Large Segmented Telescopes Research Articles

Research on piston error sensing for segmented mirrors under atmospheric turbulence.

Large aperture ground-based segmented telescopes typically use electrical edge sensors to detect co-phase errors. However, complex observing environments can lead to zero-point drift of the edge sensors, making it challenging to maintain the long-term co-phase of the segmented primary mirror using only edge sensors. Therefore, employing optical piston error detection methods for short-term calibration of edge sensors can address the issue of zero-point drift in the sensors. However, atmospheric turbulence can affect calibration accuracy based on the observational target. To achieve high-precision calibration of electrical edge sensors, this study investigates the impact of atmospheric turbulence on optical piston error detection. Based on simulated results, it is found that the actual measured piston error in the presence of atmospheric turbulence is the difference between the average phases of the two segments. Subsequently, optical piston error detection experiments were conducted in a segmented mirror system under simulated turbulent conditions with varying turbulence intensities. Experimental studies have shown that the detection accuracy of the optical method is almost the same as without turbulence when using a detection aperture size that is 0.82 times the atmospheric coherence length and an exposure time of at least 40 ms. The root mean square of the cross-calibration is better than 3nm. These experimental results indicate that under conditions of good atmospheric seeing, the optical piston error detection method can meet the short-term calibration requirements of edge sensors by setting reasonable detection area size and exposure time. It may even be possible to directly use optical detection methods to replace edge sensors for real-time detection of piston errors.

Coronagraphic detection of Earth-like planets with large, actively controlled space telescopes

We explore the capabilities of large segmented telescopes with active and adaptive optics, with a particular focus on a system view, which includes use of approaches that are routine for current large ground-based telescopes. Using a physically motivated order-of-magnitude model, we show that continuous control of telescope misalignments using adjustable optics in an exoplanet imaging instrument significantly relaxes stability requirements for the entire observatory. We start with the recent analysis by Nemati et al., (2020, JATIS 6, id. 039002), which asserts that small monolithic mirrors have an engineering advantage over larger segmented mirrors when it comes to obtaining images stable enough for direct exoplanet imaging and characterization, i.e., picometer stability. When we fold these results into our model of closed-loop operations and properly partition engineering challenges by optimizing error budget allocations, we find that even for the most sensitive modes, allowable drifts are actually of the order of nanometer over an hour, well within easily engineered tolerances. While this order-of-magnitude analysis does not include full end-to-end modeling or proper engineering margins, it showcases the importance of considering continuous wavefront sensing and control when discussing the feasibility of future exoplanet missions. We also quantify how large segmented architectures, in spite of appearing more complex at the observatory level, facilitate closed-loop operations due to their large photon collection abilities. We place our work in the context of larger discussions on aperture size that highlight a more fundamental challenge: the deeper uncertainties in performance of an exo-earth characterizing telescope primarily reside in our knowledge of the frequency of exo-earths; the effects of geological age on the resulting atmospheres; and, most importantly, on the likelihood of detectable life arising on such planets. A mission that sets out to establish whether we are alone among the nearby stars must adopt a mission architecture that is resilient against such intrinsic uncertainties: uncertainties that only direct observations can resolve. Large apertures enabled by segmented telescope designs historically have demonstrated such resilience.

New capabilities for laser holographic testing during assembly and collimation of large segmented telescope mirrors

