The fifth Sloan Digital Sky Survey Local Volume Mapper (LVM) is a wide-field integral field unit survey that uses an array of four 160 mm fixed telescopes with siderostats to minimize the number of moving parts. An individual telescope observes the science or calibration field independently and is synchronized with the science exposure. We developed the LVM Acquisition and Guiding Package (LVMAGP)-optimized telescope control software program for LVM observations, which can simultaneously control four focusers, three K-mirrors, one fiber selector, four mounts (siderostats), and seven guide cameras. This software is built on a hierarchical architecture and the SDSS framework and provides three key sequences: autofocus, field acquisition, and autoguide. We designed and fabricated a proto-model siderostat to test the telescope pointing model and LVMAGP software. The mirrors of the proto-model were designed as an isogrid open-back type, which reduced the weight by 46% and enabled reaching thermal equilibrium quickly. In addition, deflection due to bolting torque, self-gravity, and thermal deformation was simulated, and the maximum scatter of the pointing model induced by the tilt of optomechanics was predicted to be 4′.4, which can be compensated for by the field acquisition sequence. We performed a real sky test of LVMAGP with the proto-model siderostat and obtained field acquisition and autoguide accuracies of 0″.38 and 1″.5, respectively. It met all requirements except for the autoguide specification, which will be resolved by more precise alignment among the hardware components at Las Campanas Observatory.
Optical design of the wide field of view telescope is limited by obscuration of the secondary mirror in onaxis system and by serious linear astigmatism in conventional common off-axis system. We have developed an innovative off-axis reflective system in which the optical design is based on the confocal off-axis to completely compensate the linear astigmatism. The main objective of this paper is to compare alignment sensitivity of the confocal system to those of on-axis and common off-axis systems. All three optical designs are based on the classical Cassegrain reflecting telescope and have identical entrance pupil diameter of 1000 mm and F/8 focal ratio. Tilt and decenter of each optical component, and despace which indicates inter-mirror distance are considered as tolerancing parameters that are explored within fabrication tolerance ranges. Encircled energy diameter is evaluated as a criterion of the analysis while tilts of the secondary mirror and the focal position are set to the compensator. The statistical tolerancing method based on Monte-Carlo simulation is also performed to analyze system tolerances. From sensitivity analysis and Monte-Carlo simulation, we concluded that the confocal off-axis system is more sensitive than on-axis and common off-axis systems but it is a feasible system in terms of fabrication and alignment errors.
Linear Astigmatism Free - Three Mirror System (LAF-TMS) is a confocal off-axis system that eliminates linear astigmatism, which is the most critical aberration especially in the large field angle, and therefore, enables the telescope to have a wide field of view. Based on our experience with the telescope, we optimized the LAF-TMS for wavelength ranges of mid-wavelength infrared (3-5 µm) and long-wavelength infrared (8-12 µm) sensors onboard Unmanned Aerial Vehicles (UAVs). It has an entrance pupil diameter of 70 mm, a focal ratio of 1.4, and a wide field of view (FoV) of 6.20° × 4.68°, matching 10.9 mm × 8.2 mm sensor with 17 µ m sized pixels (LAF-TMS D70F1.4). The freeform mirrors of LAF- TMS D70F1.4 are optimized to eliminate the high order aberration. As a result, LAF-TMS D70F1.4 can achieve high- quality optical performance over a wide FoV without any additional correcting lenses. We performed the sensitivity analysis and the Monte-Carlo simulations as the feasibility study. During the sensitivity analysis and the Monte-Carlo simulation, decenter, tilt, despace, and surface RMS errors of three mirrors were analyzed. From the sensitivity analysis, we investigated 80% Energy Encircled Diameter by single factor perturbations. The system tolerance limits were calculated using the Monte-Carlo method with a normal distribution of errors. According to the results, we confirmed that the LAF-TMS D70F1.4 was feasible considering general fabrication and alignment tolerances.
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