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ESPaDOnS: performances of Fresnel rhomb retarders http://webast.ast.obs-mip.fr/magnetisme/espadons_new/rhombs.html
ESPaDOnS
performances of Fresnel rhomb retarders
Rhomb characteristics
Given the large wavelength domain of ESPaDOnS, the first natural idea is to use superachromatic retarders designed along
Serkowsky’s ideas, like those manufactured by Halle. However, previous experience with them demonstrated that they generate
large amplitude fringing in the intensity and polarisation spectra and thus drastically reduce the polarisation accuracy of any
potential measurements obtained with them. We therefore decided to use Fresnel rhombs, that were proven to be much more
achromatic and producing almost no fringing patterns in high resolution spectra.
A bunch of 24 single bk7 rhombs (with birefringence smaller than 0.2nm/cm) was ordered and constructed along detailed
specifications, to construct 8 quarter-wave (single) rhombs and 8 half-wave (double) rhombs, with different thicknesses of MgF2
coating to study the effect on the rhomb retardance. The rhombs are mounted in a specific barrel filled with helium (to avoid
oxydation of the totally reflecting surfaces) and sealed with a soft joint. A dedicated and fully automated optical bench was also
designed and constructed to measure the rhombs retardance with an accuracy of 0.1deg. The best rhombs were selected for the
polarimeter modules of ESPaDOnS and NARVAL (the copy of ESPaDOnS in construction for the 2m Bernard Lyot telescope
atop Pic du Midi).
Retardance accuracy
The graph on the right shows the retardance curves of 2
different quarter wave rhombs in the series (full line: first
rhomb; other lines: independant measurements of second
rhomb, taken over a few months). It demonstrates that Fresnel
rhombs can be designed and constructed so that their
retardance is nominal to better than 0.3% throughout the
whole optical domain, while their optical axis remains stable to
better than 0.1deg. This is much better in particular than
superachromatic waveplates, the retardance and optical axis of
which vary by about 2% and 4deg respectively in the same
wavelength interval.
These curves also demonstrate that the retardance
measurement is repeatable to better than 0.1deg, and does not
evolve significantly with time, at least on a timescale of a few
months. The optimal thickness of the MgF2 film deposited on
the rhombs to achieve such performances is found to be very
close to the theoretical value (of 24nm for our bk7 rhombs).
Amplitude of fringing paterns
We also estimated the amplitude of fringing patterns induced
in polarised spectra by Fresnel rhombs by taking sequences of
flat field exposures in different rhomb azimuths, in exactly the
same way as one observer recording polarisation spectra with
ESPaDOnS. The graph on the right shows the achieved signal
to noise ratio as a function of wavelength for one such
polarisation sequence. The full line depicts the signal to noise
ratio expected from the number of counts on the ccd detector,
and the dash-dot line the signal to noise ratio measured from
the check spectrum (derived from spectra recorded in the same
rhomb azimuths and thus free of any fringing patterns from
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