The CanariCam Imaging Polarimetry Mode

To set up the CanariCam polarimetry mode, one inserts either the 10 micron or the 20 micron half-wave plate (modulator) just after the entrance window (the 20 micron polarimetric capability is under consideration). Observations are made with the appropriate modulator at four different angles. A field mask is inserted at the telescope focal plane: when using a dual-beam analyser, this mask is required so that extended objects can be observed without overlapping the two orthogonally polarised beams. A Wollaston prism is inserted into the beam after the collimator mirror M2. The purpose of the Wollaston prism is to separate the orthogonally polarized components of the light, which are then imaged separately onto the detector.

Polarimetry Requirements

Polarimetry has one fundamental requirement, which is the ability to measure degrees of polarisation as small as ~0.1% in the 10 micron atmospheric window.

Why Polarimetry?

• Polarimetry is photon starved and so is really most suitable for 10-m-class telescopes.
• High angular resolution (diffraction limited) leads to a higher degree of measured polarization, since polarization is a vector quantity.
• GTC will allow diffraction-limited polarimetry from Day 1, thus giving rapid science returns.
• Numerous science drivers include solar system physics, young stars and disk studies, active galaxies, ULIRGS and quasars
• Mid-IR polarimetry is not obviously a high priority on other 8-10-m telescopes.
• CC will fill a significant but under-addressed scientific niche.
• Little mid-IR polarimetric work has been done so far and only on 4-m class telescopes

The optical elements that are required for CC-Pol and their current position in the cryostat (these may change somewhat with changes in the optical design during the Detailed Design phase) are shown below. These elements are described in the slide show link.

Preliminary optical design of CC-Pol.
 
 

Click here to see a slide how of CC-Pol implementation will work.