Seunghyuk Chang

FOCAL REDUCER FOR CQUEAN (Camera for QUasars in EArly uNiverse)

A focal reducer is developed for CQUEAN (Camera for QUasars in EArly uNiverse), which is a CCD imaging system on the 2.1 m Otto Struve telescope at the McDonald observatory. It allows CQUEAN to secure a wider field of view by reducing the effective focal length by a factor of three. The optical point spread function without seeing effects is designed to be within one pixel (0.28300´´) over the field of view of 4.820´ × 4.820´ in optimum wavelength ranges of 0.8 ? 1.1 μm. In this paper, we describe and discuss the characteristics of optical design, the lens and barrel fabrications and the alignment processes. The observation results show that the image quality of the focal reducer confirms the expectations from the design.

AUTO-GUIDING SYSTEM FOR CQUEAN (CAMERA FOR QUASARS IN EARLY UNIVERSE)

To perform imaging observations of optically red objects such as high redshift quasars and brown dwarfs, the Center for the Exploration of the Origin of the Universe (CEOU) recently developed an optical CCD camera, Camera for QUasars in EArly uNiverse (CQUEAN), which is sensitive at 0.7-1.1 ㎛. To enable observations with long exposures, we develop an auto-guiding system for CQUEAN. This system consists of an off-axis mirror, a baffle, a CCD camera, a motor and a differential decelerator. To increase the number of available guiding stars, we design a rotating mechanism for the off-axis guiding camera. The guiding field can be scanned along the 10 arcmin ring offset from the optical axis of the telescope. Combined with the auto-guiding software of the McDonald Observatory, we confirm that a stable image can be obtained with an exposure time as long as 1200 seconds.

Elimination of linear astigmatism in off-axis three-mirror telescope and its applications

A simple closed-form equation for the elimination of linear astigmatism in off-axis three-mirror telescope and imaging system is presented. Several practical design examples of telescope and auxiliary optics based on the equation are also presented and compared to the similar designs reported previously.

PROTO-MODEL OF AN INFRARED WIDE-FIELD OFF-AXIS TELESCOPE

We develop a proto-model of an ofi-axis re∞ective telescope for infrared wide-fleld observations based on the design of Schwarzschild-Chang type telescope. With only two mirrors, this design achieves an entrance pupil diameter of 50 mm and an efiective focal length of 100 mm. We can apply this design to a mid-infrared telescope with a fleld of view of 8 ‐ £ 8 ‐ . In spite of the substantial advantages of ofi-axis telescopes in the infrared compared to refractive or on-axis re∞ective telescopes, it is known to be di‐cult to align the mirrors in ofi-axis systems because of their asymmetric structures. Ofi-axis mirrors of our telescope are manufactured at the Korea Basic Science Institute (KBSI). We analyze the fabricated mirror surfaces by fltting polynomial functions to the measured data. We accomplish alignment of this two-mirror ofi-axis system using a ray tracing method. A simple imaging test is performed to compare a pinhole image with a simulated prediction.

Linear astigmatism of confocal off-axis reflective imaging systems and its elimination

Linear astigmatism of a confocal off-axis reflective imaging system when the object plane is tilted and located at a finite distance from the imaging system is derived. We show that linear astigmatism can be eliminated by proper configuration of the parent mirror axes in confocal off-axis two-mirror systems. The tilt angle of the image plane is also derived. The developed theory is verified by ray-tracing analysis of an example system.

Off-axis reflecting telescope with axially-symmetric optical property and its applications

The basic concept and fundamental result of a recently developed geometric aberration theory for classical off-axis reflecting telescopes and imaging systems are presented. It is shown that a classical off-axis reflecting telescope can be designed to have practically axially-symmetric optical property by eliminating the dominant aberration (linear astigmatism) caused by the asymmetric geometry. A simple closed-form equation for elimination of linear astigmatism is presented. Also, to show how the developed aberration theory can be applied to current and future telescopes, several off-axis reflecting telescopes and imaging systems are designed and analyzed.

Fabrication of electroless nickel plated aluminum freeform mirror for an infrared off-axis telescope

Freeform mirrors can be readily fabricated by a single point diamond turning (SPDT) machine. However, this machining process often leaves mid-frequency errors (MFEs) that generate undesirable diffraction effects and stray light. In this work, we propose a novel thin electroless nickel plating procedure to remove MFE on freeform surfaces. The proposed procedure has a distinct advantage over a typical thick plating method in that the machining process can be endlessly repeated until the designed mirror surface is obtained. This is of great importance because the sophisticated surface of a freeform mirror cannot be optimized by a typical SPDT machining process, which can be repeated only several times before the limited thickness of the nickel plating is consumed. We will also describe the baking process of a plated mirror to improve the hardness of the mirror surface, which is crucial for minimizing the degradation of that mirror surface that occurs during the polishing process. During the whole proposed process, the changes in surface figures and textures are monitored and cross checked by two different types of measurements, as well as by an interference pattern test. The experimental results indicate that the proposed thin electroless nickel plating procedure is very simple but powerful for removing MFEs on freeform mirror surfaces.