Progress on the California Extremely Large Telescope (CELT)

In order for the California Extremely Large Telescope (CELT) to achieve the required optical performance, each of its 1000 primary mirror segments must be positioned relative to adjacent segments with nanometer-level accuracy. This can be accomplished using three actuators for each segment to actively control the segment in tip, tilt, and piston. The Keck telescopes utilize a segmented primary mirror similar to CELT employing a highly successful actuator design. However, because of its size and the shear number of actuators (3000 vs. 108 for Keck), CELT will require a different design. Sensitivity to wind loads and structural vibrations, the large dynamic range, low operating power, and extremely reliable operation, all achieved at an affordable unit cost, are the most demanding design requirements. This paper examines four actuator concepts and presents a trade-off between them. The concept that best met the CELT requirements is described along with an analysis of its performance. The concept is based on techniques that achieve the required accuracy while providing a substantial amount of vibration attenuation and damping. A prototype actuator has been built to validate this concept. Preliminary tests confirm predicted behavior and future tests will establish a sound baseline for final design and production.

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.

We have designed a 30 cm cryogenic space infrared telescope for astronomical observation. The telescope is designed to observe in the wavelength range of 0.5~2.1 <TEX>${\mu}m$</TEX>, when it is cooled down to 77 K. The result of the preliminary design of the support structure and support method of the mirror of a 30 cm cryogenic space infrared telescope is shown in this paper. As a Cassegrain prescription, the optical system of a 30 cm cryogenic space infrared telescope has a focal ratio of f/3.1 with a 300 mm primary mirror (M-1) and 113 mm secondary mirror (M-2). The material of the whole structure including mirrors is aluminum alloy (Al6061-T6). Flexures that can withstand random vibration were designed, and it was validated through opto-mechanical analysis that both primary and secondary mirrors, which are assembled in the support structure, meet the requirement of root mean square wavefront error < <TEX>${\lambda}/8$</TEX> for all gravity direction. Additionally, when the M-1 and flexures are assembled by bolts, the effect of thermal stress occurring from a stainless steel bolt when cooled and bolt torque on the M-1 was analyzed.

The 60-cm diameter, f/2 DCWS telescope is a modified Ritchey Chretein configuration which collects radiation in three spectral bands, 0.4 to 0.9/micron (visible), 5.0 to 9.0/microns (MWIR), and 9.0 to 12.0 microns (LWIR). The visible and infrared radiation are separated by a dichroic beamsplitter and focused on to separate focal planes; the MWIR and LWIR radiation share a common focal plane with separate detector arrays residing side-by-side. The conic constants of the primary and secondary mirrors are that of a Ritchey Chretein telescope; to improve the image quality at the edges of the field of view, a set of zero power correcting optics have been introduced to each optical train. The Ritchey Chretein telescope is corrected to have zero third order spherical aberration and zero third order coma; the DCWS telescope is further configured such that the positive power in the primary mirror is approximately equal to the negative power in the secondary mirror, resulting in a telescope with very little field curvature.

This is part six of a series describing the ongoing optical modeling activities for the James Webb Space Telescope (JWST). The first two discussed modeling JWST on-orbit performance using wavefront sensitivities to predict line of sight motion induced blur and stability during thermal transients. The third investigates the aberrations resulting from alignment and figure errors after compensation of the controllable degrees of freedom of the observatory (i.e. primary and secondary mirrors). This paper follows the theme of the third, but considers the highly unlikely case where a gross figure error occurs on the secondary mirror. We then compensate for this error using figure control on the primary mirror. The level of compensation as well as the total motions required are evaluated and reported. Impact to image quality over the full field of the observatory is also discussed.

The Ritchey-Chretien (RC) optical design of Thirty Meter Telescope (TMT) calls for a 3.1m diameter secondary mirror (M2M) and an elliptical tertiary mirror (M3M) of 3.5m along its major axis and 2.5m along its minor axis. The M3M is a thin, large, flat, solid elliptical mirror which directs the f/15 beam from the M2M to the multiple instruments on both Nasmyth platforms. The M3M will weigh approximately two metric tons and the mirror support system will maintain the mirror figure at different gravity orientations. A recent reduction of the field of view to 15 arc minutes allows a reduction in the size of the M3M, which in turn requires re-optimization of the mirror support system. The proposed M3M optimized support system consists of 60 tri-axial supports mounted at the mirror back surface. These tri-axial supports accommodate motions of M3M in three gravity directions. The print-though RMS surface errors of M3M are 10nm for axial gravity loadings and 1nm for lateral gravity loadings. The M3 system (M3S) has an active optics (aO) capability to accommodate potential mechanical or thermal errors; its ability to correct low-order aberrations has been analyzed. A structure function (SF) of the axial gravity support print-through was calculated.

