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Abstract:

A key science goal for the Planetary Camera and Spectrograph (ELT-PCS) instrument is to image rocky exoplanets in reflected light around nearby M- and K-dwarfs. These observations will require a contrast of 10−9 at 100 mas angular separation, and 10−8 at 10 mas. To achieve such an extreme performance, every instrument design choice must be carefully evaluated. For ELT-PCS’s science instruments, perhaps the most critical design choice is which integral field unit (IFU) technology to use. To this end, we have developed a modular, lab-based integral field spectrograph test bench with which to compare the performance of two popular IFU technologies: image slicers and lenslet arrays. The test bench implements a Lyot coronagraph and a spatial light modulator to simulate a high-contrast imaging system, which feeds each IFU with a diffraction-limited PSF image. Each IFU is coupled to the spectrograph, which achieves R≈3000 with the image slicer and R≈100 with the lenslet array. The calibration and data reduction for each IFU design are described and the initial results of a contrast comparison are presented. The lenslet array achieves a deeper contrast at angular separations of less than 6λ/D. The image slicer, however, gains more at small separations from applying spectral deconvolution, with a 3.4× greater gain at 4λ/D. Spectral deconvolution further yields a contrast curve close to the detector noise floor. Further work will focus on carrying out a fully-controlled characterisation of the lenslet array IFU and a more comprehensive study into the effects of adaptive optics residuals on the contrast.

Abstract:

<jats:p> Atmospheric optical turbulence (OT) monitoring is crucial for site characterization at astronomical observatories and optical communications ground stations. The Shack–Hartmann image motion monitor (SHIMM) instrument implements a fast, infrared Shack–Hartmann sensor to measure a low-resolution OT profile continuously throughout the day and night. This work presents advances made in Shack–Hartman optical turbulence profiling techniques implemented on the SHIMM, including a derivation and validation of Z-tilt weighting functions, implementation of methods for correcting for non-zero exposure times, and for estimating the coherence time of optical turbulence using the profile coupled with the fast defocus method. These techniques were tested via end-to-end Monte Carlo simulations of the SHIMM instrument using real turbulence profiles from the Paranal stereo-SCIDAR instrument with an augmented ground layer. All measurements of integrated OT parameters were in strong agreement with the simulation inputs, evidenced by correlation coefficients close to one, small RMS error, and bias. The accuracy of the four-layer SHIMM model was also investigated, which showed high correlation with simulation inputs for all layers even in daytime OT conditions. This study suggests that, for the turbulence database used, and under realistic daytime noise conditions, a <jats:italic>C</jats:italic> <jats:sub> <jats:italic>n</jats:italic> </jats:sub> <jats:sup>2</jats:sup> ( <jats:italic>h</jats:italic> )d <jats:italic>h</jats:italic> sensitivity limit in the region of 2×10 <jats:sup>−15</jats:sup> m <jats:sup>1/3</jats:sup> was encountered in the highest altitude layer. There was also evidence of a cross-talk effect between the strong ground layer and the first atmospheric layer. </jats:p>