Primary mirror segment shape correction via Warping Harness (WH) control adjustment is key to obtaining the required image performance of the Thirty Meter Telescope (TMT). We analyzed two separate experimental activities to better predict the segment WH performance. First, we took measurements of WH influence functions and Singular Value Decomposition (SVD) modes on a prototype TMT segment and compared these to model predictions. Second, we applied the TMT control algorithm on-sky at the Keck Observatory during their segment exchange and warping activities. We then used these measurements to improve our WH control simulations to include the observed effects. Altogether, the prototype segment measurements, on-sky TMT control algorithm measurements, and detailed simulation helped to better predict segment correction performance for TMT.
We present an estimate of the optical performance of the Thirty Meter Telescope (TMT) after execution of the full telescope alignment plan. The TMT alignment is performed by the Global Metrology System (GMS) and the Alignment and Phasing System (APS). The GMS first measures the locations of the telescope optics and instruments as a function of elevation angle. These initial measurements will be used to adjust the optics positions and build initial elevation look-up tables. Then the telescope is aligned using starlight as the input for the APS at multiple elevation angles. APS measurements are used to refine the telescope alignment to build elevation and temperature dependent look-up tables. Due to the number of degrees of freedom in the telescope (over 10,000), the ability of the primary mirror to correct aberrations on other optics, the tight optical performance requirements and the multiple instrument locations, it is challenging to develop, test and validate these alignment procedures. In this paper, we consider several GMS and APS operational scenarios. We apply the alignment procedures to the model-generated TMT, which consists of various quasi-static errors such as polishing errors, passive supports errors, thermal and gravity deformations and installation position errors. Using an integrated optical model and Monte-Carlo framework, we evaluate the TMT's aligned states using optical performance metrics at multiple instrument and field of view locations. The optical performance metrics include the Normalized Point Source Sensitivity (PSSN), RMS wavefront error before and after Adaptive Optics (AO) correction, pupil position change, and plate scale distortion.
Alignment and Phasing System (APS) is responsible for the optical alignment via starlight of the approximately 12,000 degrees of freedom of the primary, secondary and tertiary mirrors of Thirty Meter Telescope (TMT). APS is based on the successful Phasing Camera System (PCS) used to align the Keck Telescopes. Since the successful APS conceptual design in 2007, work has concentrated on risk mitigation, use case generation, and alignment algorithm development and improvement. Much of the risk mitigation effort has centered around development and testing of prototype APS software which will replace the current PCS software used at Keck. We present an updated APS design, example use cases and discuss, in detail, the risk mitigation efforts.
KEYWORDS: Systems engineering, Observatories, Computer aided design, Control systems, Data modeling, Systems modeling, Performance modeling, Databases, Interfaces, Thirty Meter Telescope
This paper provides an overview of the system design, architecture, and construction phase system engineering processes of the Thirty Meter Telescope project. We summarize the key challenges and our solutions for managing TMT systems engineering during the construction phase. We provide an overview of system budgets, requirements and interfaces, and the management thereof. The requirements engineering processes, including verification and plans for collection of technical data and testing during the assembly and integration phases, are described. We present configuration, change control and technical review processes, covering all aspects of the system design including performance models, requirements, and CAD databases.
We have developed an integrated optical model of the semi-static performance of the Thirty Meter Telescope. The model includes surface and rigid body errors of all telescope optics as well as a model of the Alignment and Phasing System Shack-Hartmann wavefront sensors and control algorithms. This integrated model allows for simulation of the correction of the telescope wavefront, including optical errors on the secondary and tertiary mirrors, using the primary mirror segment active degrees of freedom. This model provides the estimate of the predicted telescope performance for system engineering and error budget development. In this paper we present updated performance values for the TMT static optical errors in terms of Normalized Point Source Sensitivity and RMS wavefront error after Adaptive Optics correction. As an example of a system level trade, we present the results from an analysis optimizing the number of Shack-Hartmann lenslets per segment. We trade the number of lenslet rings over each primary mirror segment against the telescope performance metrics of PSSN and RMS wavefront error.
Modeling is an integral part of systems engineering. It is utilized in requirement validation, system verification, as well as for supporting design trade studies. Modeling highly complex systems poses particular challenges, including the definition and interpretation of system performance, and the combined evaluation of physical processes spanning a wide range of time frames. Our solution is based on statistical interpretation of system performance and a unique image quality metric developed by TMT. The Stochastic Framework and Point Source Sensitivity allow us to properly estimate and combine the optical effects of various disturbances and telescope imperfections.
