The optical system of the entire mechanical and optical components consist of all silicon carbide (SiC) is designed, manufactured and aligned. The Korsch type Cassegrain optical system has 3-mirrors, the primary mirror (M1), the secondary mirror (M2), the folding mirror (FM) and the tertiary mirror (M3). To assemble the M3 and the FM to the rear side of the M1 bench, the optical axis of the M3 is 65.56 mm off from the physical center. Due to the limitation of the mass budget, the M3 is truncated excluding its optical axis. The M2 was assigned to the coma compensator and the M3 the astigmatism respectively as per the result of the sensitivity analysis. Despite of the difficulty of placing these optical components in their initial position within the mechanical tolerance, the initial wave front error (WFE) performance is as large as 171.4 nm RMS. After the initial alignment, the sensitivity table method is used to reach the goal of WFE 63.3 nm RMS in all fields. We finished the alignment with the final WFE performance in all fields are as large as 55.18 nm RMS.
Today, CVD SiC mirrors are readily available in the market. However, it is well known to the community that the key surface fabrication processes and, in particular, the material removal characteristics of the CVD SiC mirror surface varies sensitively depending on the shop floor polishing and figuring variables. We investigated the material removal characteristics of CVD SiC mirror surfaces using a new and patented polishing tool called orthogonal velocity tool (OVT) that employs two orthogonal velocity fields generated simultaneously during polishing and figuring machine runs. We built an in-house OVT machine and its operating principle allows for generation of pseudo Gaussian shapes of material removal from the target surface. The shapes are very similar to the tool influence functions (TIFs) of other polishing machine such as IRP series polishing machines from Zeeko. Using two CVD SiC mirrors of 150 mm in diameter and flat surface, we ran trial material removal experiments over the machine run parameter ranges from 12.901 to 25.867 psi in pressure, 0.086 m/sec to 0.147 m/sec in tool linear velocity, and 5 to 15 sec in dwell time. An in-house developed data analysis program was used to obtain a number of Gaussian shaped TIFs and the resulting material removal coefficient varies from 3.35 to 9.46 um/psi hour m/sec with the mean value to 5.90 ± 1.26(standard deviation). We report the technical details of the new OVT machine, of the data analysis program, of the experiments and the results together with the implications to the future development of the OVT machine and process for large CVD SiC mirror surfaces.
Silicon carbide (SiC) is the best material for various spaceborne telescope mirrors and structural pieces since it is high stiffness, low thermal expansion, high thermal conductivity and fine surface optical quality. This paper describes the optimum all-SiC telescope trade for a SiC primary mirror with a predesigned lightweight configuration on the back and a metering structure. A trade study involves the utilization of finite element structural analysis in evaluating thermal as well as static and dynamic loadings. The paper presented herein describes the trade studies involved in the optimum design of lightweighting cell patterns, selection of mounting method, facesheet/rib thickness and mass for a primary mirror as well as selection of supporting method, for a metering structure with respect to static deflections, dynamic characteristics and thermal aspects.
An airborne sensor is developed for remote sensing on an unmanned aerial vehicle (UAV). The sensor is an optical
payload for an eletro-optical/infrared (EO/IR) dual band camera that combines visible and IR imaging capabilities in a
compact and lightweight manner. It adopts a Ritchey-Chrétien telescope for the common front end optics with several
relay optics that divide and deliver EO and IR bands to a charge-coupled-device (CCD) and an IR detector, respectively.
For the easy assemble of such a complicated optics, a computer-aided alignment program (herein called simulator) is
developed. The simulator first estimates the details of the misalignments such as locations, types, and amounts from the
test results such as modulation transfer function (MTF), Zernike polynomial coefficients, and RMS wavefront errors at
different field positions. Then it recommends the compensator movement(s) with the estimated optical performance. The
simulator is coded on Matlab with the hidden connection to optical analysis/design software Zemax. By interfacing
ZEMAX and MATLAB, the GUI-based alignment simulator, will help even those not familiar with the two programs to
obtain accurate results more easily and quickly.
The spaceborne telescope assembly, which incorporates the primary mirror and secondary mirrors, baseplate, and metering structure, must retain the alignment after launch and during operation in space. A trend in spaceborne telescopes has been an increase in collection area and in resolution. Both of these demands in performance require larger primary mirror aperture sizes. Since the primary mirror often dominates the mass budget of the telescope, either of these options imply larger mass for the overall system. Technologies that enable lighter primary mirror will enable lower mass telescopes with larger apertures to reduce the launch costs of these missions. Primary mirror has much to gain from a significant reduction in areal density. Areal density is often limited by the stiffness to weight ratio of the primary mirror. Two key factors drive these criteria: telescope strucural characteristics and fabrication requirements. Major efforts in spaceborne telescopes have inspired research in lightweight optics. Also, metering strucure requires high stiffness, high dimensional stability and minimum obstruction of the telescope aperture to get high quality images. This paper describes a conceptual design trade studies that explores the structural views for the lightweight primary mirror as well as the metering structure in sub-metric class spaceborne telescope.
Conference Committee Involvement (2)
Material Technologies and Applications to Optics, Structures, Components, and Sub-Systems III
7 August 2017 | San Diego, California, United States
Material Technologies and Applications to Optics, Structures, Components, and Sub-Systems II
11 August 2015 | San Diego, California, United States