Application of a damped Locally Optimized Combination of Images method to the spectral characterization of faint companions using an Integral Field Spectrograph (1111.6102v1)

Abstract: High-contrast imaging instruments are now being equipped with integral field spectrographs (IFS) to facilitate the detection and characterization of faint substellar companions. Algorithms currently envisioned to handle IFS data, such as the Locally Optimized Combination of Images (LOCI) algorithm, rely upon aggressive point-spread-function (PSF) subtraction, which is ideal for initially identifying companions but results in significantly biased photometry and spectroscopy due to unwanted mixing with residual starlight. This spectro-photometric issue is further complicated by the fact that algorithmic color response is a function of the companion's spectrum, making it difficult to calibrate the effects of the reduction without using iterations involving a series of injected synthetic companions. In this paper, we introduce a new PSF calibration method, which we call "damped LOCI", that seeks to alleviate these concerns. By modifying the cost function that determines the weighting coefficients used to construct PSF reference images, and also forcing those coefficients to be positive, it is possible to extract companion spectra with a precision that is set by calibration of the instrument response and transmission of the atmosphere, and not by post-processing. We demonstrate the utility of this approach using on-sky data obtained with the Project 1640 IFS at Palomar. Damped-LOCI does not require any iterations on the underlying spectral type of the companion, nor does it rely upon priors involving the chromatic and statistical properties of speckles. It is a general technique that can readily be applied to other current and planned instruments that employ IFS's.