Secondary Mirror Research Articles

High precision automated alignment procedure for two-mirror telescopes.

A significant challenge in the production of Earth observation satellites is the precise alignment of the telescope optical components. We have developed a strategy to perform automated alignment of two-mirror telescopes for use in a realistic factory-based setting. A Ritchey-Chrétien telescope was used as an example. The secondary mirror was mounted on a high precision hexapod and its misalignment inferred from the Zernike coefficients for tilt, defocus, and coma, as measured by a phase-shifting interferometer. The required corrections to the position of the secondary mirror were implemented using an integral controller and alignment was achieved within minutes, compared to within days when using a manual alignment process. The Zernike coefficient for each aberration was reduced to within one standard deviation of the fluctuations due to residual instability (48nm).

Experimental evaluation of solar thermal performance of linear Fresnel reflector

The solar thermal performance of a 102 kW rated thermal capacity linear Fresnel reflector system was evaluated experimentally under fixed and variable airflow rate through the oil-air heat exchanger. The solar thermal system consisted of 240 mirrors (linear Fresnel solar reflectors), each of 1485 by 625 mm size to heat the thermal oil, which was used to heat the air using the oil to air heat exchanger (AFfE). The results show that the actual maximum efficiency of the solar collector lies in the range of 0.28 to 0.34, whereas the calculated one is about 0.54 for the variable airflow rates. However, for a fixed airflow rate, the actual maximum efficiency is between 0.26 and 0.30. It implies that the actual solar collector efficiency is much lower than the calculated values. One of the major factors, which is not accounted for in the theoretical efficiency calculation, is the effect of dust on the primary and secondary mirrors. The investigated site is observing considerable sand movement, which is affecting the thermal output of the solar collector. The maximum heat exchanger effectiveness, e, obtained for variable and fixed airflow conditions is 0.9 and 0.8, respectively.

ELT - Where the Secondary Mirror Becomes a Giant

ELT - Where the Secondary Mirror Becomes a Giant

ExoEarth yield landscape for future direct imaging space telescopes

The expected yield of potentially Earth-like planets is a useful metric for designing future exoplanet-imaging missions. Recent yield studies of direct-imaging missions have focused primarily on yield methods and trade studies using “toy” models of missions. Here, we increase the fidelity of these calculations substantially, adopting more realistic exoplanet demographics as input, an improved target list, and a realistic distribution of exozodi levels. Most importantly, we define standardized inputs for instrument simulations, use these standards to directly compare the performance of realistic instrument designs, include the sensitivity of coronagraph contrast to stellar diameter, and adopt engineering-based throughputs and detector parameters. We apply these new high-fidelity yield models to study several critical design trades: monolithic versus segmented primary mirrors (PMs), on-axis versus off-axis secondary mirrors, and coronagraphs versus starshades. We show that as long as the gap size between segments is sufficiently small (<0.1 % of telescope diameter), there is no difference in yield for coronagraph-based missions with monolithic off-axis telescopes and segmented off-axis telescopes, assuming that the requisite engineering constraints imposed by the coronagraph can be met in both scenarios. We show that there is currently a factor of ∼2 yield penalty for coronagraph-based missions with on-axis telescopes compared to off-axis telescopes, and note that there is room for improvement in coronagraph designs for on-axis telescopes. We also reproduce previous results in higher fidelity, showing that the yields of coronagraph-based missions continue to increase with aperture size while the yields of starshade-based missions turnover at large apertures if refueling is not possible. Finally, we provide absolute yield numbers with uncertainties that include all major sources of astrophysical noise to guide future mission design.

Relay optical function and pre-construction results of a Giant Steerable Science Mirror for a thirty meter telescope.

The Giant Steerable Science Mirror (GSSM) is the tertiary mirror system of the Thirty Meter Telescope (TMT) that relays optical beams from the secondary mirror to active instruments on Nasmyth platforms. One of the key technologies involved in GSSM functions is the error budget allocation from the system engineering of TMT. A novel approach of error analysis and allocation with strong adaptability, which is based on normalized Point Source Sensitivity (PSSn), is proposed. The relay optical function including the quality of the wavefront, the rationality of the mechanism, and the stability of the light were achieved based on the proposed method. The experiments validate the proposed method.

