Myung-Seok Kang

Reverse-optimization Alignment Algorithm using Zernike Sensitivity

When aligning catoptric or catadioptric telescopes for space cameras, it is difficult to align precisely if the field of view is large or there are several reflective surfaces. The quantitative knowledge of mirror misalignments greatly helps align a misaligned telescope precisely, and also reduce the alignment time. This paper describes a generalized reverse-optimization alignment solution algorithm using Zernike sensitivity, and proposes the minimum number of fields to take interferograms. This method was successfully applied on a Cassegrain telescope design for Earth observation from space with arbitrary misalignments and a model including some primary mirror deformation.

Medium-sized aperture camera for Earth observation

Satrec Initiative and ATSB have been developing a medium-sized aperture camera (MAC) for an earth observation payload on a small satellite. Developed as a push-broom type high-resolution camera, the camera has one panchromatic and four multispectral channels. The panchromatic channel has 2.5m, and multispectral channels have 5m of ground sampling distances at a nominal altitude of 685km. The 300mm-aperture Cassegrain telescope contains two aspheric mirrors and two spherical correction lenses. With a philosophy of building a simple and cost-effective camera, the mirrors incorporate no light-weighting, and the linear CCDs are mounted on a single PCB with no beam splitters. MAC is the main payload of RazakSAT to be launched in 2005. RazakSAT is a 180kg satellite including MAC, designed to provide high-resolution imagery of 20km swath width on a near equatorial orbit (NEqO). The mission objective is to demonstrate the capability of a high-resolution remote sensing satellite system on a near equatorial orbit. This paper describes the overview of the MAC and RarakSAT programmes, and presents the current development status of MAC focusing on key optical aspects of Qualification Model.

Medium-sized aperture camera for Earth observation from space

Medium-sized Aperture Camera (MAC) for earth observation on a small satellite is being developed by Satrec Initiative and ATSB. Designed as a cost-effective high-resolution camera, this push-broom type camera has 1 panchromatic and 4 multispectral channels using all-CCDs-in-one focal plane, and it does not split the channels by prisms. The panchromatic channel has 2.5m, and multispectral channels have 5m of ground sampling distance at a nominal altitude of 685km. The 300mm modified Ritchey-Chretien telescope contains two aspheric mirrors and two spherical correction lenses. MAC is the main payload of RazakSAT (formerly known as MACSAT) to be launched in 2005. RazakSAT is a 180kg (including MAC) small satellite, designed to provide high-resolution imagery of 20km swath width on a near equatorial orbit (NEqO). The mission objective is to demonstrate the capability of a high-resolution small remote sensing satellite system on a near equatorial orbit. This paper describes the status report on the development of the MAC Qualification Model and technical issues.

Optical alignment of a high-resolution optical earth observation camera for small satellites

Spaceborne earth observation or astronomical payloads often use Cassegrain-type telescopes due to the limits in mass and volume. Precision optical alignment of such a telescope is vital to the success of the mission. This paper describes the simulated optical alignment methods using interferograms, wavefront error, and reverse-optimization method for different levels of alignment accuracy. It concludes with the alignment experiment results of a Cassegrain type spaceborne camera with 300mm entrance pupil diameter.

Development and Verification of Thermal Control Subsystem for High Resolution Electro-Optical Camera System, EOS-D Ver.1.0

Satrec Initiative successfully developed and verified a high-resolution electro-optical camera system, EOS-D Ver.1.0. We designed this system to give improved spatial and radiometric resolution compared with EOS-C series systems. The thermal control subsystem (TCS) of the EOS-D Ver.1.0 uses heaters to meet the opto-mechanical requirements during in-orbit operation and uses different thermal coatings and multi-layer insulation (MLI) blankets to minimize the heater power consumption. Also, we designed and verified a refocusing mechanism to compensate the misalignment caused by moisture desorption from the metering structure. We verified the design margin and workmanship by conducting the qualification level thermal vacuum test. We also performed the verification of thermal math model (TMM) by comparing with thermal balance test results. As a result, we concluded that it faithfully represents the thermal characteristics of the EOS-D Ver.1.0.

Integration and Testing of Optical Imaging System for RASAT

An advanced imaging system, the optical imaging system (OIS) is being developed for a small Earth observation satellite, RASAT. OIS will produce high-resolution images in panchromatic and multi-spectral bands and it consists of one optical and one electronics units. Recently, we have completed the development of the OIS engineering/qualification model (EQM). As part of verification of EQM, functional and environmental tests were performed. Key system features of OIS and test result for EQM are summarized.

Integration and testing of a high-resolution camera for small satellites

The Medium-sized Aperture Camera system is a high-resolution imaging sensor developed for small satellites. The development of its flight model was recently completed. During its integration, testing, and characterization, we measured the modulation transfer function, the response linearity of detectors and electronics, and the line-of-sight difference between spectral bands. As part of the final functional test, we performed an end-to-end imaging test for the complete system. The methodology used in the integration, testing and characterization of the flight model is presented in this paper along with the results.

Development of Cassegrain type 0.9-m collimator

Collimator is essential to evaluate and assemble the other telescopes. Its diameter should be larger than that of the target telescope for the correct use. We are currently developing the Cassegrain type collimator of which diameter is 0.9 m. The primary mirror is light-weighted so that its weight is only 70 kg. Due to its structure, the primary mirror can be supported only at the backside of the mirror. This mirror is tested with the combination of null Hartmann test and interferometer. The secondary mirror is tested with a Hindle method. This method requires 600 mm high quality spherical mirror. The distance between the primary and secondary mirror is maintained by the Carbon composite material. The assembly of two mirrors is carried out by the computer aided alignment method. The whole structure is designed to maintain the performance of the collimator under +/-5 degrees of temperature variation.

Computer-Aided Alignment of an Earth Observation Camera

Spaceborne earth observation or astronomical payloads often use Cassegrain-type telescopes due to limits in mass and volume. Precision optical alignment of such a telescope is vital to the success of the mission. This paper describes the alignment simulation and experiment of computer-aided alignment method during the assembly of MAC (Medium-sized Aperture Camera) telescope for spaceborne earth observation.

Development of in-orbit refocusing mechanism for SpaceEye-1 electro-optical payload

SpaceEye-1 earth observation satellite, developed by Satrec Initiative Co. Ltd., is a 300 kg scale spacecraft with high resolution electro-optical payload (EOS-D) which performs 1 m GSD, 12 km swath in low earth orbit. Metering structure of EOS-D is manufactured with Carbon Fiber Reinforced Plastic (CFRP). Due to the moisture emission from CFRP metering structure, this spaceborne electro-optical payload undergoes shrinkage after orbit insertion. The shrinkage of metering structure causes change of the distance between primary and secondary mirror. In order to compensate the moisture shrinkage effect, two types of thermal refocusing mechanism were developed, analyzed and applied to EOS-D. Thermal analysis simulating in-orbit thermal condition and thermo-elastic displacement analysis was conducted to calculate the performance of refocusing mechanism. For each EOS-D telescope, analytical refocusing range (displacement change between primary and secondary mirror) was 2.5 um and 3.6 um. Thus, the refocusing mechanism can compensate the dimensional instability of metering structure caused by moisture emission. Furthermore, modal, static and wavefront error analysis was conducted in order to evaluate natural frequency, structural stability and optical performance. As a result, it can be concluded that the refocusing system of EOS-D payload can perform its function in orbit.