KEYWORDS: Mirrors, Telescopes, Optical fabrication, Telescopes, Control systems, Prototyping, Control systems design, Space telescopes, Surface finishing, Manufacturing, Profilometers
The Giant Magellan Telescope (GMT) will be featured with two Gregorian secondary mirrors, an adaptive secondary mirror (ASM) and a fast-steering secondary mirror (FSM). The FSM has an effective diameter of 3.2 m and built as seven 1.1 m diameter circular segments, which are conjugated 1:1 to the seven 8.4m segments of the primary. Each FSM segment contains a tip-tilt capability for fine co-alignment of the telescope sub-apertures and fast guiding to attenuate telescope wind shake and mount control jitter. This tip-tilt capability thus enhances performance of the telescope in the seeing limited observation mode. As the first stage of the FSM development, Phase 0 study was conducted to develop a program plan detailing the design and manufacturing process for the seven FSM segments. The FSM development plan has been matured through an internal review by the GMTO-KASI team in May 2016 and fully assessed by an external review in June 2016. In this paper, we present the technical aspects of the FSM development plan.
A number of techniques are available for acoustic source localization in isotropic plates without knowing the material properties of the plate. However, for a highly anisotropic plate acoustic source localization requires some knowledge of the plate material properties or its group velocity profile. In absence of this information one requires a large number of sensors to predict the acoustic source point in the plate. All proposed techniques for acoustic source localization with a few sensors assume the straight line propagation of waves from the source to the receiving sensor with an average group velocity when the plate material properties are not known. However, this assumption is not true for an anisotropic plate. Although the currently available techniques work well for weakly anisotropic plates since the wave path does not deviate significantly from the straight line propagation they fail miserably for highly anisotropic plates.
In this paper acoustic source is localized in an anisotropic plate when non-circular wave front is generated. Direction vectors of wave fronts are obtained from the Time-Difference-Of-Arrivals (TDOA) at three sensors placed in a cluster. Four such direction vectors are then utilized in geometric vector analysis to accurately obtain the acoustic source location. The proposed technique is illustrated on an orthotropic plate that generates rhombus shaped wave front. It should be noted that the proposed technique does not require wave propagation along a straight
A Prototype of Fast-steering Secondary Mirror (FSMP) for the Giant Magellan Telescope (GMT) has been developed by the consortium consisting of institutes in Korea and the US. In 2014 the FSMP development was finalized by combining the two major sub-systems, the mirror fabricated and the mirror cell with the tip-tilt control parts. We have developed an assembly procedure in which potential difficulties, such as handling without contacting mirror surface, and optimizing bonding process, have been resolved. Supporting jigs were produced, and optimized bonding techniques have been developed. The assembled FSMP system was installed in a test tower, and stability of the system were checked. Performance of the FSMP system will be evaluated in static and dynamic environments for the validation of the FSMP system operation as the future works.
The Giant Magellan Telescope (GMT) is one of Extremely large telescopes, which is 25m in diameter featured with two Gregorian secondary mirrors, an adaptive secondary mirror (ASM) and a fast-steering secondary mirror (FSM). The FSM is 3.2 m in diameter and built as seven 1.1 m diameter circular segments conjugated 1:1 to the seven 8.4m segments of the primary. The guiding philosophy in the design of the FSM segment mirror is to minimize development and fabrication risks ensuring a set of secondary mirrors are available on schedule for telescope commissioning and early operations in a seeing limited mode. Each FSM segment contains a tip-tilt capability for fine co-alignment of the telescope subapertures and fast guiding to attenuate telescope wind shake and mount control jitter, thus optimizing the seeing limited performance of the telescope. The final design of the FSM mirror and support system configuration was optimized using finite element analyses and optical performance analyses. The optical surface deformations, image qualities, and structure functions for the gravity print-through cases, thermal gradient effects, and dynamic performances were evaluated. The results indicated that the GMT FSM mirror and its support system will favorably meet the optical performance goals for residual surface error and the FSM surface figure accuracy requirement defined by encircled energy (EE80) in the focal plane. The mirror cell assembly analysis indicated an excellent dynamic stiffness which will support the goal of tip-tilt operation.
