[Paper Review] Automation of PRL's Astronomical Optical Polarimeter with a GNU/Linux based distributed control system

S. Ganesh, Joshi, U. C.+4 more|ArXiv.org|Dec 1, 2009

Astronomical Observations and Instrumentation2 references3 citations

TL;DR

This paper presents the automation of PRL's astronomical optical polarimeter using a GNU/Linux-based distributed control system, replacing manual operations with CCD imaging and real-time control via RTAI-enabled kernels. The system enables remote, networked operation from anywhere on the local network, significantly improving signal-to-noise ratio through smaller apertures (6–10 arcsec), reducing background noise and enabling efficient, high-precision polarization measurements of faint sources like blazars and quasars.

ABSTRACT

Physical Research Laboratory's (PRL) Optical Polarimeter has been used on various telescopes in India since its development in-house in the mid 1980s. To make the instrument more efficient and effective we have designed the acquisition and control system and written the software to run on the GNU/Linux Operating System. CCD cameras have been used, in place of eyepieces, which allow to observe fainter sources with smaller apertures. The use of smaller apertures provides dramatic gains in the signal-to-noise ratio. The polarimeter is now fully automated resulting in increased efficiency. With the advantage of networking being built-in at the operating system level in GNU/Linux, this instrument can now be controlled from anywhere on the PRL local area network which means that the observer can be stationed in Ahmedabad / Thaltej as well or via ssh anywhere on the internet. The current report provides an overview of the system as implemented.

Motivation & Objective

• To overcome limitations in signal-to-noise ratio caused by background sky light in optical polarimetry by enabling smaller apertures (6–10 arcsec) than previously feasible.
• To eliminate human error and improve observational efficiency by replacing visual eyepiece-based source acquisition with CCD imaging and automated control.
• To enable remote operation of the polarimeter from any location on the local area network (LAN) or via SSH over the internet.
• To develop a fully integrated, distributed control system using in-house hardware and software, including real-time kernel drivers and custom firmware.
• To achieve high-precision polarization measurements by ensuring system linearity across a wide dynamic range (from low to several million counts).

Proposed method

• Deployed a PC/104-based embedded system with an RTAI-enabled GNU/Linux kernel for real-time control of mechanical and optical components.
• Integrated Onyx PC/104 counter/timer boards to record photon counts from photomultiplier tubes (PMTs) in photon-counting mode.
• Used a custom in-house stepper motor driver board to control a half-wave plate for fast modulation of the incoming light beam.
• Employed Atmel microcontroller-based subsystems (five AVR Atmega 8 chips) to control mechanical operations such as aperture and filter changes, temperature monitoring, and LCD display.
• Connected a Starlight Xpress SXV-H9 and SXVF-M25C CCD camera to a separate PC/104 board for real-time imaging and source centering via a GUI.
• Developed custom kernel-space device drivers for stepper motor boards, Onyx counter boards, and USB CCD devices, along with user-space GUIs (OPAL Controls and CCD Controls) for system operation.

Experimental results

Research questions

• RQ1 Can a distributed, networked control system based on GNU/Linux improve the efficiency and accuracy of astronomical optical polarimetry?
• RQ2 To what extent does replacing visual eyepiece-based source acquisition with CCD imaging reduce background noise and improve signal-to-noise ratio?
• RQ3 How effectively can real-time, remote control of a polarimeter be achieved using a PC/104-based embedded system with RTAI?
• RQ4 What level of system linearity and measurement accuracy can be achieved across a wide dynamic range of photon counts?
• RQ5 Can in-house developed firmware and drivers for microcontrollers and hardware components ensure reliable, repeatable operation in a remote observatory setting?

Key findings

• The system achieved a signal-to-noise ratio improvement by enabling the use of smaller apertures (6–10 arcsec) compared to the previous 15–20 arcsec, significantly reducing background sky contribution.
• The instrument demonstrated high system linearity, with 100% polarization measurements ranging from 97.5% to 99.5% when using a Glan prism, confirming accurate response across a wide dynamic range (from low to several million counts).
• Remote operation via the LAN or over the internet was successfully implemented, allowing observers to control the instrument from Ahmedabad or any remote location using SSH.
• The use of CCD cameras enabled precise, repeatable centering of faint sources (e.g., blazars and quasars) in the aperture, which was previously difficult with visual methods due to low contrast.
• The entire control system, including kernel drivers, user-space GUIs, and AVR microcontroller firmware, was developed in-house, ensuring full customization and integration.
• Observation data, including 24-fold half-wave plate position counts and telescope parameters, were saved incrementally to text files named by date, enabling systematic data logging and post-observation analysis.