Subject of study. Several novel options for interferometric testing of large aspheric segmented primary telescope mirrors during all phases of construction (assembly, collimation, and performance assessment) are proposed. In this paper, we will describe and validate new engineering design solutions for interferometric surface testing of large concave aspheric segmented telescope mirrors. Methods. All proposed test configurations are based on a reflecting optical compensator in the form of an axial synthetic holographic optical element or a reflective convex aspheric surface of revolution. We propose a quasi-autocollimative light path in which an optical compensator reverses the object wavefront; however, the object wavefront itself does not directly produce the image of the surface being tested in the plane where the interference and shadow patterns are recorded, thereby eliminating significant distortion-like errors. Main results. We provide the calculated parameters for quasi-autocollimative systems used to test the surfaces of the aspheric concave primary mirror segments of four well-known world-class telescopes: the Millimetron telescope, diameter 10 m; the James Webb Space Telescope, diameter 6.5 m; the Extremely Large Telescope (ELT), diameter 39.3 m; and the postponed European Southern Observatory conceptual design known as the Overwhelmingly Large Telescope (OLT), diameter 100 m. We show that simultaneous use of a series of similar coaxial optical compensators on the object leg of the interferometer will almost completely eliminate any restrictions related to the size, asphericity, and curvature of aspheric segments of primary mirrors in telescopes. Using a conical substrate for the working surface of the holographic compensator will substantially reduce the maximum spatial frequency of the compensator. The calculations were performed using Mathcad and Zemax. We propose that traditional non-autocollimative laser holographic test configurations without wavefront reversal be used on the object leg during the initial assembly of the primary mirror segments. Laser holographic topography is also suitable for this purpose. Practical significance. The novel ideas, methods, and systems engineering solutions proposed here are based on wavefront reversal by a reflecting optical compensator or a series (“chain”) of such compensators and will support in-process testing and certification testing of aspheric primary mirror segments, with interferometric accuracy, for any of the optical telescopes currently operating or under development, whether ground based or space based. These capabilities will support full-scale interferometric testing of the 12 m diameter Millimetron aspheric segmented primary mirror initially proposed by Academician N. S. Kardashev, but will also undoubtedly be especially important for space-based telescopes.

A Coronagraph with a Sub-λ/D Inner Working Angle and a Moderate Spectral Bandwidth

Future high-contrast imaging spectroscopy with a large segmented telescope will be able to detect atmospheric molecules of Earth-like planets around G- or K-type main-sequence stars. Increasing the number of target planets will require a coronagraph with a small inner working angle (IWA), and wide spectral bandwidth is required if we enhance a variety of detectable atmospheric molecules. To satisfy these requirements, in this paper, we present a coronagraphic system that provides an IWA less than 1λ 0/D over a moderate wavelength band, where λ 0 is the design-center wavelength and D denotes the full width of the rectangular aperture included in the telescope aperture. A performance simulation shows that the proposed system approximately achieves a contrast below 10−10 at 1λ 0/D over the wavelengths of 650–750 nm. In addition, this system has a core throughput ≥10% at input separation angles of ∼0.7–1.4λ 0/D; to reduce telescope time, we need prior information on the target’s orbit by other observational methods to a precision higher than the width of the field of view. For some types of aberration including tilt aberration, the proposed system has a sensitivity less than ever-proposed coronagraphs that have IWAs of approximately 1λ 0/D. In future observations of Earth-like planets, the proposed coronagraphic system may serve as a supplementary coronagraphic system dedicated to achieving an extremely small IWA.

Fresnel phasing of segmented mirror telescopes

Shack-Hartmann (S-H) phasing of segmented telescopes is based upon a physical optics generalization of the geometrical optics Shack-Hartmann test, in which each S-H lenslet straddles an intersegment edge. For the extremely large segmented telescopes currently in the design stages, one is led naturally to very large pupil demagnifications for the S-H phasing cameras. This in turn implies rather small Fresnel numbers F for the lenslets; the nominal design for the Thirty Meter Telescope calls for F=0.6. For such small Fresnel numbers, it may be possible to eliminate the lenslets entirely, replacing them with a simple mask containing a sparse array of clear subapertures and thereby also eliminating a number of manufacturing problems and experimental complications associated with lenslets. We present laboratory results that demonstrate the validity of this approach.

