To cater the need of growing astronomical community of India, there is a proposal to install 10-12m size optical-NIR telescope, equipped with state of the art back-end instruments . A telescope of this size is possible only, when primary mirror is made of smaller mirror segments. In order to get acquainted with segmented mirror telescope technology, at Indian Institute of Astrophysics Bangalore, we have initiated a project to develop a small prototype telescope made of small mirror segments. The proposed prototype telescope will use seven hexagonal mirrors, which will be supported by simple mirror support assembly and driven by indigenously developed voice coil based actuators. We also plan to make use of in-house developed inexpensive inductive edge sensor, which can precisely sense inter-segment relative displacement. The telescope mount is supposed to be Alt-Az and secondary mirror will be supported by trusses made of steel. The primary axes like elevation, azimuth and field de-rotator will be driven by direct drive motors. Though the primary objective of this telescope is to demonstrate the segmented mirror technology, however, we have designed the telescope in such way that it can also be used to a few dedicated science cases. The telescope is planned to be installed at Hanle, Ladakh India which is also a potential site for India's large telescope project. In this paper, we will present the progress made in opto-mechanical design as well development of other sub-systems required for the PSMT. The prototyping effort is one step toward realization of a large telescope in India and it is expected to be completed in two years period.
Segmented mirror telescopes (SMT) are built using several small hexagonal mirrors positioned and aligned by the three actuators and six edge sensors per segment to maintain the shape of the primary mirror. The actuators are responsible for maintaining and tracking the mirror segments to the desired position, in the presence of external disturbances introduced by wind, vibration, gravity, and temperature. The present paper describes our effort to develop a soft actuator and the actuator controller for prototype SMT at Indian Institute of Astrophysics, Bangalore. The actuator designed, developed, and validated is a soft actuator based on the voice coil motor and flexural elements. It is designed for the range of travel of ±1.5 mm and the force range of 25 N along with an offloading mechanism to reduce the power consumption. A precision controller using a programmable system on chip (PSoC 5Lp) and a customized drive board has also been developed for this actuator. The close loop proportional-integral-derivative (PID) controller implemented in the PSoC gets position feedback from a high-resolution linear optical encoder. The optimum PID gains are derived using relay tuning method. In the laboratory, we have conducted several experiments to test the performance of the prototype soft actuator as well as the controller. We could achieve 5.73- and 10.15-nm RMS position errors in the steady state as well as tracking with a constant speed of 350 nm/s, respectively. We also present the outcome of various performance tests carried out when off-loader is in action as well as the actuator is subjected to dynamic wind loading.
The Segmented Mirror Telescopes (SMT) are built using small hexagonal mirror segments placed side by side to form a monolithic primary mirror of very large size. The effective figure of such a segmented primary mirror is maintained against external disturbances introduced by gravity, temperature, wind and vibration with the help of primary mirror active control system. This active control system consists of two levels of control – global and local level. At the global scale, three actuators per segment and two edge sensors per intersegment sides are used to maintain the shape of the primary mirror. At the local level, actuator control system executes the commands generated by the global control loop. Every mirror segment is controlled with the help of three actuators, where the major role of these actuators is to provide a tip, tilt, and piston to the mirror segments. In this paper, we describe the actuator developed for 1.5m diameter Prototype Segmented Mirror Telescope (PSMT). The actuator for this telescope is a soft actuator based on the voice coil mechanism. This actuator is designed for with the range of travel of ±1.5mm and the force range of 25N along with an offloading capability to reduce the power consumption. The prototype actuator is undergoing different tests at Indian Institute of Astrophysics (IIA), Bangalore. The tracking rate of 324nm/s is achieved with the tracking error of 22.5 nm RMS.