The Giant Magellan Telescope (GMT) Fast Steering Secondary Mirror (FSM) is one of the GMT two Gregorian secondary mirrors. The FSM is 3.2 m in diameter and built as seven 1.06 m diameter circular segments. The conceiving philosophy used on the design of the FSM segment mirror is to minimize development and fabrication risks ensuring a set of secondary mirrors are available on schedule for telescope commissioning and early operations in a seeing limited mode, thereby mitigating risks associated with fabrication of the Adaptive Secondary Mirrors (ASM). This approach uses legacy design features from the Magellan Telescope secondary mirrors to reduce such risks. The final design of the substrate and support system configuration was optimized using finite element analyses and optical performance analyses. The optical performance predictions of the FSM are based on a substrate with a diameter of 1.058m (on-axis), 1.048m (off-axis), a depth of 120mm, and a face plate thickness of 20mm leading to a mass of approximately 90kg. The optical surface deformations, image qualities, and structure functions for the axial and lateral gravity print-through cases, thermal gradient effects, and dynamic performances were evaluated. The results indicated that the GMT FSM mirror and its support system will favorably meet the optical performance goals for residual surface error and the FSM surface figure accuracy requirement defined by encircled energy in the focal plane. The mirror cell assembly analysis indicated an excellent dynamic stiffness which will support the goal of 20 Hz tip-tilt motion.
The Giant Magellan Telescope (GMT) will be one of the next class of extremely large segmented mirror telescopes. The GMT will utilize two Gregorian secondary mirrors, and Adaptive Secondary Mirror (ASM) and a Fast-steering Secondary Mirror (FSM). The FSM consists of six off-axis mirrors surrounding a central on-axis circular segment. The segments are 1.1 m in diameter and conjugated 1:1 to the seven 8.4 m segments of the primary. A prototype of the FSM mirror (FSMP) has been developed, analyzed and tested in order to demonstrate the mechanical and optical responses of the mirror assembly when subjected to structural and thermal loadings. In this paper, the mechanical and thermal performances of the FSMP were evaluated by performing finite element analyses (FEA) in NX Nastran. The deformation of the mirror’s lateral flexure was measured when the FSMP was axially loaded and the temperature response of the mirror assembly was measured when exposed to a sample thermal environment. In order to validate the mirror/lateral flexure design concept, the mechanical, optical and thermal measurements obtained from the tests conducted on mirrors having two different lateral flexures were compared to the responses calculated by FEA.
The fast steering mirror (FSM) is a key element in astronomical telescopes to provide real-time angular correction of line-of-sight error due to telescope jitter and wind-induced disturbance. The Giant Magellan Telescope (GMT) will utilize a FSM as secondary mirror under unfavorable wind conditions that excites the telescope at the lowest resonance frequency around 8Hz. A flexure in the center of the mirror constrains lateral displacements, while still allowing tip-tilt motion to steer. Proper design of this central flexure is challenging to meet lateral loading capability as well as angular and axial flexibility to minimize optical surface distortion forced by redundant constraints at the flexure. We have designed the lateral flexure and estimated its performance from a variety of design case studies in a finite element analysis tool. A carefully designed finite element model at the sub-system level including the flexure, lightweight mirror and 3 point axial supports allows evaluating whether the designed flexure is qualified within specifications. In addition, distorted surface maps can be achieved as a function of forces that could be induced in telescope operation or due to misalignment errors during assembling. We have also built a test set-up to validate the finite element analysis results. Optical quality was measured by a phase shifting interferometer in various loading conditions and the measurements were decomposed by standard Zernike polynomials to concentrate specific surface shapes and to exclude low order shapes as measurement uncertainties.
The Giant Magellan Telescope (GMT) Fast-steering secondary mirror (FSM) is one of the GMT two Gregorian
secondary mirrors. The FSM is 3.2 m in diameter and built as seven 1.1 m diameter circular segments conjugated 1:1 to
the seven 8.4m segments of the primary. A parametric study and optimization of the FSM mirror blank and central
lateral flexure design were performed. For the optimized FSM configuration, the optical image qualities and structure
functions for the axial and lateral gravity print-through cases, thermal gradient effects, and dynamic performances will be
discussed. This paper reports performance predictions of the optimized FSM. To validate our lateral flexure design
concept, mechanical and optical tests were conducted on test mirrors installed with two different lateral flexures.