Active Optics of Large Segmented Mirrors: Dynamics and Control

This paper examines the active optics of future large segmented telescopes from the point of view of dynamic simulation and control. The first part of the paper is devoted to the modeling of the mirror. The model has a moderated size and separates the quasi-static behavior of the mirror (primary response) from the dynamic response (secondary or residual response). The second part of the paper is devoted to control. The control strategy considers explicitly the primary response ofthe telescope through a singular value controller. The control-structure interaction is addressed with the general robustness theory of multivariable feedback systems, where the secondary response is considered as uncertainty. This approach is very fast and allows extensive parametric studies. The study is illustrated with an example involving 90 segments, 270 inputs, and 654 outputs.

H2 Control of Large Segmented Telescopes

In this paper we conduct a preliminary study of control of the primary mirror of a large segmented telescope, i.e. aligning the segments to form a paraboloid. This work is motivated by the need of astronomers to study faint objects and to reach the highest possible angular resolution. First, we present background information about large segmented telescopes and our collaborative research program on large telescopes. Second, we formulate a model of the primary mirror system in state space. Third, we design a centralized controller using H2 methods for a seven-segment system which establishes the best possible performance characteristics for the laboratory-type of unit that we plan to build. Simulation results are displayed. Finally, we apply spatially-invariant distributed control techniques to an infinite segmented system that approximates a large mirror. Through the H2 norm, we calculate upper bounds for the relative displacements between adjacent segments with either a spatially-invariant infinite controller or a truncated local controller. Simulation results are also presented with a truncated local controller applied to a 19-segment system. In all cases, the results indicate that the required mirror-to-mirror surface position accuracy of 10-8 m can be achieved.

Segment aberration effects on contrast

High-contrast imaging, particularly the direct detection of extrasolar planets, is a major science driver for the next generation of telescopes. This science requires the suppression of scattered starlight at extremely high levels and that telescopes be correctly designed today to meet these stringent requirements in the future. The challenge increases in systems with complicated aperture geometries such as obscured, segmented telescopes. Such systems can also require intensive modeling and simulation efforts in order to understand the trade-offs between different optical parameters. The feasibility and development of a contrast prediction tool for use in the design and systems engineering of these telescopes is described. The performance of a particular starlight suppression system on a large segmented telescope is described analytically. These analytical results and the results of a contrast predictor are then compared with the results of a full wave-optics simulation.

Spatial frequency analytical modeling of adaptive optics for diffraction limited coronagraphs studies on extremely large segmented telescopes

In this paper, we briefly describe the principle of analytical modelization of the adaptive optics (AO) corrected point spread function in the spatial frequency domain, both for the turbulent and the telescope static aberrations, and demonstrate how this can be used for simulating AO corrected phase screens on extremely large hexagonal segmented telescopes, required for performance analysis of diffraction limited coronagraphs.

Analytic diffraction analysis of a 32-m telescope with hexagonal segments for high-contrast imaging

Large segmented telescopes cannot be modeled accurately with fast-Fourier-transform techniques since small features such as gaps between the segments will be inadequately sampled. An analytic Fourier-transform method can be used to model any pupil configuration with straight edges, including tolerance analysis and some types of apodization. We analytically investigated a 32-m segmented primary with 18 hexagonal segments for high-contrast imaging. There are significant regions in the image in which extrasolar planets could be detected. However, the hexagonal profile of the pupil was not as useful as expected. The gaps between the segments, the secondary obscuration, and the secondary spiders must be as small as possible and their edges must be apodized. Apodizing the edges of the individual segments reduced the useful regions in the image since the gaps appeared to be wider.

Analytical study of diffraction effects in extremely large segmented telescopes.

We present an analysis of the diffraction effects from a segmented aperture with a very large number of segments-prototype of the next generation of extremely large telescopes. This analysis is based on the point-spread-function analytical calculation for Keck-type hexagonal segmentation geometry. We concentrate on the effects that lead to the appearance of speckles and/or a regular pattern of diffraction peaks. These effects are related to random piston and tip-tilt errors on each segment, gaps between segments, and segment edge distortion. We deliver formulas and the typical numerical values for the Strehl ratio, the relative intensity of higher-order diffraction peaks, and the averaged intensity of speckles associated with each particular case of segmentation error.