Recent world events have underscored the need for a satellite based persistent global surveillance capability. To be useful, the satellite must be able to continuously monitor objects the size of a person anywhere on the globe and do so at a low cost. One way to satisfy these requirements involves a constellation of satellites in low earth orbit capable of resolving a spot on the order of 20 cm. To reduce cost of deployment, such a system must be dramatically lighter than a traditional satellite surveillance system with a high spatial resolution. The key to meeting this requirement is a lightweight optics system with a deformable primary and secondary mirrors and an adaptive optic subsystem correction of wavefront distortion. This proposal is concerned with development of MEMS micromirrors for correction of aberrations in the primary mirror and improvement of image quality, thus reducing the optical requirements on the deployable mirrors. To meet this challenge, MEMS micromirrors must meet stringent criteria on their performance in terms of flatness, roughness and resolution of position. Using Sandia's SUMMIT foundry which provides the world's most sophisticated surface MEMS technology as well as novel designs optimized by finite element analysis will meet severe requirements on mirror travel range and accuracy.

Earth observation satellite with a large mirror telescope has been studied in Japan Aerospace Exploration Agency to offer a fine ground sampling distance from geostationary orbit. Our latest optical design has a 3.6-m primary mirror and thus aims to obtain a ground sampling distance of sub-10 m from an altitude of 36,000 km at visible wavelengths. To achieve diffraction-limited performance in such optics on orbit, the telescope equips with not only actuators to control primary and secondary mirrors but also a deformable mirror (DM) at the exit pupil plane for the fine phasing. For on-orbit wavefront correction with a deformable mirror, a wavefront sensorless image-based aberration correction scheme is advantageous from the viewpoint of severely limited hardware resources in satellites. Phase diversity (PD) and stochastic parallel gradient descent (SPGD) optimization are known for promising image-based approaches. The former is model-based and thus the estimation accuracy of wavefront aberration significantly depends on the model accuracy, while the other requires many measurements to compensate for large aberration. To alleviate these issues, we propose sequential use of PD and SPGD optimization to efficiently reduce wavefront correction. We first developed an optical testbed with an incoherent light source, a MEMS DM, and extended image targets, and then a wavefront correction experiment was carried out. As a result, the proposed method successfully achieved diffraction-limited imaging performance with a small number of measurements. We will also discuss the image dependence of the wavefront correction performance.

California Institute of Technology and University of California have begun conceptual design studies for a new telescope for astronomical research at visible and infrared wavelengths. The California Extremely Large Telescope (CELT) is currently envisioned as a filled-aperture, steerable, segmented telescope of approximately 30 m diameter. The key to satisfying many of the science goals of this observatory is the availability of diffraction-limited wavefront control. We describe potential observing modes of CELT, including a discussion of the several major outstanding AO system architectural design issues to be resolved prior to the initiation of the detailed design of the adaptive optics capability.

In this paper, two new techniques are proposed for manipulation of the microsatellite imaging structures and sensors in order to reduce the micro-satellite's weight and improve the image quality. Theses satellites generally include three mirrors. First, replacing primary mirror by deformable mirror with appropriate actuators is suggested. In this design, the primary mirror is replaced with deformable mirror (DM) and the secondary mirror can be aligned in the cassegrain design and tertiary mirror could be ignored. Second, by changing the position of sensor, the image quality of different pixels could be changed. Normally, when the sensor is fixed, parts of the image might be blurred, noisy and distorted. Therefore, if the sensor is capable of changing its position, the quality of the distorted pixels will be improved but other parts will become blurred. In this case blurred pixels should be omitted and improved pixels should be saved and final image would be taken form processed pixels. In this paper a new concept of "local focusing" is introduced. This concept aims to process images at a variable distance of a sensor, which can cause the final image quality to become better than the fixed sensor.

