Qichang An

Estimation of the GSSM calibration error

The calibration of the tertiary mirror of the Thirty Meter Telescope, also known as the giant science steering mirror (GSSM), is a step of great significance during its testing process. Systematic, drift, and random errors constitute the major limitations to the accuracy of the calibration measurements. In this article, we estimated the errors in the calibration of the GSSM with a laser tracker. For the systematic error, a measurement strategy based on the standard bar method was successfully designed and applied. At the same time, we can distinguish between the drift and random errors by means of a correlation analysis. The systematic error, which depends strongly on the configuration of the system formed by the GSSM and the laser tracker, was estimated to be 20 μm for the GSSM prototype. The random error, averaging 15 min, was about 4 μm. The correlation coefficients among three different noise measurements are all lower than 0.1, which indicates that the noise is dominated by random errors. Finally, the error can be sufficiently suppressed by rearranging the position of the spherically mounted retroreflectors. The result shows that the accuracy of the measurement can be improved by 21.4% with the new arrangement method.

Study of tilt axis bearing arrangement for M3S of TMT project

The tertiary mirror positioned assembly (M3PA) of the thirty meters telescope (TMT) is the largest tertiary mirror pointing system in the world. The tracking and pointing performance of M3PA is better than any other telescopes which have been built, and the working condition is even worse, so the designers face an enormous challenge. The tracking system includes the bottom rotator shaft and the tilt shaft. The study of this paper focuses on the tilt shaft. There are mainly three forms. The first form is one end fixed with the other unrestrained in axial direction. The second form uses two pairs of angular contact ball bearing. The last form lays two tape roller bearings. All of them can meet the requirements when the M3PA is vertical. But the first one becomes invalid when the M3PA is horizontal. We pay our attention on the study for the second arrangement method.. This bearing arrangement can produce a good stiffness, and increase the first modal frequency to 15.1Hz. In addition, some analysis were down to study the load applied on the balls. The results show that the maximum load is up to 5000N with the stress of 2300MPa.

Pre-construction results of giant steerable science mirror for TMT

The Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) team is developing Giant Steerable Science Mirror (GSSM) for Thirty Meter Telescope (TMT) which has got into the preliminary design phase in 2017. To develop the passive support structure system for the largest elliptic-plane flat mirror and a smoothest tracking mechanism for the gravity-variant condition, CIOMP had developed a 1/4 scale, functionally accurate version of the GSSM prototype as the pre-construction of GSSM. The prototype incorporates the same optical-mechanical system and servo control system as GSSM. The size of the prototype mirror is 898.5mm×634mm×12.5 mm with elliptic-plane figure and is supported by 18 points whiffletree on axial and 12 points whiffletree on lateral. The main objective of the preconstruction includes validate the conceptual design of GSSM and increase more confidence when meet the challenge during the development of GSSM. The assembling, integration and verification of the prototype have been completed based on the test results. CIOMP has got the sufficient test results during the pre-construction phase and got into the preliminary design for GSSM.

Performance improvement of the Giant Steerable Science Mirror prototype: calibration, added-on damping treatment, and warping harness

The Giant Steerable Science Mirror (GSSM), the tertiary mirror of the Thirty Meter Telescope, is designed to meet complicated requirements. Calibration, added-on damping treatment, and warping harness will lower the cost to meet those strict requirements. A laser tracker and sphere-mounted retro-reflector (SMR) were used to calibrate the GSSM prototype (GSSMP). Use of non-uniform distribution SMRs will lower the systematic metrology error. The frequency response function between input excitation and dummy mirror responses is investigated to realize the design of tuned mass damping, which will be added on the GSSMP as a damping treatment to improve settling time and tracing performance. Finally, we utilized the warping harness, combining Zernike mode and bending mode, to relax the requirements of GSSM for low-order mirror figure aberrations.

