Wide Area Linear Optical Polarimeter (WALOP) lab development and Characterization

dhruv_walop

WALOP is an ambitious astronomical polarimeter which is going to be built as a part of the Polar-Areas Stellar Imaging Polarization High-Accuracy Experiment (PASIPHAE) collaboration between the University of Crete in Greece, the Inter-University Center for Astronomy and Astrophysics (IUCAA) in Pune, India, the California Institute of Technology (Caltech in the US, including the Caltech Optical Observatories and the Owens Valley Radio Observatory), the South African Astronomical Observatory, and the Institute of Theoretical Astrophysics (at the University of Oslo).

I am the only undergraduate student working on this project. I work on lab testing and optical design characterization of WALOP with my guide Prof. A. N. Ramaprakash at IUCAA, Pune. As an engineering undergraduate it is quintessential to understand the construction of a polarimeter along with a thorough understanding of Stokes Parameters used to find the state of polarization of light. WALOP makes use of 2 Wollaston Prisms (whose wedge angles differ by 45 degrees to each other) to split an incoming beam of light into 4 spots – 2 from each prism. Earlier polarimeters used identicial Wollaston Prisms in their design. The unique aspect of the instrument is that unlike earlier polarimeters such as RoboPol it would not make use of a Half Wave Retardation plate and so will not have any moving parts in the instrument.  This paper provides a fantastic review of RoboPol and how the polarization state of light is calculated.  Below is a diagram from the same paper showing the optical design principle for RoboPol. In WALOP, the Half Wave Retarder before the Wollaston Prisms (WP) would be absent.

robopol design

Although I cannot write the details of my work on my website, I can summarize my work primarily focused on 3 areas:

1)  Developing the lab setup for testing the optical design  for WALOP with off axis and on axis light sources, R band response and sources with varying polarization states.

2) Finding out the accuracy and precision for measurements of polarization percentage of low polarized light (<1% polarization) and the polarization angles using data analysis tools such as Image Reduction and Analysis Facility (IRAF) and Aperture Photometry Tool. I strongly recommend my juniors who are keen to work in these areas to look up these tools and learn how to use them as they provide a detailed statistical and mathematical insight into how astronomical data reduction takes place.

3) Investigating sources of errors such as due to the instrument and correcting for those in measurements.

Following is an image from the lab setup developed showing the 4 spots formed by the extraordinary and ordinary rays from the 2 Wollaston Prisms. Notice the 4 spots are approximating a Gaussian PSF which is important for good data to be produced.

 

walop_screenshot

An actual image of the sky that would be produced by WALOP is shown below. This is an image of stars being photographed by RoboPol. A star is split into 4 points through which the polarization state can be found out. Note that in RoboPol only a single CCD detector was used whereas in WALOP there would be 4 separate CCD detectors each for 1 spot to avoid confusion. Also, the IUCAA Digital Sampler Array and Controller project would be used to simultaenously read the 4 CCD detectors.

cog.png  The curve of growth plot for our selected software aperture around the artificial star.

radial

The radial profile of the image star is calculated as a function of the distance from the centre of the software aperture.

robopol.png Source: The RoboPol pipeline and control system

 

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