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EPICS-EPOL

A number of European countries, united in the European Southern Observatory, are working towards building the largest optical telescope in the world: The European Extremely Large Telescope or E-ELT. The primary mirror of this telescope, consisting of no less than 798 hexagonal segments, will have an effective diameter of 39,3 meters. This means that this new telescope will have more light-gathering power than all the large telescopes available to astronomers today, combined! The reason for building such a large telescope is twofold: The incredible light-gathering power means that faint objects in the farthest reaches of the universe can be observed. The large mirror also means that the resolving power of the telescope will be tremendous, allowing much greater detail in the observations. The images above and at right are artist’s impressions of the telescope. Note the small objects in front of the telescope: These are cars!

Work has commenced on a number of different advanced instruments that will exploit the unique capabilities of the E-ELT. One of the proposed instruments, EPICS, will be dedicated to detecting and characterising exoplanets, including rocky ones. Based on the results and the experience gained with ExPo, my collaborators and I have participated in the EPICS phase-A study, performing the initial design for an imaging polarimeter arm, EPOL, for this instrument. This design, pictured at left, is very similar to ExPo, but with improved optics and detectors to gain maximum benefit from the enormous jump in spatial resolution and light gathering offered by the E-ELT and its Extreme Adaptive Optics system. Combined with a coronagraph, the sensitive dual-beam polarimetry will offer a contrast approaching 10-9 at an inner working angle of 0.01 arcseconds (see below left). A distinguishing feature of EPOL compared to other instruments is that, thanks to the polarimetry, it is more sensitive to rocky exoplanets (below).

To increase the technological maturity, the polarimetric IFU concept will be tested on-sky as a (very substantial) modification of the ExPo polarimeter. This IFU will sample a 10 x 10 arcsecond field with 75 x 75 microlenses and offer a spectral resolution of R=22 in the wavelength range 500-900 nm. The goal is a polarimetric sensitivity equal to ExPo (10-4 ) or better. Below  are images taken in the lab with this instrument of unpolarised (left) and orthogonally polarised (middle and right) broadband light.

While the baseline EPOL design is essentially an extrapolation of the ExPo design, I am currently developing a new concept: A spectro-polarimetric integral field unit. This will sample the field using a microlens array, as a regular IFU, but will add dual-beam polarimetry for every spaxel. Two orthogonally polarised spectra are dispersed for every micropupil. The key advantage of this concept is that it offers wavelength resolution in combination with the sensitive imaging polarimetry.