An off-axis Golay3 sparse aperture telescope with a freeform secondary mirror

The application of freeform surface in an optical system can effectively increase the freedom of optical design and reduce the number of optical elements. In this work, a two-mirror off-axis sparse aperture telescope is proposed. The primary mirror is made of three sub-mirrors arranged in the Golay3 configuration while the secondary is a freeform surface defined by a polynomial of two variables X and Y. The off-axis configuration is used to remove the obstruction of the secondary mirror. The results indicate that the fill factor of the Golay3 primary mirror increases to 58.4%, which is significantly higher than that of the on-axis configuration. The image quality is improved within 2.5° full field of view from its spot diagram, distortion, modulation transfer function, and encircled energy.

Optical characterization of two-stage non-imaging solar concentrator for active daylighting system

A novel prototype of two-stage non-imaging solar concentrator (2S-NISC) for active daylighting in a building has been constructed and presented in our previous paper. In this work, the optical characteristics of the 2S-NISC have been explored to analyze the overall performance embracing maximum solar concentration ratio, percentage of energy in uniform illumination area, spillage loss, tolerance angle towards tracking error etc. using analytical formulas assisted by ray-tracing software based on designed parameters including the focal distance, f/D ratio and size of the receiver. In our simulation, we consider the case of 2S-NISC consisted of eighty primary facet mirrors of 5 cm × 5 cm each, twenty secondary facet mirrors of 8 cm × 8 cm each, and plastic optical fibers as a daylight transmitter. When focal distance is changed from 50 to 100 cm, the maximum solar concentration ratio increases from 65.1 to 70.1 suns; whilst the percentage of energy in the uniformly illuminated area shows the overall trend of decreasing when focal distance is changed from 50 to 100 cm except at focal distances 60 cm and 100 cm due to the slope error. Finally, the total tolerance angle of the 2S-NISC that allows at least 95% of energy to be collected with respect to perfect tracking is determined as 0.54˚.

Development of an isotropic Stewart platform for telescope secondary mirror

Abstract For large aperture telescopes, active control of the secondary mirror is essential for correction of the aberrations. In this paper, an isotropic Stewart platform is developed for the telescope secondary mirror control system, providing 6-DOF high-precision pointing and positioning. The isotropic configuration is designed and one experimental prototype of the isotropic Stewart platform is constructed. Then, the isotropic characteristic is investigated through the verification experiments based on the statics principle. Furthermore, the dynamics model of the isotropic Stewart platform is established and the model properties are analyzed. Finally, the experimental prototype is developed and the decoupled controller is adopted. The experimental results demonstrate the effectiveness of the decoupled control strategy and further validate the proposed isotropic drive mechanism for the telescope secondary mirror.

Design and analysis for annular focus antenna with off-axial parabolic and slanting hyperbolic rotating surfaces configuration.

We propose a novel annular focus antenna with an off-axial parabolic primary mirror and slanting hyperbolic secondary mirror. Ray tracing based on three-dimensional vector reflection theory is utilized to design and analyze the structural parameters of the antenna. In ideal conditions, there is no geometric loss in theory and the transmitting efficiency increases by 24.54% compared with the conventional Cassegrain optical antenna. The transmitting efficiency is also calculated in the conditions of off-axial and on-axial defocus. The simulated results demonstrate that this design can increase transmission efficiency of the optical antenna effectively.

Omnidirectional emission from micro-notched Bragg fiber

A micro-notched Bragg fiber is proposed as a simplified method to achieve omnidirectional emission. The output beam can retain Gauss-like distribution when the size of the notch is appropriate. Simulation results show that the width and the depth of the notch significantly affect the features of the radial beam, such as the divergence angle and output energy. According to the result that the initial phase can directly control the phase of the output beam, the notched Bragg fiber is suggested as being suitable for simplifying a Cassegrain optical antenna system, and making up for the energy loss (about 25%) caused by the central obstruction of the secondary mirror.

Use of coordinate measuring machines for the assembly of axisymmetric two-mirror objectives with aspherical mirrors

The error for the relative alignment of aspherical mirrors was analyzed using coordinate measuring machines for assembling Ritchey–Chretien two-mirror objectives. It is shown that, in addition to the measurement error of linear and angular decentering of mirrors, the error in determining the position of the secondary and primary mirrors in the housing of the objective has a significant impact on the build quality. Requirements for lens design are formulated to ensure the required accuracy of assembly.

