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The new high precision polarimeter for the “Observatoire du Mont Mégantic” (POMM) is an instrument designed to
observe exoplanets and other targets in the visible and near infrared wavebands. The requirements to achieve these
observation goals are posing unusual challenges to structural and mechanical designers.
In this paper, the detailed design, analysis and laboratory results of the key mechanical structure and sub-systems are
presented.
First, to study extremely low polarization, the birefringence effect due to stresses in the optical elements must be kept to
the lowest possible values. The double-wedge Wollaston custom prism assembly that splits the incoming optical beam is
made of bonded α-BBO to N-BK-7 glass lenses. Because of the large mismatch of coefficients of thermal expansion and
temperatures as low as -40°C that can be encountered at Mont-Mégantic observatory, a finite element analysis (FEA)
model is developed to find the best adhesive system to minimize stresses.
Another critical aspect discussed in details is the implementation of the cascaded rotating elements and the twin rotating
stages. Special attention is given to the drive mechanism and encoding technology. The objective was to reach high
absolute positional accuracy in rotation without any mechanical backlash.
As for many other instruments, mass, size and dimensional stability are important critera for the supporting structure.
For a cantilevered device, such as POMM, a static hexapod is an attractive solution because of the high stiffness to
weight ratio. However, the mechanical analysis revealed that the specific geometry of the dual channel optical layout
also added an off-axis counterbalancing problem. To reach an X-Y displacement error on the detector smaller than 35μm
for 0-45° zenith angle, further structural optimization was done using FEA. An imaging camera was placed at the
detector plane during assembly to measure the actual optical beam shift under varying gravitational loading.
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