Sug-Whan Kim

Galaxy evolution explorer ultraviolet color-magnitude relations and evidence of recent star formation in early-type galaxies

We have used the Galaxy Evolution Explorer UV photometric data to construct a first near-UV (NUV) color-magnitude relation (CMR) for the galaxies preclassified as early-type by Sloan Digital Sky Survey studies. The NUV CMR is a powerful tool for tracking the recent star formation history in early-type galaxies, owing to its high sensitivity to the presence of young stellar populations. Our NUV CMR for UV-weak galaxies shows a well-defined slope and thus will be useful for interpreting the rest-frame NUV data of distant galaxies and studying their star formation history. Compared to optical CMRs, the NUV CMR shows a substantially larger scatter, which we interpret as evidence of recent star formation activities. Roughly 15% of the recent epoch (z < 0.13) bright [M(r) < -22] early-type galaxies show a sign of recent (1 Gyr) star formation at the 1%-2% level (lower limit) in mass compared to the total stellar mass. This implies that low-level residual star formation was common during the last few billion years even in bright early-type galaxies.

Static tool influence function for fabrication simulation of hexagonal mirror segments for extremely large telescopes

We present a novel simulation technique that offers efficient mass fabrication strategies for 2m class hexagonal mirror segments of extremely large telescopes. As the first of two studies in series, we establish the theoretical basis of the tool influence function (TIF) for precessing tool polishing simulation for non-rotating workpieces. These theoretical TIFs were then used to confirm the reproducibility of the material removal foot-prints (measured TIFs) of the bulged precessing tooling reported elsewhere. This is followed by the reverse-computation technique that traces, employing the simplex search method, the real polishing pressure from the empirical TIF. The technical details, together with the results and implications described here, provide the theoretical tool for material removal essential to the successful polishing simulation which will be reported in the second study.

The galaxy evolution explorer

The Galaxy Evolution Explorer (GALEX), a NASA Small Explorer Mission planned for launch in Fall 2002, will perform the first Space Ultraviolet sky survey. Five imaging surveys in each of two bands (1350-1750Å and 1750-2800Å) will range from an all-sky survey (limit mAB~20-21) to an ultra-deep survey of 4 square degrees (limit mAB~26). Three spectroscopic grism surveys (R=100-300) will be performed with various depths (mAB~20-25) and sky coverage (100 to 2 square degrees) over the 1350-2800Å band. The instrument includes a 50 cm modified Ritchey-Chrétien telescope, a dichroic beam splitter and astigmatism corrector, two large sealed tube microchannel plate detectors to simultaneously cover the two bands and the 1.2 degree field of view. A rotating wheel provides either imaging or grism spectroscopy with transmitting optics. We will use the measured UV properties of local galaxies, along with corollary observations, to calibrate the UV-global star formation rate relationship in galaxies. We will apply this calibration to distant galaxies discovered in the deep imaging and spectroscopic surveys to map the history of star formation in the universe over the red shift range zero to two. The GALEX mission will include an Associate Investigator program for additional observations and supporting data analysis. This will support a wide variety of investigations made possible by the first UV sky survey.

Precessions aspheric polishing: new results from the development program

The Precessions process for producing aspheric and other optical surfaces is undergoing rapid development. In this paper, we summarise the considerable success achieved in controlling the repeatability of the process on both the 200mm and 600mm machines, and illustrate this with examples of aspherics that have been produced. We particularly describe our approach to fine form-control. This has required the development of various strategies to moderate the volumetric removal rates, in order to give the required sensitivity of removal. We conclude with a discussion of the scaling laws that apply when adapting the process to smaller and larger sized parts. This is illustrated by predicting the process-parameters for mass-producing segments for extremely large telescopes.

