Jingxu Zhang

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 on the sensitivity of temperature gradient for large aperture SiC lightweight mirror based on active optics

With the aperture of large ground-based telescopes increasing, thermal issues appear more evidently. As a relatively large thermal expansion coefficient SiC (about 2.5x10-6 /K), it makes large aperture SiC lightweight primary mirror more sensitive to temperature gradient. Firstly, discuss thermal issue of the mirror seeing induced by the temperature difference between the mirror surface and ambient theoretically. Then analyze the mirror surface deformation under seven different steady-state temperature fields with a unit temperature load. A uniform axial gradient can cause a mirror surface deflection RMS which can reach 438.4 nm. According to the simulation results, it shows that the primary is most sensitive to a uniform axial gradient and least to uniform change. Lastly, the parameter of thermal control is determined through the above analysis with the error budget to get a better image quality.

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.

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.

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.

The design and analysis of 2m telescope’s K Mirror system

During the alt-azimuth telescope tracking, due to the frame structure of tracking support and relative movement of each mirror in Coude optical path, image plane is rotating. To eliminate the effects of image rotation for imaging and subsequent image processing, dove prism or K mirror are generally used. A set of K mirror system designed for 2m telescope. Affected by various errors in the alignment process, the rotating axis K, the optical axis of the K mirror, and the optical axis of the telescope’s optical system can’t be fully coincide. This causes the track optical axis drawn on the image is not a point, but a Pascal’s limacon. The impact caused by the various sources of error were analyzed in this paper and simulation results have important guiding significance for the alignment error distribution.