results with other ray tracing software such as Zemax and CodeV. Additionally, PhoSim is capable of simulating infrared spectral imaging cases with a user-defined Spectral Energy Distribution (SED), intensity, and emissivity of each pixel. The comprehensive simulation results demonstrate the high performance of the LAF-TMS with a wide-wide FoV and multispectral capabilities.

We are developing the KASI-Deep Rolling Imaging Fast Telescope Generation 1 (K-DRIFT G1) based on the on-site performance assessment of the K-DRIFT pathfinder. The telescope is a confocal off-axis freeform three-mirror system designed for the detection of extremely low surface brightness structures in the sky. The optical specifications of the K-DRIFT G1 are as follows: the entrance pupil diameter is 300 mm, the focal ratio is 3.5, the field of view is 4.43° × 4.43°, and the image area is 81.2 mm × 81.2 mm with 10 μm pixels. We performed sensitivity analysis and tolerance simulations to integrate and align the system. We present the analysis results and development plan of the K-DRIFT G1.

We have developed the KASI-Deep Rolling Imaging Fast Telescope (K-DRIFT), adopting a 300 mm aperture off-axis freeform three-mirror design to detect faint and diffuse low-surface-brightness structures. By conducting the on-sky test observations and performing a series of simulations to analyze the performance of the K-DRIFT, we confirmed three main error sources causing optical performance degradation. The imaging performance of the K-DRIFT has successfully improved by correcting low-to-mid spatial frequency wavefront errors based on performance analysis results. This paper presents the K-DRIFT’s optical performance analysis algorithm and the optical performance improvement.