Abstract
Measuring the orbits of directly imaged exoplanets requires precise astrometry at the milliarcsec level over long periods of time due to their wide separation to the stars (≳10 au) and long orbital period (≳20 yr). To reach this challenging goal, a specific strategy was implemented for the instrument Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE), the first dedicated exoplanet imaging instrument at the Very Large Telescope of the European Southern Observatory (ESO). A key part of this strategy relies on the astrometric stability of the instrument over time. We monitored for five years the evolution of the optical distortion, pixel scale, and orientation to the True North of SPHERE images using the near-infrared instrument IRDIS. We show that the instrument calibration achieves a positional stability of ∼1 mas over 2″ field of views. We also discuss the SPHERE astrometric strategy, issues encountered in the course of the on-sky operations, and lessons learned for the next generation of exoplanet imaging instruments on the Extremely Large Telescope being built by ESO.
Highlights
Orbital monitoring of exoplanetary and stellar systems is fundamental for analyzing their architecture, their dynamical stability and evolution, and even tracing back their mechanisms ofJ
(i.e., companions detected with S/N larger than the ratio of the PSF width over the calibration uncertainty), we expect that sub-mas precisions should be achieved with exoplanet imaging instruments on the Extremely Large Telescopes (ELTs)
We described in this paper the astrometric strategy and a five year analysis of the astrometric calibration of the Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE) instrument, the first instrument dedicated to exoplanet imaging at European Southern Observatory (ESO)/Very Large Telescope (VLT)
Summary
Orbital monitoring of exoplanetary and stellar systems is fundamental for analyzing their architecture, their dynamical stability and evolution, and even tracing back their mechanisms of. Precise and robust relative astrometric measurements over time in direct imaging require a good knowledge of the instrumental limitations and dedicated observing strategies. We outline key lessons learned for high-precision relative astrometry with SPHERE for optimizing the preparation of the exploitation of the generation of exoplanet imaging instruments especially for the ELT being built by the European Southern Observatory (ESO, Sec. 5). Extensive tests using injections of synthetic point sources in laboratory data processed with spectral differential imaging[30,31] showed that for separation measurements, astrometric accuracy is better than 1.5 to 2 mas over a FOVof 1.6′′ for detections at signal-to-noise ratios (S/N’s) above 10,32 within the requirements. The dithering stage has a finite positioning accuracy of 0.74 mas,[38] which has to be taken into account in the astrometric error budget when the calibration spots are not used during the whole science observation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
