Characterizing the Performance of the NIRC2 Vortex Coronagraph at W. M. Keck Observatory

Abstract

The NIRC2 vortex coronagraph is an instrument on Keck II designed to directly image exoplanets and circumstellar disks at mid-infrared bands L (3.4-4.1 µm) and M s (4.55-4.8 µm). We analyze imaging data and corresponding adaptive optics telemetry, observing conditions, and other metadata over a three year time period to characterize the performance of the instrument and predict the detection limits of future observations. We systematically process images from 359 observations of 304 unique stars to subtract residual starlight (i.e., the coronagraphic point spread function) of the target star using two methods: angular differential imaging (ADI) and reference star differential imaging (RDI). We find that for the typical parallactic angle (PA) rotation of our dataset (∼10 • ), RDI provides gains over ADI for angular separations smaller than 0.25 . Furthermore, we find a power-law relation between the angular separation from the host star and the minimum PA rotation required for ADI to outperform RDI, with a power-law index of -1.18±0.08. Finally, we use random forest models to estimate ADI and RDI post-processed detection limits a priori. These models, which we provide publicly on a website, explain 70%-80% of the variance in ADI detection limits and 30%-50% of the variance in RDI detection limits. Averaged over a range of angular separations, our models predict both ADI and RDI contrast to within a factor of 2. These results illuminate important factors in high-contrast imaging observations with the NIRC2 vortex coronagraph, help improve observing strategies, and inform future upgrades to the hardware.

Near-infrared Imaging of a Spiral in the CQ Tau Disk

“…Details of this data set as well as other ALMA data of CQ Tau are described in Ubeira Gabellini et al (2019). Table 1 (Mawet et al 2017;Serabyn et al 2017;Xuan et al 2018) combined with angular differential imaging (ADI; Marois et al 2008). The observation achieved angular rotation of ∼111 • .…”

Keck/NIRC2 L’-band Imaging of Jovian-mass Accreting Protoplanets around PDS 70

“…We performed initial preprocessing of the data using a general pipeline developed for NIRC2 vortex observations (Xuan et al 2018;Ruane et al 2019). We will briefly summarize the steps here, and we refer the reader to Xuan et al (2018) and Ruane et al (2019) for details. First, we corrected bad pixels and flat-field effects in each image.…”

Wind-driven halo in high-contrast images

“…If the instrument additionally provides simultaneous images at two or more wavelengths, the spectral diversity can be used as the QSS expands radially at increasing wavelength, while the circumstellar signals remain at a fixed position: this is the spectral differential imaging (SDI, Racine et al 1999) technique. Finally, if a sufficient number of images of various targets are available from the instrument, multiple reference differential imaging (MRDI, Lafrenière et al 2009;Soummer et al 2011b;Xuan et al 2018;Ruane et al 2019, with space-based instruments and with ground-based instruments resp.) can be applied using the correlation between the images to be processed and the image library from the instrument.…”