The Giant Magellan Telescope (GMT) will be a 25m class telescope currently in the design and development phase. The
GMT will be a Gregorian telescope and equipped with a fast-steering secondary mirror (FSM). This secondary mirror is
3.2 m in diameter and built as seven 1.1 m diameter circular segments conjugated 1:1 to the seven 8.4m segments of the
primary. The prototype of FSM (FSMP) development effort is led by the Korea Astronomy and Space Science Institute
(KASI) with several collaborators in Korea, and the National Optical Astronomy Observatory (NOAO) in USA. The
FSM has a tip-tilt feature to compensate image motions from the telescope structure jitters and the wind buffeting. For
its dynamic performance, each of the FSM segments is designed in a lightweight mirror. Support system of the
lightweight mirror consists of three axial actuators, one lateral support at the center, and a vacuum system. A parametric
design study to optimize the FSM mirror configuration was performed. In this trade study, the optical image qualities
and structure functions for the axial and lateral gravity print-through cases, thermal gradient effects, and dynamic
performances will be discussed.
The scanning long-wave optical test system (SLOTS) is under development at the University of Arizona to provide rapid
and accurate measurements of aspherical optical surfaces during the grinding stage. It is based on the success of the
software configurable optical test system (SCOTS) which uses visible light to measure surface slopes. Working at long
wave infrared (LWIR, 7-14 μm), SLOTS measures ground optical surface slopes by viewing the specular reflection of a
scanning hot wire. A thermal imaging camera collects data while motorized stages scan the wire through the field.
Current experiments show that the system can achieve a high precision at micro-radian level with fairly low cost
equipment. The measured surface map is comparable with interferometer for slow optics. This IR system could be
applied early in the grinding stage of fabrication of large telescope mirrors to minimize the surface shape error imparted
during processing. This advantage combined with the simplicity of the optical system (no null optics, no high power
carbon dioxide laser) would improve the efficiency and shorten the processing time.
Understanding the Earth spectral bio-signatures provides an important reference datum for accurate de-convolution of
collapsed spectral signals from potential earth-like planets of other star systems. This study presents a new ray tracing
computation method including an improved 3D optical earth model constructed with the coastal line and vegetation
distribution data from the Global Ecological Zone (GEZ) map. Using non-Lambertian bidirectional scattering
distribution function (BSDF) models, the input earth surface model is characterized with three different scattering
properties and their annual variations depending on monthly changes in vegetation distribution, sea ice coverage and
illumination angle. The input atmosphere model consists of one layer with Rayleigh scattering model from the sea level
to 100 km in altitude and its radiative transfer characteristics is computed for four seasons using the SMART codes. The
ocean scattering model is a combination of sun-glint scattering and Lambertian scattering models. The land surface
scattering is defined with the semi empirical parametric kernel method used for MODIS and POLDER missions. These
three component models were integrated into the final Earth model that was then incorporated into the in-house built
integrated ray tracing (IRT) model capable of computing both spectral imaging and radiative transfer performance of a
hypothetical space instrument as it observes the Earth from its designated orbit. The IRT model simulation inputs include
variation in earth orientation, illuminated phases, and seasonal sea ice and vegetation distribution. The trial simulation
runs result in the annual variations in phase dependent disk averaged spectra (DAS) and its associated bio-signatures
such as NDVI. The full computational details are presented together with the resulting annual variation in DAS and its
associated bio-signatures.
Computer controlled optical surfacing (CCOS) requires accurate knowledge of the tool influence function (TIF) for the
polishing tool. The linear Preston's model for material removal has been used to determine the TIF for most cases. As
the tool runs over the edge of the workpiece, however, nonlinear removal behavior needs to be considered to model the
edge TIF. We reported a new parametric edge TIF model in a previous paper.** This model fits 5 parameters to
measured data to accurately predict the edge TIF. We present material from the previous paper, and provide a library of
the parametric edge TIFs for various tool shape and motion cases. The edge TIF library is a useful reference to design an
edge figuring process using a CCOS technique.
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