Telescope Control System Research Articles

Cooling air flow field in the primary mirror temperature control system of a solar telescope

The temperature distribution and the difference with the ambient temperature of the primary mirror are crucial factors affecting the observational quality of the solar telescope. We designed a flow field structure of the primary mirror temperature control system based on the 2.5-meter Wide-field and High-resolution Solar Telescope(WeHoT), analyzed the flow conditions and heat transfer capacity of the overall flow field, and then carried out five stages of optimization iteration and simulation guided by the analysis results. A flow field structure for the primary mirror temperature control system with high efficiency and high uniformity is summarized. The simulation results for the final solution indicate an average surface temperature of the primary mirror at 20.064°C, with a maximum temperature difference within the reflective surface of 0.809°C, and a reduced difference with the ambient temperature of 0.411°C. Surface thermal deformation is less than 0.2 μm, with an RMS value of 18.05 nm, achieving the ideal state. This work can provide a theoretical foundation and valuable insights for future research on the temperature control system of the primary mirror in large-aperture solar telescopes.

Precision space telescope attitude determination and control system design principles

In order to preclude the impact of atmospheric factors on the quality of received data, state of the art astronomy research projects require spacecraft mounted telescopes. However, such designs entail the issue of sustaining the accuracy of telescope boresight attitude relative to the object of observation for the required period of time. This paper is a summary of design principles for an attitude determination and control system of a space telescope mounted rigidly onboard a spacecraft. The paper formulates requirements for attitude accuracy, and proposes a viable equipment configuration as well as a three- stage attitude control algorithm. The first stage of the algorithm ensures primary boresight pointing towards the target object prior to the object being captured in the telescope’s FoV based on star tracker and angular rate sensor data. The second stage of the algorithm ensures boresight positioning relative to the target object direction up to an error value allowing for the required image quality, based on telescope readings. The third stage of the algorithm ensures telescope boresight holding relative to the target object direction with a required accuracy for the duration of exposure. Interferences from operating satellite equipment are reviewed, and the principles of reducing their impact on sighting accuracy are formulated. Low inclination, low eccentricity geostationary orbit is recommended as the best operating environment for the space telescope’s scientific instrumentation, as well as for providing a continuous data interface between spacecraft and ground segment.

Improving the telescope guiding with field stabilization on the Very Large Telescope/Unit Telescopes

Acquisition and guiding services have been provided at major telescopes in order to improve the quality and efficiency of scientific observations. Field stabilization (FS) is an essential component of the telescope control system. With the arrival of new-generation instruments, the stabilization requirements have increased. Higher correction frequency is required to minimize the residuals. A major re-engineering of the CCD acquisition software now enables an increase of the operational frequency from 25 to 87 Hz. We analyze and discuss the performances of FS at various frequencies on the Very Large Telescope with its 8-m telescopes including the Adaptive 8-m telescope equipped with a deformable secondary mirror. We will report on the analysis of the residuals in function of several operational key parameters, e.g., the loop frequency, the magnitude of the selected telescope guide star, etc. We will also discuss the possible automatization of the FS during night operations.

RACS2: a framework of a remote autonomous control system for telescope observation and its application

With the increasing demand for astronomical observations, telescope systems are becoming increasingly complex. Thus, the observatory control software needs to be more intelligent. It has to control each instrument inside the observatory, finish the observational tasks autonomously, and report the information to users if needed. We developed a distributed autonomous observatory control framework named the Remote Autonomous Control System 2nd (RACS2) to meet these requirements. Rich features are integrated into the RACS2 framework. The RACS2 is a modular framework, in which each device control software and system services are implemented as different components. Furthermore, the RACS2 framework assimilates new techniques, such as a lightweight message and serialization mechanism. RACS2 also has good compatibility with other frameworks or ecosystems such as Python and the EPICS (Experimental Physics and Industrial Control System). The RACS2 framework can communicate with EPICS-based and Python-based software. With the help of these features, key system components like executor, scheduler are developed. The executor component can support sophisticated tasks. Autonomous observation can be achieved by the scheduler component. A set of web-based graphical user interface (GUI) is designed to help control and manage the framework remotely. Based on the RACS2 framework, we have implemented the Dome A Twins (DATs) telescope’s observation system and the space object observation system. The systems have been operated for more than 1 year. The RACS2 framework also has been used in our other projects like camera control system of Wide Field Survey Telescope (WFST) and its observatory control system, and the design can be a useful reference for the development of other frameworks.

