Extremely Large Telescope

One of the highest scientific priorities for the ELT is to characterise exo-planets and to take images of Earth-like planets. Therefore, a dedicated planetary camera and spectrograph (ELT-PCS, also called EPICS) is part of the planned suite of ELT instruments. It will provide a complete toolkit to characterise planetary systems through, e.g., orbital motion, spectrophotometry of planetary atmospheres, and planet-disk interactions. Only ELT-PCS can realise the full potential of the ELT in exo-planet characterisation providing ≳100× better contrast at smaller inner working angles than the general-purpose instruments HARMONI and MICADO, reaching rocky planets in the habitable zone. Detailed design and construction (Phase B) of ELT-PCS will start as soon as the R&D for specific components reaches the required technology readiness level.

The University of Oxford is responsible for the integral field spectrograph (IFS) work package of the post Phase A R&D program. The University of Oxford led the integral field spectrograph (IFS) work package of the EPICS Phase A study investigating both lenslet-array and image-slicer based IFSs. While the former are used in current planet finding instruments (e.g., SPHERE/VLT and GPI/Gemini), an image-slicer based design provides a wider spectral range and makes more efficient use of detector pixels. These lead to a cheaper instrument with more efficient speckle suppression at smaller inner working angles. Thus the type of IFS is a critical decision for the ELT-PCS concept.

At phase A, we designed a novel, compact image-slicer based IFS with an unprecedented quarter million spaxels that met the requirements of the FoV and volume constraints for EPICS (Tecza at al. 2010). In parallel, we carried out simulations and experimental tests that demonstrated that an image-slicer based IFS can provide the required contrast. More specifically, we achieved a speckle suppression factor of ≥250 (limited only by the experimental setup, Salter et al. 2010b), ≥10× above the contrast achieved by lenslet-array based IFSs (Crepp et al. 2011).

In the current ELT-PCS R&D phase we hope to confirm these very promising results with an in-depth comparison of the two IFS concepts. Oxford will be responsible for the image-slicer based IFS, and we will use a common test setup at ESO (Garching), based on the ESO GHOST adaptive optics bench, for the comparative tests that produces speckles in the focal plane of the IFS.

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