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Wednesday, August 04, 2021

Explained: How PASIPHAE will peep into the unknown regions of the sky

The development of a vital instrument, which will be used in upcoming sky surveys to study stars, is being led by an Indian astronomer. What is PASIPHAE, and why is it important?

Written by Anjali Marar | Pune |
Updated: June 14, 2021 10:24:04 am
The polarimeter is being built at the instrumentation facility at IUCAA, Pune. (Photo via IUCAA)

The mysteries surrounding the origin of the universe continue to draw human curiosity. The development of a vital instrument, which will be used in upcoming sky surveys to study stars, is being led by an Indian astronomer. The project has been funded by the world’s leading institutions, signalling India’s growing expertise in building complex astronomical instruments.


Polar-Areas Stellar-Imaging in Polarisation High-Accuracy Experiment (PASIPHAE) is an international collaborative sky surveying project. Scientists aim to study the polarisation in the light coming from millions of stars.

The name is inspired from Pasiphae, the daughter of Greek Sun God Helios, who was married to King Minos.

The survey will use two high-tech optical polarimeters to observe the northern and southern skies, simultaneously.

It will focus on capturing starlight polarisation of very faint stars that are so far away that polarisation signals from there have not been systematically studied. The distances to these stars will be obtained from measurements of the GAIA satellite.

By combining these data, astronomers will perform a maiden magnetic field tomography mapping of the interstellar medium of very large areas of the sky using a novel polarimeter instrument known as WALOP (Wide Area Linear Optical Polarimeter).

Scientists from the University of Crete, Greece, Caltech, USA, Inter-University Centre for Astronomy and Astrophysics (IUCAA), India, the South African Astronomical Observatory and the University of Oslo, Norway, are involved in this project, steered by the Institute of Astrophysics, Greece.

The Infosys Foundation, India, Stavros Niarchos Foundation, Greece and USA’s National Science Foundation have each provided a grant of $1 million, combined with contributions from the European Research Council and the National Research Foundation in South Africa.

Why is PASIPHAE important?

Since its birth about 14 billion years ago, the universe has been constantly expanding, as evidenced by the presence of Cosmic Microwave Background (CMB) radiation which fills the universe.

Immediately after its birth, the universe went through a short inflationary phase during which it expanded at a very high rate, before it slowed down and reached the current rate. However, so far, there have only been theories and indirect evidence of inflation associated with the early universe.

A definitive consequence of the inflationary phase is that a tiny fraction of the CMB radiation should have its imprints in the form of a specific kind of polarisation (known scientifically as B-mode signal).

All previous attempts to detect this signal met with failure mainly due to the difficulty posed by our galaxy, the Milky Way, which emits copious amounts of polarised radiation.

Besides, it contains a lot of dust clouds that are present in the form of clusters. When starlight passes through these dust clouds, they get scattered and polarised.

“It is like trying to see faint stars in the sky during daytime. The galactic emission is so bright that the polarisation signal of CMB radiation is lost,” said S Maharana, a PhD student at IUCAA who is involved in this project.

The PASIPHAE survey will measure starlight polarisation over large areas of the sky. This data along with GAIA distances to the stars will help create a 3-Dimensional model of the distribution of the dust and magnetic field structure of the galaxy. Such data can help remove the galactic polarised foreground light and enable astronomers to look for the elusive B-mode signal.

What is WALOP?

Wide Area Linear Optical Polarimeter (WALOP) is an instrument, when mounted on two small optical telescopes, that will be used to detect polarised light signals emerging from the stars along high galactic latitudes.

A WALOP each will be mounted on the 1.3-metre Skinakas Observatory, Crete, and on the 1-metre telescope of the South African Astronomical Observatory located in Sutherland.

“Once built, they will be unique instruments offering the widest ever field of view of the sky in polarimetry. It will be capable of capturing images within ½ ° by ½ ° area of the sky during every exposure,” said A N Ramaprakash, senior IUCAA scientist and fellow at IA, Crete.

In simple terms, the images will simultaneously have the finest of details of a star along with its panoramic background.

WALOP will operate on the principle that at any given time, the data from a portion of the sky under observation will be split into four different channels. Depending on the manner in which light passes through the four channels, the polarisation value from the star is obtained. That is, each star will have four corresponding images which when stitched together will help calculate the desired polarisation value of a star.

As the survey will focus on sky areas where very low polarisation values (<0.5 per cent) are expected to emerge, a polarimeter with high sensitivity and accuracy clubbed with a large field of view was needed, so WALOP was planned sometime in 2013.

This was after the success of the RoboPol experiment survey during 2012-2017, in which some PASIPHAE collaborators were involved. Since then, the design, fabrication and assembly, led by Ramaprakash, is underway.

WALOP and its predecessor RoboPol share the single shot photometry principle. But the 200 kg weighing WALOP will be capable of observing hundreds of stars concurrently present both in the northern and the southern skies as opposed to RoboPol, which has a much smaller field of view in the sky.

Development of the instrument is in an advanced stage currently and progressing at the instrumentation facility in IUCAA.

Why WALOP will be deployed on 1-metre class optical telescopes

A major limitation while using large optical telescopes is that they cover a relatively smaller area of the sky, defeating the overall purpose of PASIPHAE.

Whereas the 1-metre-class telescopes enable both larger fields of view of the sky combined with the minutest details of distant stars.

Since the sky survey will continue for four years, it will be a challenge to devote a sizable amount of observation time of any large telescope solely towards studying star polarisation.

“So, the maximum observation time offered by the smaller telescopes will be diverted for the PASIPHAE sky survey using WALOP,” added Ramaprakash, also a visiting faculty at Caltech.

The attempt to press-in the 1-metre-class telescopes is also to demonstrate that breakthrough science and challenging experiments can be undertaken using smaller telescopes, even in the era of large and extremely large telescopes.

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