The HETDEX Instrumentation: Hobby–Eberly Telescope Wide-field Upgrade and VIRUS

Abstract The Hobby–Eberly Telescope (HET) Dark Energy Experiment (HETDEX) is undertaking a blind wide-field low-resolution spectroscopic survey of 540 deg2 of sky to identify and derive redshifts for a million Lyα-emitting galaxies in the redshift range 1.9 < z < 3.5. The ultimate goal is to measure the expansion rate of the universe at this epoch, to sharply constrain cosmological parameters and thus the nature of dark energy. A major multiyear Wide-Field Upgrade (WFU) of the HET was completed in 2016 that substantially increased the field of view to 22′ diameter and the pupil to 10 m, by replacing the optical corrector, tracker, and Prime Focus Instrument Package and by developing a new telescope control system. The new, wide-field HET now feeds the Visible Integral-field Replicable Unit Spectrograph (VIRUS), a new low-resolution integral-field spectrograph (LRS2), and the Habitable Zone Planet Finder, a precision near-infrared radial velocity spectrograph. VIRUS consists of 156 identical spectrographs fed by almost 35,000 fibers in 78 integral-field units arrayed at the focus of the upgraded HET. VIRUS operates in a bandpass of 3500−5500 Å with resolving power R ≃ 800. VIRUS is the first example of large-scale replication applied to instrumentation in optical astronomy to achieve spectroscopic surveys of very large areas of sky. This paper presents technical details of the HET WFU and VIRUS, as flowed down from the HETDEX science requirements, along with experience from commissioning this major telescope upgrade and the innovative instrumentation suite for HETDEX.

From main axes control to flexible body control of large telescope structures

AbstractAstronomers have to point their telescopes with extreme high precision to their targets on the sky. The early craftsmen, which built their telescope mounts, were masters in mechanics and developed sophisticated axis mechanisms based on clockwork motors for the hour angle. The situation changed with the growth of the size of the optical telescopes, the upcoming of large radio telescopes, the related transition to elevation over azimuth mounts and the introduction of electrical main axes drives. The new telescopes were equipped with gears of high transmission ratios, and the dynamics of the motor control was limited by mechanical resonances between the rotors of the motors and the mass of the telescope structure. Early layout tools for the controllers were lumped mass models for the telescope structure including gear stiffnesses and inertias of rotors and brakes, with the free rotor and locked rotor frequencies as critical characteristics influencing the servo loop performance. Nowadays, static and dynamic analysis is performed with the help of finite element models, and the axes controllers are optimized with end‐to‐end models. The emergence of large radio telescopes initiated the development of additional compensation methods for environmental influences as temperature and wind on the shape of the reflectors and their pointing to the sky. Additional active elements on the telescope as surface actuators and subreflector positioners, and additional state sensors on the structure allow the identification and compensation of structural deformations. The method is called flexible body control (FBC). The increasing size of the optical telescopes initiated the development of new mirror technologies as thin meniscus or segmented mirrors in sizes not feasible with the passive supported thick meniscus mirrors. These mirrors need active elements as shape actuators, segment positioners, and wavefront and edge sensors. The method is called active optics (AcO). Astronomical observations in the visible are disturbed by atmospheric blur, and the operability is depending of the active compensation of that blur, based on a fast wavefront sensor analyzing an artificial star and an additional fast wavefront corrector. The method is called adaptive optics (AdO). The following text gives an overview on the historic development of telescope control systems from the viewpoint of the telescope mount and its axes control systems, finally culminating in FBC applications. Actual examples are the large millimeter telescope LMT/GTM in Mexico, the advanced technology solar telescope DKIST in Hawaii, and the airborne telescope of SOFIA, the Stratospheric Observatory for Infrared Astronomy with its base in Palmdale, California.

Azimuth Control for Large Aperture Telescope Based on Segmented Arc Permanent Magnet Synchronous Motors

A tracking control algorithm based on active disturbance rejection controller (ADRC) is proposed to overcome the telescope’s mount fluctuation. The fluctuations are caused by internal and external disturbance when the large aperture telescope runs at ultra-low speed with direct drive. According to the high-precision and high-stability requirements of a large aperture telescope, the ADRC position controller is designed based on segmented arc Permanent Magnet Synchronous Motors (arc PMSMs). The tracking differentiator of ADRC is designed to undergo a transition process to avoid overshoot in the position loop. The speed of target tracking process is observed by the extended state observer and the position information in the system is estimated in real time. The current control variable of the segmented arc PMSM is generated by implementing a non-linear state error feedback control law. The simulation results demonstrate that the proposed control strategy can not only accurately estimate the position and speed of the tracking target, but also estimate the disturbance to compensate the control variables. Experiments showed that the speed error is less than 0.05′ s−1 when using the ADRC, and it can realize high tracking performance when compared with PID controller, which improves the robustness of a large aperture telescope control system.