Inverse Chop Addition: Thermal IR Background Subtraction without Nodding

Due to the large size and mass of the secondary mirror on next generation extremely large telescopes it will not be possible to provide classical chopping and nodding as is used during mid-IR observations today. As a solution to this we propose an alternative approach to thermal background reduction called `inverse chop addition`. Here we use the symmetries of the thermal background to replace nodding, which allows us to get nearly identical background reductions while only using a special chopping pattern. The performance of this method was tested during technical time observations on VLT/VISIR. With this method, a higher observational efficiency can be obtained than with `classical chopping and nodding`, while achieving equally good reduction results. These results suggest that `inverse-chop addition` could be a good alternative for classical chopping and nodding on both current and next generation ground-based facilities.

Telescopes Alignment Using the Sharpness Function Method Based on Undersampled Images

In astronomy, the images are sometimes undersampled. In the previous research works, image reconstructions have to be employed to recover the lost information from a set of undersampled images to achieve a high-resolution image using dithering or drizzle methods, which increase the complexity of alignment processes and make the real-time correction impossible. In this paper, the telescope is aligned by changing the positions of the secondary mirror using sharpness function method combined with the stochastic parallel gradient descent algorithm based on both well-sampled and undersampled images without image reconstructions. To improve the accuracy and robustness of the alignment, a new metric called relative root mean square error is proposed. Both numerical simulations and experiments are implemented, and the alignment precisions are measured by wavefront residual errors using Shack-Hartmann wavefront sensors. The results show that the correction processes can converge stably and quickly whether the images are well-sampled or undersampled. In experiments, the average wavefront error is 0.0595 λ for undersampled images and 0.0548 λ for well-sampled images after the telescope alignment, indicating that the misalignments are well compensated for both well-sampled and undersampled images.

Design of conical Wolter-I geometry with sectioned secondary mirrors for x-ray telescopes

Conical Wolter-I geometry is employed for many x-ray telescopes to lower their cost and fabrication difficulty at the expense of angular resolution. Owing to the conic error, the angular resolution of conical Wolter-I geometry is much worse than that of Wolter-I geometry, especially for the telescopes with large diameter. We optimized the conical Wolter-I geometry to significantly improve the angular resolution. We designed a conical Wolter-I geometry with sectioned secondary mirrors. Based on the normal conical Wolter-I geometry, we divided the secondary mirror into two equal sections along the optical axis. In this case, the collecting area was reduced by 5% because of the interval between the two sections. Meanwhile, the conic error was reduced by about 50%, indicating a great improvement in angular resolution. Regarding our improvement in the thermal slumping technique, it is feasible to fabricate sectioned mirrors, thus improving the angular resolution by 50% at the cost of a 5%-reduction in collecting area. In addition, a hybrid geometry, comprising the sectioned and nonsectioned geometries, is proposed as an alternative for x-ray telescopes with a large amount of nested shells, to obtain both a large collecting area and decent angular resolution.

Compact, Wide-Angle Catadioptric Telescope Design for MWIR Imaging

In this work, the optical design of a compact diffraction-limited F/3.0 MWIR (3-5 µm) catadioptric telescope which consists of an aspheric primary mirror, a spherical secondary mirror, a ZnSe-ZnSe-Ge triplet lens and an air-spaced Si-Ge-Si corrector triplet lens placed before the focus is given. The calculated effective focal length, full field of view (FFOV) and angular magnification values for the designed telescope are 600 mm, 2.6° and -9.2, respectively. Zemax® calculations for the telescope MTF (modulation transfer function), PSF (point spread function), RMS (root-mean-square) wavefront error (WFE), optical path difference (OPD), Seidel aberrations, encircled energy, distortion and field curvature are discussed. The telescope performance analysis indicates that the telescope can achieve diffraction-limited image quality performance within the 2.6° full field-of-view. Additionally, the designed telescope can resolve at 50% contrast spatial frequency equal to 18.5 cycles/mm (at the on-axis FOV). Finally, a detailed tolerance analysis for the telescope design employing Monte Carlo and sensitivity analysis simulation features in Zemax® is provided.

