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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:

The correlation between optical starlight polarization and polarized thermal dust emission can be used to infer intrinsic dust properties. This correlation is quantified by the ratio R P / p , which has been measured by the Planck Collaboration to be 5.42 ± 0.05 MJy sr −1 at 353 GHz when averaged over large areas of the sky. We investigated this correlation using newly published stellar polarimetric data densely sampling a continuous sky region of about four square degrees at intermediate Galactic latitude. We combined RoboPol optical polarization measurements for 1430 stars with submillimeter data from the Planck satellite at 353 GHz. We performed linear fits between the Planck ( Q s , U s ) and optical ( q v , u v ) Stokes parameters, taking into account the differences in resolution between the two datasets as well as the distribution of clouds along the line of sight. We find in this region of the sky that the R P / p value is 3.67 ± 0.05 MJy sr −1 , indicating a significantly shallower slope than that found previously using different stellar samples. We also find significant differences in the fitted slopes when fitting the Q s – q v and U s – u v data separately. We explore two explanations using mock data: the miscalibration of the polarization angle and the variations in R P / p along the line of sight due to multiple clouds. We show that the former can produce differences in the correlations of Q s – q v and U s – u v , but large miscalibration angles would be needed to reproduce the magnitude of the observed differences. Our simulations favor the interpretation that R P / p differs between the two dominant clouds that overlap on the sky in this region. The difference in R P / p suggests that the two clouds may have distinct dust polarimetric properties. With knowledge from the tomographic decomposition of the stellar polarization, we find that one cloud appears to dominate the correlation of U s – u v , while both clouds contribute to the correlation of the Q s – q v data.