Методика побудови первинної матриці похибок радіотелескопа РТ-32 в автоматизованому режимі

2020 was the year of introduction of the Ukrainian new generation radio telescope RT-32 into the experimental operation. The test results of maser hydrogen and hydroxyl lines obtained during the experimental operation confirmed the correctness of the calculations and technological solutions of Ukrainian scientists and manufacturers Consortium. One of the further development directions of RT-32 as a radio astronomical research tool is to increase the accuracy of pointing the radio telescope to radio astronomical sources. One of the further development directions of RT-32 as a radio astronomical research tool is to increase the accuracy of pointing the radio telescope to astronomical radio sources. The latter is to be achieved by automating the processes of guidance error matrices formation and their integration during the observations. The formation of such a matrix presupposes taking into account the structural features of the antenna system and weather condition. The paper presents the results of geodetic measurements of the antenna system surface on different elevation angle, construction of the 3D model of the reflector. The method of constructing the error matrix, which at this stage of research provides the necessary simplicity of perception and interpretation of the obtained results by the human operator, is proposed. The results of the developed method verification using reference radio sources are given and the error matrices of elevation and azimuth pointing (dimension 81x81 elements) obtained with the use of said method are presented. The introduction of the results presented in the article into the radio telescope control system allowed increasing the accuracy of RT-32 radio telescope pointing in the C- and K- bands to the value of ~36″. This work partially was supported by Latvian Council of Science project "Joint Latvian-Ukrainian study of peculiar radio galaxy “Perseus A” in radio and optical bands. Nr: lzp-2020/2-0121".

Trends in Architecture and Middleware of Radio Telescope Control System

The control system is the central control unit of the radio telescope. It is used to monitor, control, coordinate, and manage software and hardware systems so as to satisfy the requirements of high-precision control in astronomical observation of radio telescope. The control system architecture is the foundation for the implementation of the control system, which determines the stability, scalability, and maintainability of the control system. Furthermore, the architecture design of the control system is closely geared towards the technological development of radio telescope and computer software architecture. In this article, we analyze the characteristic of the control system of a radio telescope in various steps and discuss the development of their architecture and middleware framework. System architecture and middleware framework of control system also serve as a useful reference for the design of other radio telescope control systems.

Efficient calculation of solar position using rectangular coordinates

There have been many algorithms published for calculating the position of the Sun. Most of them are simplified in some way, offering either reduced accuracy or a limited range of valid times. A different approach to the problem is presented here, derived from the approach used by some large astronomical telescope control systems, in which rectangular coordinates are used to transform the celestial coordinates into a direction as seen from a site (or multiple sites) on the surface of the Earth. This reduces the computational complexity for the conversion to topocentric coordinates. For applications in which calculations are repeated at a high rate, such as for solar tracking, this approach also enables a simple interpolation to be used for the Sun’s equatorial position vector, which brings a further dramatic reduction in the computational complexity of the most detailed published algorithms, with a smaller loss of accuracy compared with any of the simpler published algorithms.

Friction compensation for an m-Level telescope based on high-precision LuGre parameters identification

Friction torque severely weakens the tracking accuracy and low-speed stability of an m-level TCS (telescope control system). To solve this problem, a friction compensation method is proposed, based on high-precision LuGre friction model parameters identification. Together with dynamometer calibration, we first design a DOB (disturbance observer) to acquire high-accuracy TCS friction value in real time. Then, the PSO-GA (a hybrid algorithm combined particle swarm optimization algorithm and genetic algorithm) optimization algorithm proposed effectively and efficiently realizes the LuGre model parameters identification. In addition, we design a TCS controller including DOB and LuGre model parameters identification based on double-loop PID controller for practical application. Engineering verification tests indicate that the accuracy of DOB calibrated can reach 96.94%of the real measured friction.When azimuth axis operates in the speed cross-zero work mode, the average positive peak to tracking error reduces from 0.8926″ to 0.2252″ and the absolute average negative peak to tracking error reduces from 0.8881″ to 0.3984″. Moreover, the azimuth axis tracking MSE reduces from 0.1155″ to 0.0737″, which decreases by 36.2%. Experimental results validate the high precision, facile portability and high real-time ability of our approach.

