A polarimeter, to observe exoplanets in the visible and infrared, was built for the “Observatoire du Mont Mégantic”
(OMM) to replace an existing instrument and reach 10-6 precision, a factor 100 improvement. The optical and
mechanical designs are presented, with techniques used to precisely align the optical components and rotation axes to
achieve the targeted precision. A photo-elastic modulator (PEM) and a lock-in amplifier are used to measure the
polarization. The typical signal is a high DC superimposed to a very faint sinusoidal oscillation. Custom electronics
was developed to measure the AC and DC amplitudes, and characterization results are presented.
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
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.
The alignment method of a fast catadioptric optical module with very large field of view is presented in this paper. The
module is made of three aspheric optical components: a primary mirror, a secondary mirror and a field lens. To achieve
the 22.6 degrees field of view, the secondary mirror makes a large obscuration requiring an F/0.75 working f-number to
achieve the effective F/1.05. The catadioptric optical module was integrated with the IRXCAM-640 uncooled camera
module made by INO. System spatial resolution is improved with the use of a 4-position microscan mechanism.
The needs of surveillance/detection operations in the infrared range, for industrial, spatial and military applications
continuously tend toward larger field of view and resolution while maintaining the system as compact as possible. To
answer this need, INO has developed a 1280x960 pixel thermal imager, said HRXCAM, with 22.6° field of view. This
system consists in the assembly of a catadioptric optics with microscan mechanism and a detection electronic module
based on a 640x480 25μm pitch pixel bolometric detector. The detection module, said IRXCAM, is a flexible platform
developed for fast prototyping of varied systems thanks to its ability to support a large range of infrared detectors. With
its multiple hardware and software functionalities, IRXCAM can also be used as the complete electronic module of a
finalized system. HRXCAM is an example of fast prototyping with IRXCAM and an optical lens that fully demonstrates
the imaging performance of the final system. HRXCAM provides 1280x960 pixel images at a nominal 5-15 Hz
frequency with 60 mK NETD. It can also be used in the 640x480 mode at 58 Hz with the same sensitivity. In this paper,
the catadioptric optics with integrated microscan and IRXCAM architecture and specifications are reviewed. Some
typical examples of image obtained with HRXCAM in outdoor conditions are presented.