Merit function regression method for efficient alignment control of two-mirror optical systems

The precision alignment of high-performance, wide-field optical systems is generally a difficult and often laborious process. We report a new merit function regression method that has the potential to bring to such an optical alignment process higher efficiency and accuracy than the conventional sensitivity table method. The technique uses actively damped least square algorithm to minimize the Zernike coefficient-based merit function representing the difference between the designed and misaligned optical wave fronts. The application of this method for the alignment experiment of a Cassegrain type collimator of 900mm in diameter resulted in a reduction of the mean system rms wave-front error from 0.283 lambda to 0.194 lambda;, and in the field dependent wave-front error difference from +/-0.2 lambda to +/-0.014 lambda in just two alignment actions. These results demonstrate a much better performance than that of the conventional sensitivity table method simulated for the same steps of experimental alignment.

MEMS micromirror characterization in space environments

This paper describes MEMS micromirror characterization in space environments associated with our space applications in earth observation from the International Space Station and earths orbit satellite. The performance of the micromirror was tested for shock and vibration, stiction, outgassing from depressurization and heating, and electrostatic charging effects. We demonstrated that there is no degradation of the micromirror performance after the space environment tests. A test bed instrument equipped with the micromirrors was delivered and tested in the ISS. The results demonstrate that the proposed micromirrors are suitable for optical space systems.

Three-shell-based lens barrel for the effective athermalization of an IR optical system

We have developed a new IR optical system that consists of three mirrors and four lenses, and that operates in the temperature range 8°C-32°C. This temperature range can induce thermoelastic deformation in the lenses and their mounting subassembly, leading to a large defocus error associated with the displacement of the lenses inside the barrel. We suggest using a new three-shell-based athermalization structure composed of two materials with different coefficients of thermal expansion (Invar and aluminum). A finite element analysis and the experiment data were used to confirm that this new athermalization barrel had a defocus error sensitivity of 11.6 nm/°C; this is an improvement on the widely used conventional single-shell titanium barrel model, which has a defocus error sensitivity of 29.8 nm/°C. This paper provides the technical details of the new athermalization design, and its computational and experimental performance results.

Computer-guided alignment II :Optical system alignment using differential wavefront sampling.

We present a differential wavefront sampling method for the efficient alignment of centred optical systems. Using the inter-element effects reported in our previous study, this method generates a linear symmetric matrix that relates the optical wavefront to misalignments within the system. The solution vector of this matrix equation provides a unique description of decentre and tilt misalignments of the system. We give a comparison of this approach to the existing method in the first case study and then illustrate characteristics of the new approach using the subsequent four case studies and Monte-Carlo alignment simulations. The results reveal superiority of the method over the existing one in misalignment estimation accuracy and demonstrate the practical feasibility and robustness.

Computer-guided alignment I : Phase and amplitude modulation of alignment-influenced optical wavefront

As the first part of a development programme on computer-guided alignment (CGA), we model the alignment influence on the optical wavefront in terms of the phase and amplitude modulation. This modulation is derived from the interaction between alignment parameters and influence functions, both expressed in complex form. The alignment influence model is used to approximate the ray-traced target wavefront of a randomly mis-aligned multi-element system. The approximated wavefront shows a factor of ~ 100 improvement in predicting the target, when coupled non-linear influences among elements are included. This demonstrates the significance of the inter-element effect. We discuss the possibility of adopting the model for rectifying mis-alignment of multi-element systems.

Novel laser datum system for nanometric profilometry for large optical surfaces.

We report a new laser datum system for precision point-by-point profilometry of large curved optical surfaces. The laser datum is sensed by a nulling quadrant photodiode mounted in a flexural system with hybrid actuators, which also carries interferometer reference optics for vertical and horizontal displacement measurement. The flexure characteristics such as cross-talk and hysteresis were investigated. The optimum environmental conditions for the active position-control were studied, and closed-loop control was modeled. The experimental results for compensation accuracy showed a repeatability of +/- 4 nm rms, the compensation accuracy of 10 nm (vertical channel) and 20 nm (horizontal channel).