The Thirty Meter Telescope (TMT) is a Ritchey-Chritien optical telescope with a 30-meter diameter primary mirror made up of 492 hexagonal segments. Such a large and complex optical system requires detailed modeling of the optical performance during the design phase. An optical modeling computational framework has been developed to support activities related to wavefront & image performance prediction. The model includes effects related to mirror shape sensing & control, mirror alignment & phasing, M1 segment control, low order wavefront correction, adaptive optics simulation for high order wavefront correction, and high contrast imaging. Here we give an overview of this optical simulation framework, the modeling tools and algorithms that are used, and a set of sample analyses. These tools have been used in many aspects of the system design process from mirror specification to instrument & sensor design to algorithm development and beyond.
KEYWORDS: Telescopes, Point spread functions, Vignetting, Telescope design, Distortion, Error analysis, Systems modeling, Optical transfer functions, Space telescopes, Thirty Meter Telescope
The Normalized Point Source Sensitivity (PSSN) has previously been defined and analyzed as an On-Axis
seeing-limited telescope performance metric. In this paper, we expand the scope of the PSSN definition to
include Off-Axis field of view (FoV) points and apply this generalized metric for performance evaluation of the
Thirty Meter Telescope (TMT). We first propose various possible choices for the PSSN definition and select
one as our baseline. We show that our baseline metric has useful properties including the multiplicative feature
even when considering Off-Axis FoV points, which has proven to be useful for optimizing the telescope error
budget. Various TMT optical errors are considered for the performance evaluation including segment alignment
and phasing, segment surface figures, temperature, and gravity, whose On-Axis PSSN values have previously
been published by our group.
We evaluate how well the performance of the Thirty Meter Telescope (TMT) can be maintained against thermally
induced errors during a night of observation. We first demonstrate that using look-up-table style correction for
TMT thermal errors is unlikely to meet the required optical performance specifications. Therefore, we primarily
investigate the use of a Shack-Hartmann Wavefront Sensor (SH WFS) to sense and correct the low spatial
frequency errors induced by the dynamic thermal environment. Given a basic SH WFS design, we position
single or multiple sensors within the telescope field of view and assess telescope performance using the JPL
optical ray tracing tool MACOS for wavefront simulation. Performance for each error source, wavefront sensing
configuration, and control scheme is evaluated using wavefront error, plate scale, pupil motion, pointing error,
and the Point Source Sensitivity (PSSN) as metrics. This study provides insight into optimizing the active optics
control methodology for TMT in conjunction with the Alignment and Phasing System (APS) and primary mirror
control system (M1CS).
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. In order to better
understand the likely errors and how they will impact the telescope performance we have performed detailed
simulations. We first generated unwarped primary mirror segment surface shapes that met TMT specifications.
Then we used the predicted warping harness influence functions and a Shack-Hartmann wavefront sensor model
to determine estimates for the 492 corrected segment surfaces that make up the TMT primary mirror. Surface
and control parameters, as well as the number of subapertures were varied to explore the parameter space. The
corrected segment shapes were then passed to an optical TMT model built using the Jet Propulsion Laboratory
(JPL) developed Modeling and Analysis for Controlled Optical Systems (MACOS) ray-trace simulator. The
generated exit pupil wavefront error maps provided RMS wavefront error and image-plane characteristics like
the Normalized Point Source Sensitivity (PSSN). The results have been used to optimize the segment shape
correction and wavefront sensor designs as well as provide input to the TMT systems engineering error budgets.
KEYWORDS: Telescopes, Optical transfer functions, Error analysis, Space telescopes, Point spread functions, Wavefronts, Systems modeling, Spatial frequencies, Computer simulations, Thirty Meter Telescope
We investigate a new metric, Normalized Point Source Sensitivity (PSSN), for characterizing the seeing limited
performance of the Thirty Meter Telescope. As the PSSN metric is directly related to the photometric error of
background limited observations, it truly represents the efficiency loss in telescope observing time. The PSSN
metric properly accounts for the optical consequences of wavefront spatial frequency distributions due to different
error sources, which makes it superior to traditional metrics such as the 80% encircled energy diameter. We
analytically show that multiplication of individual PSSN values due to individual errors is a good approximation
for the total PSSN when various errors are considered simultaneously. We also numerically confirm this feature
for Zernike aberrations, as well as for the numerous error sources considered in the TMT error budget using a
ray optics simulator, Modeling and Analysis for Controlled Optical Systems. We also discuss other pertinent
features of the PSSN including its relations to Zernike aberration and RMS wavefront error.
We consider high-resolution optical modeling of the Thirty Meter Telescope for the purpose of error budget and instrumentation trades utilizing the Modeling and Analysis for Controlled Optical Systems tool. Using this ray-trace and diffraction model we have simulated the TMT optical errors related to multiple effects including segment alignment and phasing, segment surface figures, temperature, and gravity. We have then modeled the effects of each TMT optical error in terms of the Point Source Sensitivity (a multiplicative image plane metric) for a seeing limited case and an adaptive optics corrected case (for the NFIRAOS). This modeling provides the information necessary to rapidly conduct design trades with respect to the planned telescope instrumentation and to optimize the telescope error budget.
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