SPHERE IRDIS and IFS astrometric strategy and calibration

TL;DR

This paper details the astrometric calibration procedures and results for the SPHERE instrument, including pixel scale, distortion, and orientation measurements over two years of observations.

Contribution

It provides the first comprehensive calibration of SPHERE's IRDIS and IFS astrometric parameters, including distortion correction and calibration strategies.

Findings

IRDIS distortion dominated by 0.60% anamorphism

Pixel scale for IRDIS is 12.255 mas/pixel

Pixel scale for IFS is 7.46 mas/pixel

Abstract

We present the current results of the astrometric characterization of the VLT planet finder SPHERE over 2 years of on-sky operations. We first describe the criteria for the selection of the astrometric fields used for calibrating the science data: binaries, multiple systems, and stellar clusters. The analysis includes measurements of the pixel scale and the position angle with respect to the North for both near-infrared subsystems, the camera IRDIS and the integral field spectrometer IFS, as well as the distortion for the IRDIS camera. The IRDIS distortion is shown to be dominated by an anamorphism of 0.60+/-0.02% between the horizontal and vertical directions of the detector, i.e. 6 mas at 1". The anamorphism is produced by the cylindrical mirrors in the common path structure hence common to all three SPHERE science subsystems (IRDIS, IFS, and ZIMPOL), except for the relative orientation of their field of view. The current estimates of the pixel scale and North angle for IRDIS are 12.255+/-0.009 milliarcseconds/pixel for H2 coronagraphic images and -1.75+/-0.08 deg. Analyses of the IFS data indicate a pixel scale of 7.46+/-0.02 milliarcseconds/pixel and a North angle of -102.18+/-0.13 deg. We finally discuss plans for providing astrometric calibration to the SPHERE users outside the instrument consortium.