Secondary Mirror Position Research Articles

Field of view-independent detection of secondary mirror position errors in coaxial reflecting space telescopes using deep learning

Field of view-independent detection of secondary mirror position errors in coaxial reflecting space telescopes using deep learning

Optical Pupil Shift Correction Method for Large Ground-Based Optical Telescopes

Cassegrain telescopes with larger apertures suffer significant optical pupil shifting errors caused by gravity and temperature gradients. This study constructs an optical model of a large Cassegrain telescope and a method to calculate and compensate its shifting error. First, the optical structure is simplified by incorporating only the most relevant telescope components, and the system optics is modeled using ray tracing. A computer-aided mounting-based method for calculating the misalignment error of primary and secondary mirrors is proposed, with the spatial position change of secondary mirrors as the input. Next, a compensation method based on the coma-free point theory of the Cassegrain system is proposed, with the main mirror’s optical axis as the reference. Finally, using a 4 m aperture telescope as an example, the gravity- and thermal-deformation-induced shifting error is simulated. Based on this simulation, the secondary mirror position is adjusted using a secondary mirror-adjustment mechanism Hexapod platform to compensate for the misalignment error. Both gravity and temperature are major causes of shift in the pupil position, with a maximum shift of 18 mm. After compensation, this shift is controlled within 1 mm. The modeling method and the simulation process mentioned in this research can also be used in the other relevant fields.

Compact Shortwave Infrared Imaging Spectrometer Based on a Catadioptric Prism.

This article demonstrates a compact prism imaging spectrometer method. A catadioptric curved prism is located at the secondary mirror position of the spectrometer and used to balance the aberrations, enlarge the dispersion width, and decrease the volume. A mathematical model of the prism and spectrometer is derived, which provides an optimal initial structure for a non-coaxial spectrometer, simplifying the optical design process and reducing the system volume. Using this method, a compact shortwave infrared imaging spectrometer with a 16° field of view is designed with an F-number/3, and the measured spectrum ranges from 0.95 to 2.5 μm. The performance is analyzed and evaluated. Laboratory testing results prove the excellent optical performance, and under the same specifications, the spectrometer length decreases by 40%.

基于多视场波前传感的次镜位置误差检测方法

基于多视场波前传感的次镜位置误差检测方法

ADRC control of a 6-DOF parallel manipulator for telescope secondary mirror

In view of the special requirements of the secondary mirror control system on large aperture telescopes, an improved 6-DOF parallel manipulator is designed and used to replace the traditional hexapod used in telescope secondary mirror position dynamic compensation. A highly robust active disturbance rejection controller (ADRC) is designed, which consists of a nonlinear tracking differentiator (NTD), an extended state observer (ESO), a nonlinear state error feedback law (NLSEF), and disturbance compensation. The ESO can track the all-order state variables, as well as estimate and compensate for unmodeled dynamics and total external disturbance of the system. The results of simulation indicate that the ADRC can improve tracking precision and control performance when it is compared with the proportion integration differentiation (PID) controller. The test results show that the absolute accuracy of the three dimensional parallel motions is about ± 4 μm, and the two dimensional tilts' is about 10 μrad. The control precision meets the system design for a telescope secondary mirror.

Self-Tuning Mechanism for the Design of Adaptive Secondary Mirror Position Control

Deformable mirrors (DMs) are electromechanical devices used in ground-based telescopes to compensate for the distortions caused by the atmospheric turbulence, the main factor limiting the resolution of astronomical imaging. Adaptive secondary mirrors (ASMs) represent a new type of DMs; two of them have been recently installed on the 8-m-class large binocular telescope (LBT). ASMs are able to jointly correct rigid and nonrigid wave-front distortions thanks to the use of force actuators distributed on the overall mirror surface. As an offset, each actuator needs to be piloted by a dedicated controller, whose parameters must be accurately tuned to obtain the desired mirror shape. At the present time, the calibration of the controller parameters is executed manually. This paper presents a novel automatic controller tuning procedure that does not rely on the modeling of the mirror dynamics. The experimental validation on a prototype reproducing the three innermost rings of the LBT ASM is reported.

The astigmatic aberration field in active primary mirror astronomical telescopes

Abstract Stemming from the exceptional results obtained with the pathfinding NTT telescope commissioned in 1989, the current generation of large astronomical telescopes, including the VLT, Gemini, Subaru, LBT, VST, and many more, use an active primary mirror in conjunction with the position of the secondary mirror to optimize performance with a cycle time typically measured in minutes over the course of acquiring a long exposure image. Recent discoveries, based in research enabled by the nodal aberration theory (NAT), have created a complete, linear model for the effect of, and the interaction of, the alignment compensators used during an exposure when an active primary mirror has a role in the process of maintaining performance in combination with the rotation of the secondary mirror around a specific external pivot point. This scenario, presented in the context of NAT, clearly illustrates important, limiting interactions between the secondary mirror position and the primary mirror astigmatic figure residual created through active control. It also points to the need for the wavefront sensor algorithm to anticipate that the astigmatic field will be binodal. The science of weak galactic lensing is exceptionally sensitive to binodal astigmatism, making this aberration field an urgent area of research.

FOCUS HISTORY OF THE HUBBLE SPACE TELESCOPE - LAUNCH TO MAY 1993

ABSTRACT Since the launch of the Hubble Space Telescope (HST) the secondary mirror of the telescope has been moved several times in order to collimate the telescope and also to define a position of best focus. In addition to these moves the focus position changes over time because of water desorption by the graphite epoxy in the metering truss. We report here the focus history of the telescope based on a knowledge of the mirror moves made and an analysis of desorption monitoring data obtained by the Faint Object Camera (FOC) in the F/96 mode and of the routine calibration data obtained by the Wide Field and Planetary Cameras. Focus values were extracted using two different methods. In the first method the distance between the center of the Point Spread Function (PSF) and the shadows of the pads supporting hte HST primary mirror are related to the focus error. In the second method an analytical formula for the PSF with variable aberration coefficients is fitted to the data. Focus positions derived from the two methods show good agreement. The data show that a desorption of about 83 microns has taken place since 1990 August 16. The desorption has not levelled off as expected from the trend of the earlier data. Long term variations of the secondary mirror position of ~3-15 microns from the "best" focus position have been observed. Variations ofthe order of 2-5 microns over an orbital period have also been noted. Focus changes resulting from secondary mirror movements greater than ~5 microns changes the point spread function significantly and makes deconvolution and quantitative measurements difficult.