Why measure the polarisation of starlight at all?

Polarisation arises from processes such as scattering off dust, alignment of grains by magnetic fields, and synchrotron radiation. Measuring it reveals magnetic field directions, the geometry of scattering material, and the nature of emission, information that the brightness and colour of a source alone cannot provide.

Why is polarimetry considered technically demanding?

Astronomical polarisation signals are often a fraction of a percent, smaller than many instrumental effects. Measuring them reliably requires modulating the signal, using dual-beam analysers, and carefully calibrating the polarisation introduced by the telescope itself.

Astronomical Polarimeters

Astronomical polarimeters measure the polarisation of light from celestial sources, revealing magnetic fields, scattering geometries, and dust properties that intensity alone cannot show.

Definition

An astronomical polarimeter is an instrument that determines the polarisation state of incoming light, typically by combining a retarder and a polarising analyser ahead of a detector and measuring how the signal changes as the polarisation is modulated.

Scope

This topic covers the Stokes parameters that describe polarisation, the optical components used to analyse it such as wave plates, polarisers, and Wollaston prisms, modulation techniques to suppress systematic errors, imaging and spectropolarimetry, and the calibration challenges of measuring polarisation fractions that are often well below one percent.

Core questions

What physical information does polarisation encode?
How is the polarisation state of light measured?
Why is modulation important for accurate polarimetry?
What limits the precision of polarisation measurements?

Key theories

Stokes parameters: The polarisation state of light is fully described by four Stokes parameters, which polarimeters recover by measuring intensities through different combinations of retarders and analysers.
Modulation and differential measurement: Rapidly switching a wave plate or using dual-beam analysers lets the polarised signal be measured differentially, cancelling many instrumental and atmospheric systematics.
Instrumental polarisation and calibration: Telescope and instrument optics introduce spurious polarisation that must be characterised with polarised and unpolarised standard stars to recover the true source polarisation.

Clinical relevance

Polarimetry maps interstellar and stellar magnetic fields, traces dust grain alignment and scattering in nebulae and circumstellar disks, diagnoses synchrotron emission in jets, and probes the geometry of unresolved sources, complementing imaging and spectroscopy.

History

Astronomical polarimetry grew from early visual and photographic measurements of starlight polarisation in the mid-twentieth century, which revealed the alignment of interstellar dust. Photoelectric and then array-based polarimeters, together with modulation techniques, raised precision to the level needed to study magnetic fields and faint polarised signals.

Key figures

John Tinbergen
David Clarke

Seminal works

tinbergen1996
clarke2010

Frequently asked questions

Why measure the polarisation of starlight at all?: Polarisation arises from processes such as scattering off dust, alignment of grains by magnetic fields, and synchrotron radiation. Measuring it reveals magnetic field directions, the geometry of scattering material, and the nature of emission, information that the brightness and colour of a source alone cannot provide.
Why is polarimetry considered technically demanding?: Astronomical polarisation signals are often a fraction of a percent, smaller than many instrumental effects. Measuring them reliably requires modulating the signal, using dual-beam analysers, and carefully calibrating the polarisation introduced by the telescope itself.