Improving signal-to-noise in the direct imaging of exoplanets and circumstellar disks with MLOCI^⋆

¹ European Southern Observatory, Alonso de Cordova 3107, Vitacura, 19001 Casilla, Santiago Chile,
e-mail: zwahhaj@eso.org
² Facultad de Ingenieria, Universidad Diego Portales. Av. Ejercito 441, Santiago, Chile
³ Sterrewacht Leiden, Universiteit Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Departamento de Física y Astronomía, Universidad de Valparaíso, Valparaíso, Chile
⁵ Departamento de Astronomá, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁶ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁷ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁸ Gemini Observatory, Southern Operations Center, c/o AURA, Casilla 603, La Serena, Chile
⁹ UMI-FCA, CNRS/INSU (UMI 3386), France
¹⁰ Millennium Nucleus “Protoplanetary Disks”, Chile

Received: 6 February 2015
Accepted: 14 May 2015

Abstract

We present a new algorithm designed to improve the signal-to-noise ratio (S/N) of point and extended source detections around bright stars in direct imaging data.One of our innovations is that we insert simulated point sources into the science images, which we then try to recover with maximum S/N. This improves the S/N of real point sources elsewhere in the field. The algorithm, based on the locally optimized combination of images (LOCI) method, is called Matched LOCI or MLOCI. We show with Gemini Planet Imager (GPI) data on HD 135344 B and Near-Infrared Coronagraphic Imager (NICI) data on several stars that the new algorithm can improve the S/N of point source detections by 30–400% over past methods. We also find no increase in false detections rates. No prior knowledge of candidate companion locations is required to use MLOCI. On the other hand, while non-blind applications may yield linear combinations of science images that seem to increase the S/N of true sources by a factor >2, they can also yield false detections at high rates. This is a potential pitfall when trying to confirm marginal detections or to redetect point sources found in previous epochs. These findings are relevant to any method where the coefficients of the linear combination are considered tunable, e.g., LOCI and principal component analysis (PCA). Thus we recommend that false detection rates be analyzed when using these techniques.