Метод автоматичної побудови матриці похибок радіотелескопа РТ-32. Методика автоматичного оцінювання похибок наведення

On March 15th, 2021, scientists of the National Space Facilities Control and Tests Center and the Radio Astronomical Institute of the National Academy of Sciences of Ukraine carried out preliminary observations with the Ukrainian new generation radio telescope RT-32 (Zolochiv, Lviv region). The extragalactic radiation of radio galaxy 3C84 (Perseus-A), masers from the galactic molecular cloud W3, radio emission of methanol maser from the galactic radio source G188.946 + 0.886 were observed and successfully recorded. Observations were performed as training in the framework of preparation for the launch of a joint Ukrainian-Latvian radio astronomy project lzp-2020/2-0121. The results of the observations confirmed the world level of RT-32 radio telescope characteristics, the efficiency of the primary error matrix and revealed several shortcomings in the functioning of the tracking system. It was found that the primary tracking error matrix has insufficient discreteness and contains errors of the first and second types. In the article, we present a method of automatic construction of the radio telescope error matrix according to the data of a radiometric receiver and receivers-recorders. The method of construction provides automatic processing of the obtained radiometric data. The results of verification of the developed method using the reference radio sources of different types and the elements of tracking errors’ matrix by the elevation and azimuth obtained when using it are presented. The results obtained with the proposed method were included in the radio telescope control system and allowed us to increase the aiming accuracy of the RT-32 radio telescope.

PI-shaped LQG control design for adaptive optics systems

Adaptive optics systems are extensively used in astronomy to obtain diffraction-limited images of celestial objects with ground-based telescopes. The crucial point is to control a deformable mirror to compensate for the distortions introduced by atmospheric turbulence in the incoming wavefront. In this paper, a modal PI-shaped LQG controller taking advantage of model knowledge is designed; it compares favorably to standard PI and Kalman-based controllers used in state-of-the-art control systems of telescopes. The proposed architecture is validated with simulations and experiments on a laboratory adaptive optics test-bench.

Unanticipated Fault Detection Technology for Tip/Tilt Mirror Control System of Large Aperture Telescope

Large Aperture telescopes are usually placed in some special environment. Once a malfunction occurs, it’s inconvenient for maintenance. This paper presents those unanticipated failures in tip/tilt mirror control system of large aperture telescopes, and tries to propose a detection strategy based on BP neural network. Firstly, the paper analyses the failure mechanism of the software and the hardware of the tip/tilt mirror control system. Then the known fault data are used to train the BP neural network in order to obtain an accurate detection model. Finally, the unanticipated data are mixed into the known fault data, and the model is used to detect unanticipated faults. The simulation results show that the model can be used to detect the unanticipated fault in tip/tilt mirror control system effectively.

IGRINS Slit-viewing Camera Software

We have developed observation control software for the Immersion GRating INfrared Spectrometer (IGRINS) slit-viewing camera module, which maintains the position of an astronomical target on the spectroscopic slit. It is composed of several packages that monitor and control the system, acquire the images, and compensate for the tracking error by sending tracking feedback information to the telescope control system. For efficient development and maintenance of each software package, we have applied software engineering methods, i.e., a spiral software development with model-based design. It is not trivial to define the shape and center of astronomical object point spread functions (PSFs), which do not have symmetric Gaussian profiles in short exposure (<4 s) guiding images. Efforts to determine the PSF centroid are additionally complicated by the core saturation of bright guide stars. We have applied both a two-dimensional Gaussian fitting algorithm (2DGA) and center balancing algorithm (CBA) to identify an appropriate method for IGRINS in the near-infrared K-band. The CBA derives the expected center position along the slit-width by referencing the spillover flux ratio of the PSF wings on both sides of the slit. In this research, we have compared the accuracy and reliability of the CBA to the 2DGA by using data from IGRINS commissioning observations at McDonald Observatory. We find that the performance of each algorithm depends on the brightness of the targets and the seeing conditions, with the CBA performing better in typical observing scenarios. The algorithms and test results we present can be utilized with future spectroscopic slit observations in various observing conditions and for a variety of spectrograph designs.

A New Mechanical Resonance Suppression Method for Large Optical Telescope by Using Nonlinear Active Disturbance Rejection Control

Aiming at solving the problem of the multi-low-frequency mechanical resonances appearing in the large optical telescope control system, this paper proposes a novel control method based on nonlinear active disturbance rejection control (NADRC) and proportional-integral (PI) control. In the proposed control framework, a nonlinear tracking differentiator (NTD)-based feedforward control is designed to improve the tracking performance of the system. Then, the principle of suppression of mechanical resonance of this method is analyzed. Compared with the most commonly used acceleration feedback control (AFC) method, the theoretical analysis shows that the proposed method is more effective for suppressing the low-frequency mechanical resonance. Finally, the proposed method is applied to a large optical telescope, and the experimental results show that the proposed method is better than AFC.

