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ESPaDOnS: examples of frames http://webast.ast.obs-mip.fr/magnetisme/espadons_new/frames.html
The example presented below illustrates the ability of ESPaDOnS to diagnose polarised light. Among the very extensive tests
carried out, the one included below depicts ESPaDOnS response to fully polarised light with north-south (Q=1) linear
polarisation. To estimate the amount of polarisation for a given polarisation state, four subexposures are successively taken with
the half-wave Fresnel rhombs set to 4 different configurations (configurations 1 and 4, and configurations 2 and 3 being roughly
equivalent by pairs). When estimating linear polarisation along north/south and east west axes (Stokes parameter Q), the
rhombs are successively rotated to position q1, q2, q3 and q4. On the frames included below, one can notice that:
the right beam disappears almost completely when rhombs are set to position q1 (middle image) compared to an
image obtained with unpolarised illumination (left image); a similar result is obtained with rhombs set to position q4;
the situation is reversed, with the right beam now almost completely extinguished, when rhombs are set to position
q2 (right image); a similar result is obtained with rhombs set to position q3.
Unpolarised illumination
Fully polarised illumination
(Q=1), with rhombs set to
position q1
Fully polarised illumination
(Q=1), with rhombs set to
position q2
Note that in both cases, the extinguished beam still shows up at a very weak intensity level (of less than 1% that of the main
beam). This is due to slight residual chromatic inaccuracies in the properties of Fresnel rhombs.
© Jean-François Donati, last update May 26 2004
3 of 3 08/07/04 11:35 PM