Design and Implementation of Control System for Polarimeter

Abstract

Optical Polarimeter has been used as a back-end instrument of 1.2m telescope at Physical Research Laboratory’s Mt. Abu Observatory since 1980. It is a scientific instrument used to measure the degree of polarization in an astronomical object such as a comet, star or galaxy. Polarization data provides information about the geometry and other physical properties of the source, properties which cannot be inferred in any other way. Supporting hardware is required to automatize the polarimeter and collect data efficiently and accurately. This instrument was developed nearly three decades ago. For each part of the polarimeter there is a separate board and all these boards are interfaced by another board which makes the system bulky and complex. Now with advancement of electronics industry, there are many boards available in the market that can handle many functionality with lower price. The aim of this project is to develop a new compact control system using a Raspberry Pi board and replace the existing system. Raspberry Pi works as a central processing unit. It has 40 general purpose input/output pins apart from the usual peripheral ports such as the USB, ethernet, hdmi, micro-usb, micro-sd etc. Other required boards like GPS breakout, USB-CTR-32, stepper-motor driver board are connected through USB ports. Thermal imaging camera is connected through GPIO pins for cloud sensing. Apart from the control electronics, in the mechanical hardware the earlier rotary stepper motor based mechanism was replaced with a simpler linear stepper motor. This linear stepper motor needs a larger current than the previous one. Hence the ULN2003 current driver chip used earlier needed to be replaced with a ULN2065B chip. As the operating system of Raspberry Pi is linux and has in built networking, the instrument can be controlled over the internet. Use of simplified and compact electronics makes next generation instrument with multiple wavelength polarization to be measured simultaneously using beam-splitters, filters and multiple detectors. In this report, design of new control system with Raspberry Pi is described which is developed in PRL, Ahmedabad and implemented.