New phasing algorithm for large segmented telescope mirrors

We propose a new phase calibration method, called the sensor-by-sensor phase calibration method (SSPC), which is applied for the phasing of a large segmented telescope mirror. Instead of making the entire segmented mirror in phase at one time and obtaining the de- sired sensor readings accordingly, we serially make the local areas in phase at each sensor location and thus obtain the desired sensor read- ings one by one. Then we run the telescope active control system to make the segments cophase. For the case of Keck telescopes, the SSPC method uses all the 168 phase measurements at 168 phase sen- sor locations; then we obtain 168 desired sensor readings for the seg- mented mirror active control system. The SSPC method is completely insensitive to the residual tilt errors from the segment alignment and the residual errors from the active control system. The validity of the SSPC method has been demonstrated in an active optics experimental system, which has three segments and a 500-mm-diam overall aperture. The accuracy of the SSPC method is better than 20-mm rms in this experi- mental system, and it is easy to obtain diffraction-limited images at a visible wavelength l5650 nm over an aperture of 220 mm. © 2002 Society

Tip-tilt error for extremely large segmented telescopes: detailed theoretical point-spread-function analysis and numerical simulation results.

We present an analysis of point-spread functions for segmented mirrors affected by random tip-tilt errors on each segment. In addition to Strehl ratio evaluation, this analysis considers key characteristics such as the intensity and the location of speckles and secondary peaks and the relative energy distribution between these features. We develop a method to describe the shape of a nonaveraged point-spread function and deduce the final expressions for ensemble-averaged characteristics. Based on Keck-type hexagonal segmentation geometry, our study is extended to an arbitrary number of segments, and we describe qualitatively the transition from the case of a mirror with few segments to that of a mirror with several hundred segments--prototype of the next generation of Extremely Large Telescopes.

Generalized figure-control algorithm for large segmented telescope mirrors

A fundamental discovery in segmented-mirror active optics is described: symmetrizing the geometry of the sensor–actuator array provides a computationally effective symmetrization of the mathematical description for the figure control of a Keck-type telescope mirror. The author establishes a universal mathematical control model and provides an efficient algorithm to solve this model equation set. This model can be applied to multifarious Keck-type mirror configurations with a similar sensor–actuator geometry design, no matter what kind of outline shapes and how many segments they have. With the underlying symmetry, a further extension of this algorithm is possible without increasing the number of parameters to be estimated for the recently proposed extremely large telescopes, such as the 30-m California Extremely Large Telescope (CELT) and the 100-m OverWhelmingly Large (OWL) telescope. Moreover, careful choice of boundary conditions in conjunction with the proper choice of minimization algorithm yields results that exceed the performance of the current existing techniques given by Nelson and Mast [Appl. Opt.21, 2631 (1982)]. This method allows noise performance analysis. Several computer simulation models for application of this algorithm are given for the Keck 10-m Telescope and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope’s (LAMOST’s) MA (Reflecting Schmidt plate) and MB (spherical primary mirror).

Large segmented telescopes

The accurate control of the segments of large segmented telescopes to achieve desired shapes could open the way for the construction of much larger telescopes with much better capabilities. In this article, we consider centralized, decentralized, and overlapping approaches to designing controllers for the shape control of a segmented telescope. In the centralized approach, all the segments and interconnections are considered to be a single dynamical system. In the decentralized case, the interconnections between the segments are neglected for control design purposes. In the overlapping decentralized approach, overlapped segments are considered to be isolated subsystems. In each case, we design controllers using the H∞∞∞ robust control approach. In the centralized control case, the disturbances are rejected over a wide frequency range and all performance requirements are met. The order of the centralized controller, however, is high, which makes it difficult to implement in real time. The decentralized control approach greatly reduces the computational and hardware requirements at the cost of performance deterioration. The overlapping approach trades off between the centralized and decentralized cases that provides sufficient flexibility to meet both performance and computational requirements.

Robust Adaptive Control of a Large Segmented Telescope

Robust Adaptive Control of a Large Segmented Telescope