An off-axis three-mirror detection system with a large field of view is designed in order to improve the space target detection capability. The optical system is a Cook-TMA with the focal length of 127mm, the F number of 2, the field angle of 25° × 25° and the spectral range of 400-700nm. The primary mirror and the tertiary mirror of the off-axis three mirror system are all designed by free form surfaces: the primary mirror is characterized by Zernike polynomial and the tertiary mirror is described by XY polynomial. At the same time, we analyze the related characteristics of Zernike polynomial and XY polynomial. The results show that the free form surfaces have great advantages in improving the field of view and the imaging quality of the off-axis optical system. The system has high energy concentration and good imaging quality, which can capture and track the target in a wide range is suitable for wide area target monitoring.

Thermal analysis for the Thirty Meter Telescope (TMT) optics (the primary mirror segment, the secondary mirror, and the tertiary mirror) was performed using finite element analysis in ANSYS and I-DEAS. In the thermal analysis, each of the optical assemblies (mirror, mirror supports, cell) was modeled for various thermal conditions including air convections, conductions, heat flux loadings, and radiations. The thermal time constant of each mirror was estimated and the temperature distributions of the mirror assemblies were calculated under the various thermal loading conditions. The thermo-elastic analysis was made to obtain the thermal deformation based on the resulting temperature distributions. The optical performance of the TMT optics was evaluated from the thermally induced mirror deformations. The goal of this thermal analysis is to establish thermal models by the FEA programs to simulate for an adequate thermal environment. These thermal models can be utilized for estimating the thermal responses of the TMT optics. In order to demonstrate the thermal responses, various sample time-dependent thermal loadings were modeled to synthesize the operational environment. Thermal responses of the optics were discussed and the optical consequences were evaluated.

The Thirty Meter Telescope (TMT) Primary Mirror (M1) is composed of 492 hexagonal aspheric segments. In order to compensate for residual polishing errors, installation errors, gravity effects and parasitic forces in the whiffletree support, each segment is equipped with 21 Warping Harness (WH) mechanisms, which allow low order corrections to the optical surface. The primary mirror segment aberrations after shape corrections with warping harness have been identified as the single largest error term in the Thirty Meter Telescope (TMT) image quality error budget [1]. The WH mechanism consists of a Linear Actuator, Ball Link and a Leaf Spring with strain gauge sensor. The Warping Harness key requirements are: high positioning accuracy, 50 years operational life time with minimum maintenance, low cost, high reliability, high resolution, low hysteresis, high stability in unpowered state, large operational temperature range, low power dissipation, vacuum compatibility and survival of accidental condensing conditions. Smooth motor has developed a robust, low cost linear actuator for the TMT M1 Warping Harness. A WH Accelerated Life Cycle Test with increased temperature extremes was successfully completed. Acceleration factors were calculated with the Coffin-Manson model for temperature. Compliance with the technical requirements for the WH system has been demonstrated.

Some results are presented of a successful attempt to correct the surface manufacturing errors on the secondary mirror of the Noto 32 m radiotelescope by means of a new primary mirror, the shape of which is controlled by 244 electromechanical actuators. During a 43 GHz Jupiter observation, the technique yielded a 22% gain increase.

In this paper, a novel two-stage dish concentrator (TSD) with a rotary secondary mirror (SM) is presented for solar thermal water/CO2 splitting. An in-house code for ray-tracing simulation of the concentrator was developed and validated. Among all feasible geometries, a hyperboloid with an upper sheet is the most popular option and is widely used as a secondary reflector, which is mainly discussed here. All para-hyperboloid geometric combinations can be categorized into three typical patterns (φ1 &lt; π/2, φ1 = π/2, φ1 &gt; π/2, φ1 = field angle of PM). The initial designs of the TSD, respective to different off-axis levels for each combination, were first designed. Then a new mathematical model was introduced to reshape the SM to reach optimal truncated designs. Finally, a new concept of an off-axis primary mirror (PM) combined with the truncated SM was evaluated by using the in-house ray-tracing code. The results include the optical efficiency, concentration ratio and intercepted radiant flux. The best solutions with the highest optical efficiency fall in the range π/2 ≤ φ1 ≤ (π − arcsin 0.8) rads and 0.4 ≤ NA2 ≤ 0.6 (NA2 = sin φ2, φ2 = field angle of SM), which vary with the concentration ratio and inclination angle.