Polarimeters at Pic du Midi

Laboratoire d'Astrophysique de Toulouse - Tarbes (LATT)
et
Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique (LESIA)


The Nematic Liquid Crystal (NLC) Polarimeter (2004 and after)


The optical bench between F1 and F2 focus of the telescope in MSDP mode (click to enlarge)

Elements of the optical bench with the NLC polarimeter in spectroscopic mode (click to enlarge)

Elements of the optical bench with the NLC polarimeter in imagery mode (click to enlarge)

The two variable retarders, both with thermal control (click to enlarge)

The retarders, from Meadowlark (USA), receive a 2 kHz square signal and the retardance depends on the amplitude of the signal (from 0 to 10 V). It can vary continuously from 0 to 800 nm. The transient nematic effect is applied to both retarders to speed up the transition time between two states. The time to switch between two states is about 20 ms, so that the modulation speed must be lower than 50 Hz (click figure to enlarge)

Calibration curve of the two retarders, as a function of wavelength, for quarter, half and three quarter waves retardance, determined with interference filters on optical bench (click to enlarge)

The Ferroelectric Liquid Crystal (FLC) Polarimeter (2003 and before)


Elements of the optical bench with FLC polarimeter (click to enlarge)

The crystal is an achromatic polarization rotator from Displaytech, USA, which acts as a half waveplate. -5 V gives 0 wave, and +5 V gives half wave. The transient nematic effect is used to speed up the transition time between 0 to half wave. The time to switch between the two states is about 0.1 ms, so that the modulation speed can be as high as 10 KHz (click to enlarge)

The polarizing beamsplitter/shifter (end 2006 and later)


Elements of the optical bench with the NLC polarimeter in spectroscopic mode with the polarizing beamsplitter/shifter (click to enlarge)

Details of the polarizing beamsplitter/shifter under construction (click to enlarge)