From gyang@njit.edu Thu Oct 20 17:31:53 2005 Date: Thu, 20 Oct 2005 17:16:26 -0400 From: Guo Yang To: Haimin Wang Subject: TCS Haimin, Attached please find a description of the TCS. Please let me know if it needs more revise. Thanks. Guo [ Part 2: "Attached Text" ] \documentclass[reqno]{article} \usepackage{ae} % or {zefonts} \usepackage[T1]{fontenc} \usepackage[ansinew]{inputenc} \usepackage{amsmath} \usepackage{amssymb} \usepackage{graphicx} \begin{document} The telescope control system (TCS) is very important to the success of the New Solar Telescope (NST) project. Due to the limitation of our manpower and other resources, our telescope control system is also taking care of the environment control system and the dome control system which can be found in some large telescope systems. Our TCS is designed to be a distributed control system, which consists of a number of subsystems and a headquarter that oversees all the subsystems. Each subsystem is specialized to control one or more aspects of the telescope and/or the dome. Currently we have four subsystems, which include Pointing and Tracking System, Active Optics Control System, Wavefront Sensing System and Thermal Control System. The function of the subsystems are obvious from their names. The functions of the headquarter system are: Receiving or polling status information and other parameters from subsystems; Getting control commands from GUI and call relevant subsystems and accomplish the requests; Getting observation commands from GUI, resolve the subsystems involved and (potential conflicts) and call corresponding subsystems. The added layer of the headquarter system greatly improves the flexibility of the TCS and make the whole system more extensible. The communication between the subsystems and the headquarter is handled by the Internet Communication Engine (Ice, http://www.zeroc.com/). Compared with CORBA (Common Object Request Broker Architecture) which was used in some of telescopes' control systems, Ice is a communication middleware that is of great feature completeness and very high performance. Right now, we have developed a communication interface that will be used in all the subsystems and headquarter. We keep the communication interface very simple deliberately to ensure high efficiency. In addition, a simple communication interface will not become a burden to the developers, some of whom are not very familiar with the Ice communication middleware. We use XML (Extensible Markup Language) as the format of the messages that are exchanged between subsystems and the headquarter. The readable message format has great advantages over binary format data that was used in older systems. Since the amount of information exchanged in the control system is not very large, this won't pose a problem for the network either. The Pointing and Tracking system (TPTS) is responsible for the pointing and tracking of the telescope. And it assumes the responsibilities of controlling the dome and the mirror cover as well. The developer has made the TPTS's kernel and the command processor working. Since the new dome will be installed very soon, the developer is focused on the dome control part now. The Adaptive Optics Control System (AOCS) is the subsystem that implements the function of active optics by adjusting the figure of the primary mirror and the position of the secondary mirror. The active optics is only responsible for correcting the low frequency optical aberrations caused by gravitation force, the residual manufacturing error, alignment error between the primary and second mirrors and perhaps the wind buffet effect. Some errors are very hard to correct by adjusting the figure of the primary mirror, such as defocus and other errors caused by misalignment between primary and secondary mirrors. The secondary mirror will be mounted on a hexapod with six degrees of freedom. By adjusting the position of the secondary mirror, the misalignment of primary and secondary mirror and the defocus error can be corrected easily. For the rest of the low order optical errors, the figure of the primary mirror will be adjusted by posing or pulling actuators installed on the back of the primary mirror. Our goal is to make a close-loop system by utilizing the wavefront aberration information from the Wavefront Sensing System. At the first step we may use a pre-calculated table to make the correction based on the position of the primary mirror. Right now the hexapod mirror control is fully working. A Shack-Hartmann wavefront sensing system will be used in the Wavefront Sensing System to provide the wavefront aberration information for the AOCS. Since the AOCS works at a relatively low frequency (1 or 2 corrections per minute), all the calculations will be done by software running on a high-end PC. Right now, the developers have finished the software calculating the Zernike polynomial coefficients. The remaining work will be to develop image acquisition part and the communication part. The Thermal Control System will include a weather station that can provide environment data including ambient temperature, wind speed and direction, humidity and pressure. Even more importantly, this subsystem will also monitor the temperature sensors that associated with the primary mirrors and with the subsequent optical elements of the telescope. It will ensure that the primary mirror be kept at a relative stable temperature by using an air-knife. This is very important for the local seeing and thus the final image quality. \end{document}