空间相机次镜调整机构的性能评价及参数优化

For meeting the requirement of complicated design restrictions, a set of performance indices was proposed, which was used to evaluate the efficiency of a space telescope secondary mirror adjusting mechanism. Based on these indices, a set of dimension parameters with high efficiency was obtained. By establishing the kinematic model, error model, statics model, dynamic model and analyzing the characters of these models, the Workspace Efficiency Index (WEI), Error Efficiency Index (EEI), Force Efficiency Index (FEI) and Energy Efficiency Index (EnEI) were proposed. The performance atlases method was used to investigate these indices. Four performance atlases of these indices were presented. Furthermore, a set of dimension parameters ((a, b, l)=(197 mm, 643 mm, 1 260 mm)) with high efficiency was obtained by optimizing these indices. The research result is helpful for engineering applications of this mechanism.

大型空间望远镜次镜调整机构精度分析与测试

大型空间望远镜次镜调整机构精度分析与测试

ARGOS at the LBT

Having completed its commissioning phase, the Advanced Rayleigh guided Ground-layer adaptive Optics System (ARGOS) facility is coming online for scientific observations at the Large Binocular Telescope (LBT). With six Rayleigh laser guide stars in two constellations and the corresponding wavefront sensing, ARGOS corrects the ground-layer distortions for both LBT 8.4 m eyes with their adaptive secondary mirrors. Under regular observing conditions, this set-up delivers a point spread function (PSF) size reduction by a factor of 2–3 compared to a seeing-limited operation. With the two LUCI infrared imaging and multi-object spectroscopy instruments receiving the corrected images, observations in the near-infrared can be performed at high spatial and spectral resolution. We discuss the final ARGOS technical set-up and the adaptive optics performance. We show that imaging cases with ground-layer adaptive optics (GLAO) are enhancing several scientific programmes, from cluster colour magnitude diagrams and Milky Way embedded star formation, to nuclei of nearby galaxies or extragalactic lensing fields. In the unique combination of ARGOS with the multi-object near-infrared spectroscopy available in LUCI over a 4 × 4 arcmin field of view, the first scientific observations have been performed on local and high-z objects. Those high spatial and spectral resolution observations demonstrate the capabilities now at hand with ARGOS at the LBT.

Design of a built-in baffle for a Ritchey–Chretien optical system

A baffle effectively prevents direct rays of stray light from entering an optical system. Existing design methods can result in very long outer baffle lengths, increasing the weight and volume of the optical system, or large secondary mirror baffle sizes, increasing the central obscuration and resulting in the reduction of the illumination beam on the image. To overcome these challenges, a design method for a built-in baffle is presented. The designed built-in baffle for a typical Ritchey-Chretien optical system is compared with a conventionally designed baffle. The results show that when the length of outer baffle is shortened from 3456 to 2349.9mm, the modulation transfer function (MTF) increases from 0.361 to 0.421 in case 1, and when the length is shortened from 3456 to 1924.2mm, the MTF increases from 0.361 to 0.408 in case 2. Meanwhile, the stray light suppression capability is close to that of the traditional method.

Increasing the SOFIA Secondary Mirror Mechanism’s Fast Steering Capability by Identification of a Structural Resonance and Its Subsequent Elimination Through Mass Re-Distribution

The Stratospheric Observatory for Infrared Astronomy (SOFIA) consists of a 2.7[Formula: see text]m infrared telescope integrated into a Boeing 747 SP. One of the most complex subsystems of the observatory is the secondary mirror assembly (SMA). This active steering mechanism is used for image stabilization and infrared chopping. Since its integration in 2002, the performance of the mechanism is limited by a structural resonance. Based on Finite Element (FE) simulations and experimental modal surveys, a ring shaped reaction mass was identified to be the causing element of this structural mode. Attenuating the resonance on the hardware level would result in a larger actuation bandwidth for faster chopping and image stabilization. Concentrating mass at the suspension points while keeping the inertia of the ring structure is expected to take strain energy out of the mode. An end-to-end simulation, including a FE model of the mechanism and a controller model was set up to predict the in-flight performance of this concept. A segmented ring made from tungsten and AlSiC (i.e. strong mass redistribution) mounted on the original suspension was selected for the design of a prototype. The prototype was manufactured and thoroughly tested on a full-scale mockup of the mechanism confirming the predicted performance. An actuation bandwidth improvement of 80% was achieved. The settling time for infrared chopping was reduced from 10 to 7[Formula: see text]ms providing about 3.3% higher efficiency for observations with 5[Formula: see text]Hz chopping.