Polarization of Light from Basics to Instruments

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Part IV: Polarimeters, dual-beam type

The measurements V/I is:

Switch the beams to compensate the gain effects

Slide 69

A real circular polarimeter Semel, Donati, Rees (1993)

Quarter-wave plate, rotated at -45º and +45º

Analyser: double calcite crystal

Part IV: Polarimeters, example of circular polarimeter

Slide 70

A real circular polarimeter free from gain (g) and atmospheric transmission () variation effects

First measurement with quarter-wave plate at -45º, signal in the (r)ight and (l)eft beams:

Second measurement with quarter-wave plate at +45º, signal in the (r)ight and (l)eft beams:

Measurements of the signals:

Part IV: Polarimeters, example of circular polarimeter

Slide 71

A real circular polarimeter free from gain and atmospheric transmission variation effects

Build a ratio of measured signals which is free of gain and variable atmospheric transmission effects:

average of the 2 measurements

Part IV: Polarimeters, example of circular polarimeter

Slide 72

Polarimeters - Summary

2 types:

polaroid-type: easy to make but ½ light is lost, and affected by variable atmospheric transmission

dual-beam type: no light lost but affected by gain differences and variable transmission problems

Linear polarimetry:

analyzer, rotatable

analyzer + half-wave plate

Circular polarimetry:

analyzer + quarter-wave plate

2 positions minimum

1 position minimum

Part IV: Polarimeters, summary

Slide 73

Part V: ESPaDOnS

Optical components of the polarimeter part :

Wollaston prism: analyses the polarization and separates the 2 (linear!) orthogonal polarization states

Retarders, 3 Fresnel rhombs:

Two half-wave plates to switch the beams around

Quarter-wave plate to do circular polarimetry

Slide 74

ESPaDOnS: circular polarimetry

Fixed quarter-wave rhomb

Rotating bottom half-wave, at 22.5º increments

Top half-wave rotates continuously at about 1Hz to average out linear polarization when measuring circular polarization

Part V: ESPaDOnS, circular polarimetry mode

Slide 75

ESPaDOnS: circular polarimetry of circular polarization

half-wave

22.5º positions

flips polarization

gain, transmission

quarter-wave

fixed