Pre-construction progress of giant steerable science mirror for TMT

The Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) team is developing the Giant Steerable Science Mirror (GSSM) for Thirty Meter Telescope (TMT) which will enter the preliminary design phase in 2016. The GSSM is the tertiary mirror of TMT and consists of the world’s largest flat telescope mirror (approximately 3.4m X 2.4 m X 100mm thick) having an elliptical perimeter positioned with an extremely smooth tracking and pointing mechanism in a gravity-varying environment. In order to prepare for developing this unique mirror system, CIOMP has been developing a 1/4 scale, functionally accurate version of the GSSM prototype during the pre-construction phase of GSSM. The prototype will incorporate the same optomechanical system and servo control system as the GSSM. The size of the prototype mirror is 898.5mm×634mm×12.5mm with an elliptical perimeter. The mirror will be supported axially by an 18 point whiffletree and laterally with a 12 point whiffletree. The main objective of the preconstruction phase includes requirement validation and risk reduction for GSSM and to increase confidence that the challenge of developing the GSSM can be met. The precision mechanism system and the optical mirror polishing and testing have made good progress. CIOMP has completed polishing the mirror, the prototype mechanism is nearly assembled, some testing has been performed, and additional testing is being planned and prepared. A dummy mirror is being integrated into the cell assembly prototype to verify the design, analysis and interface and will be used when testing the prototype positioner tilt and rotation motions. The prototype positioner tilt and rotator structures have been assembled and tested to measure each subsystem’s jitter and dynamic motion. The mirror prototype has been polished and tested to verify the polishing specification requirement and the mirror manufacturing process. The complete assembly, integration and verification of the prototype will be soon finished. Final testing will verify the prototype requirements including mounted mirror surface figure accuracy in 5 different orientations; rotation and tilt motion calibration and pointing precision; motion jitter; and internally generated vibrations. CIOMP has scheduled to complete the prototype by the end of July 2016. CIOMP will get the sufficient test results during the pre-construction phase to prepare to enter the preliminary design for GSSM.

TMT Tertiary Mirror Optical Jitter Model Metrology

To measure the jitter performance of the Tirthy Meter Telescope tertary mirror(TMT M3), the model of jitter based on the accerleration signal is established. First and foremost, the procedure TMT required to processs the jitter data is presented. Then, the jitter of the system is divided into two parts: powered jitter and unpowered jitter. The metric of the jitter is required to be in the form of displacement or angle. So the frequency integration is involved to minify the error source introduced by the tending iterm in the time domain integration. In the evaluation of the power jitter, the accelerometers are used to obtain the rigid body motion of the tertiary mirror. The random decrement technique and dominated mode theory are applied to obtain the model of unpowered jitter. A classical telescope performances a 1.6” RMS jitter metric; for a four meter scale system, the resonance frequency is around 107 Hz, powering, with 0.14% damping. Consequently, the tip (183 Hz, 0.6%) and tilt (193 Hz, 0.19%) are both be measured. Using this model, the system engineers can estimate the unpowered jitter when the system is presented in the distortion.

Pre-construction of giant steerable science mirror for TMT

The Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) team is developing the Giant Steerable Science Mirror (GSSM) for Thirty Meter Telescope (TMT) which will get into the preliminary design phase in 2016. To develop the passive support structure system for the largest elliptic-plan flat mirror and smoothest tracking mechanism for the gravity-invariant condition, CIOMP is designing and building a 1/4 scale, functionally accurate version of the GSSM prototype. The prototype will incorporate the same optical-mechanical system and electric control system as the GSSM. The size of the prototype mirror is 898.5mm×634mm×12.5mm with elliptic-plan figure and will be supported by 18 points whiffletree on axial and 12 points whiffletree on lateral. The mirror surface figure will be evaluated by SlopeRMS which is the final evaluation method used in the actual GSSM. The prototype allows the mirror point to and be tested in five specified gravity orientations and meet the requirements of SlopeRMS. The prototype testing platform will have the interfaces with direct drive systems. The jitter testing will be implemented on the prototype system to verify the bearing, the encoder, the servo control algorithm in the low speed up to 5 arcsecond per second. The total prototype system configured mirror surface figure will be better than 1 micro radian SlopeRMS in each tested orientation. The positioner jitter will be less than 0.1 arcsecond RMS for tilt and rotator axis respectively and will be analyzed with frequency domain to meet the requirements of the TMT adaptive optics system. The pre-construction will be completed at the beginning of 2016 and provide the technical support to the preliminary design of GSSM.