Eugene D. Kim

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.

Alignment methods for Cassegrain and RC telescope with wide field of view

Ritchey-Chretien type or Cassegrain type has been widely used for earth-observing space camera. As most earth-observing cameras are required to scan the wide area in a single path, they generally have a wider field of view, compared to the ground-based telescope. However, the alignment of Cassegrain or RC telescope with a wide-field of view is not easy. One reason is that it has a central hole in the primary mirror so that it is difficult to find an optical axis. Another reason is that it can introduce much off-axis aberration such as coma and astigmatism, when it is aligned at on-axis with zero-coma condition. In this paper, we calculate the alignment accuracy using a conventional method for a RC type telescope of which diameter is 300 mm and field of view is 2.08 degrees. We suggest that the most effective alignment method for wide field of view system is a computer-aided alignment. With this method, it was found that the variation of rms wavefront error of the telescope over the entire field of view was less than 10 %.

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.

Spatial Characterization of MAC, a High-Resolution Optical Earth Observation Camera for Small Satellites

Spatial calibrations have been performed on the Medium-sized Aperture Camera (MAC) of the RazakSAT satellite. Topics discussed in this paper include the measurements of system modulation transfer function (MTF), relative pixel line-of-sight (LOS), and end-to-end imaging tests. The MTF measurements were made by capturing the scanned knife-edge image on a pixel, and an issue in the MTF calculation algorithm is discussed. The method used to place the focal plane at the correct focal position is described, since they make use of MTF measurements. Relative LOS measurements are done by theodolite measurements of the telescope. Qualitative ground test result of end-to-end imaging is given.

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.

Sub-half meter imaging satellite, SpaceEye-X

The demand on very high-resolution Earth observation data has steadily increased and the satellite image market has shown substantial growth in past decades. Recently, many private enterprises have formed or plan to form constellations of small satellites to serve various needs. This paper describes the key features of an imaging satellite with resolution better than 0.5 m, SpaceEye-X, which is under development in Satrec Initiative based on qualified designs. Its advanced features allow timely delivery of high-quality image data, high agility, and high accuracy for demanding operations. The development status and future plan are also summarized.

Pre-flight performance characterization of RazakSAT medium-sized aperture camera (MAC)

Pre-flight performance has been characterized on the Medium-sized Aperture Camera (MAC) of the RazakSAT: capable of Earth observation at 2.5 m resolution and 20 km swath width. Topics discussed in this paper include measurements of system modulation transfer function (MTF) and pixel lines-of-sight (LOS); characterization of focal plane assembly (FPA) and signal processing electronics; end-to-end imaging. The MTF was obtained with knife-edge scanning technique, which is also used to align the FPA. For band-to-band registration, relative pixel LOS was measured using theodolite and effective focal length of the telescope was derived from the measurement. For the FPA and signal processing module, dark reference, pixel-to-pixel response variation and response linearity have been quantified. The end-to-end imaging tests were done to check the imaging function before the launch, by scanning a slide target at the focus of the collimator.

Development of Small-Sized Aperture Camera (SAC) for high- performance imaging on small satellites

SAC is a compact camera for imaging in visible-NIR spectral ranges. SAC provides high-resolution images over the wide geometric and spectral ranges: 10 m GSD and 50 km swath-width in the spectral ranges of 520 ~ 890 nm. The missions incorporate various imaging operations: multi-spectral imaging; super swath-width imaging with cameras in parallel; along-track stereo imaging with slanted 2 cameras. In this paper, SAC is introduced with design and performance. Though developed for small satellites, presenting development status and test results will demonstrate the potential capability for worldwide remote sensing groups: short development period, cost-effectiveness, and high performance.