Image Size and Control System Developments of the Airborne Telescope SOFIA

The SOFIA telescope is a worldwide unique observatory that enables infrared astronomy aboard a Boeing 747SP at altitudes of up to 45[Formula: see text]kft. Contrary to any ground-based telescope, SOFIA is exposed not only to aerodynamic forces but also aircraft motion and excitation. Nevertheless, the ambitious scientific goals require a stable platform and very precise pointing. As of now, the telescope can be considered diffraction-limited in the far-infrared wavelengths beyond 50[Formula: see text][Formula: see text]m. A careful study of the different sources of blur revealed that image jitter is among the most influential. During the course of a flight, the telescope is exposed to various excitation levels, leading to deformation of its flexible structure and vibrations in a wide range of frequencies. Since SOFIA entered its operational phase, continuous efforts have been made to develop and implement upgrades in the pointing and control system. The original design was a robust and conservative structure aimed to ensure safe operations with several different science instruments and many unknown parameters. In recent years, more and more agile system components have followed to tackle the residual image motion. This paper introduces the telescope control system followed by a summary of recently installed upgrades and ends with an outlook on future developments on our way to diffraction-limited imaging beyond 25[Formula: see text][Formula: see text]m.

Analysis of the Subdivision Errors of Photoelectric Angle Encoders and Improvement of the Tracking Precision of a Telescope Control System.

Photoelectric angle encoders, working as position sensors, have a great influence on the accuracy and stability of telescope control systems (TCS). In order to improve the tracking precision of TCS, a method based on subdivision error compensation for photoelectric angle encoders is proposed. First, a mathematical analysis of six types of subdivision errors (DC error, phase error, amplitude error, harmonic error, noise error, and quantization error) is presented, which is different from the previously used analysis based on the Lissajous figure method. In fact, we believe that a mathematical method is more efficient than the figure method for the expression of subdivision errors. Then, the distribution law and period length of each subdivision error are analyzed. Finally, an error compensation algorithm is presented. In a real TCS, the elevation jittering phenomenon occurs, which indicates that compensating for the amplitude error is necessary. A feed-forward loop is then introduced into the TCS, which is position loop- and velocity loop-closed, leading to a decrease of the tracking error by nearly 54.6%, from 2.31” to 1.05”, with a leading speed of 0.25°/s, and by 40.5%, from 3.01” to 1.79”, with a leading speed of 1°/s. This method can realize real-time compensation and improve the ability of TCS without any change of the hardware. In addition, independently of the environment and the kind of control strategy used, this method can also improve the tracking precision presumably because it compensates the measuring error inside the photoelectric angle encoder.

Rapid telescope pointing calibration: a quaternion-based solution using low-cost hardware

A telescope control system relies on a pointing model to determine the gimbal angles that aim the telescope toward a desired target. High-accuracy telescope pointing models include parameters that describe the mount/telescope orientation as well as common mechanical effects. For professional telescopes, calibrating the pointing model requires careful initial alignment around a nominal orientation (e.g., leveling) followed by sightings of dozens to hundreds of stars to fit the model parameters. While this approach is effective for observatories, applications such as transportable optical ground stations for communications, space situational awareness, or astronomy using low-cost telescope networks can benefit from a more rapid calibration approach. We formulate a quaternion-based pointing model that utilizes measurements from an externally mounted star camera to compromise between calibration speed and accuracy. A key aspect of this formulation is that it is completely agnostic to the orientation of the telescope/mount so that no manual prealignment is required. We derive angle and rate commands for telescope pointing and tracking based on the model. We present results from a 15-min calibration procedure on a very low-cost telescope that demonstrated pointing to an accuracy of 53 arc sec RMS in azimuth and 66 arc sec RMS between 20-deg and 70-deg altitude.

Reliability Analysis of Main-axis Control System of the Antarctic Equatorial Astronomical Telescope Based on Fault Tree

The Antarctic astronomical telescopes work chronically on the top of the unattended South Pole, and they have only one chance to maintain every year. Due to the complexity of the optical, mechanical, and electrical systems, the telescopes are hard to be maintained and need versatile expedition teams, which means that an excessive awareness is essential for the reliability of the Antarctic telescopes. Based on the fault mechanism and fault mode of the main-axis control system for the Antarctic equatorial astronomical telescope AST3-3 (Antarctic Schmidt Telescope 3-3), the method of fault tree analysis is introduced in this article, and we obtain the importance degree of the top event from the importance degree of the bottom event structure. From the above result, the hidden problems and weak links of the system can be effectively found out, which will indicate the direction for improving the stability of the system and optimizing the design of the system.