Updated optical modeling of JWST coronagraph performance contrast, stability, and strategies

Abstract

We update performance simulations and contrast predictions for JWST's coronagraphs based on the latest information on the as-built telescope and instrument properties, including both static and dynamic contributions to wavefront error. By combining optical modeling of the telescope, instruments and coronagraph optics along with STScI's rigorously-validated exposure time calculation engine, we develop updated contrast models including contributions from effects such as target acquisition residuals, stellar color differences, etc. We present assessments of the impact of wavefront error changes over time between science and PSF reference stars, using modeled wavefront drifts on various timescales based on available observatory structural/thermal/optical modeling and tested performance during the OTIS cryo test, extrapolated to on-orbit conditions. For NIRCam we explore tradeoffs between different occulting masks at a given wavelength. Between now and the start of Cycle 1 science, these and other updated simulations will enable the science community to prepare analysis tools and PSF subtraction software to hit the ground running with JWST coronagraphic observations.

Direct imaging of sub-Jupiter mass exoplanets with James Webb Space Telescope coronagraphy

“…Additionally, we do not consider the MASK210R round mask as it is only compatible with filters below 2.3 m, a wavelength range that ground-based telescopes are likely to have superior performance across. Of the two remaining round masks we select the MASK335R for all of the NIRCam simulations due to its smaller inner working angle (IWA) compared to the MASK430R and with the knowledge that any potential improvement in contrast with the MASK430R is relatively small following PSF subtraction (Perrin et al 2018). As there are a large number of filters compatible with NIRCam coronagraphic imaging (see Fig.…”

“…All simulations are performed using the package PanCAKE 1 (Van Gorkom et al 2016;Perrin et al 2018;Girard et al 2018), which is based on the official JWST exposure time calculator Pandeia (Pontoppidan et al 2016). Given a desired input scene, PanCAKE is capable of producing corresponding 2D simulated images for all coronagraphic observations with the NIRCam and MIRI instruments aboard JWST.…”

“…For each target, simulated observations are performed on the target star and a reference assumed to be identical in spectral type and magnitude to that of the target. Whilst mismatches in spectral type can lead to non-optimal PSF subtraction, differences on the order of a few subtypes are unlikely to significantly impact the estimated contrast performance (Perrin et al 2018) and we therefore assume that a viable reference can be found for every target in the sample. Unlike the target observations, we repeat the reference star observations nine times following a circular small-grid dither (SGD) pattern.…”

“…to be the primary driver of variations in JWST optical telescope element (Perrin et al 2018). Any such variations will inhibit the ability to perform an accurate PSF correction and therefore will affect the achievable contrast.…”

“…Linder et al 2019;Phillips et al 2020), enabling a more accurate determination of their predicted luminosities. Recently, Perrin et al (2018) provided a significant, target unspecific, update to the contrast predictions for JWST, Sallum & Skemer (2019) gave an account of the non-redundant masking and kernel phase capabilities of JWST, and finally Brande et al (2020) have explored the feasibility of JWST mid-infrared coronagraphic imaging of field-aged exoplanets around the nearest stars. With these studies in mind, we instead focus our efforts towards determining the overall detection limits of JWST coronagraphic imaging towards sub-Jupiter mass exoplanets for a set of observations of the previously discussed nearby young moving groups.…”