NIRCam Coronagraphy was declared ready for science in the early summer 2022. Several impactful science results have since been obtained using the NIRCam coronagraphs, mainly on known exoplanetary systems. In this contribution we give an update on all improvements we have implemented to make this mode more efficient and perform better. With tight timing constraints in commissioning, we focused on the long wavelengths occulter MASK335R. Here we describe how we improved the target acquisition for all five masks, the distortion correction and global alignment, the absolute flux calibration, etc. We also implemented the default dual channel operations mid-Cycle 1 (simultaneous short and long wavelengths). While not trivial, this new capability improves the efficiency and the impact NIRCam Coronagraphy can have in the field of exoplanets. We discuss the current on-sky contrasts and astrometric performances which are now better understood and can be compared to other high contrast facilities. We demonstrate that NIRCam Coronagraphy is transformative in characterizing known objects but also discovering colder and/or more mature exoplanets.
KEYWORDS: Coronagraphy, Stars, James Webb Space Telescope, Point spread functions, Distortion, Telescopes, Signal to noise ratio, Calibration, Target acquisition, Exoplanets, Astronomical imaging, Near infrared, Direct methods, Astronomical instrumentation
In a cold and stable space environment, the James Webb Space Telescope (JWST or ”Webb”) reaches unprecedented sensitivities at wavelengths beyond 2 microns, serving most fields of astrophysics. It also extends the parameter space of high-contrast imaging in the near and mid-infrared. Launched in late 2021, JWST underwent a six month commissioning period. In this contribution we focus on the NIRCam Coronagraphy mode which was declared ”science ready” on July 10 2022, the last of the 17 JWST observing modes. Essentially, this mode enables the detection of fainter/redder/colder (less massive for a given age) self-luminous exoplanets as well as other faint astrophysical signal in the vicinity of any bright object (stars or galaxies). Here we describe some of the steps and hurdles the commissioning team went through to achieve excellent performances. Specifically, we focus on the Coronagraphic Suppression Verification activity. We were able to produce firm detections at 3.35µm of the white dwarf companion HD 114174 B which is at a separation of ' 0.500and a contrast of ' 10 magnitudes (104 fainter than the K∼5.3 host star). We compare these first on-sky images with our latest, most informed and realistic end-to-end simulations through the same pipeline. Additionally we provide information on how we succeeded with the target acquisition with all five NIRCam focal plane masks and their four corresponding wedged Lyot stops.
The geometric distortion of the CCD detectors used in the Hubble Space TelescopeWide Field Camera 3 (WFC3)
and Advanced Camera for Surveys (ACS) instruments is characterized by both large and fine-scale distortions.
The large-scale distortion, due to the complexity of the HST optical assembly, can be modeled by a high-order
polynomial. The majority of fine-distortion is inherent to the CCD detectors themselves, which manifests itself
as fine-scale, correlated systematic offsets in the residuals from the best-fit polynomial solution. Such systematic
offsets across the CCD chip introduce astrometric errors at the level of about 0.1 pix (up to 1.5 μm within the 15
μm pixels). These fine-scale and low-amplitude distortions apparently arise from the spatial irregularities in the
pixel grid. For the WFC3/UVIS CCD chips, there is a clear pattern of periodic skew in the lithographic-mask
stencil imprinted onto the detector. Similar irregularities in the pixel grid of ACS/WFC CCD chips are even
more pronounced by the narrow (68×2048 pixel) lithographic-mask stencil. To remove these distortions, a 2-D
correction in the form of a look-up table has been developed using HST images of very dense stellar fields. The
post-correction of fine-scale astrometric errors can be removed down to the level of 0.01 pix (0.15 μm) or better.
Installed in the Hubble Space Telescope (HST) in May 2009, the Wide Field Camera 3 (WFC3) is performing extremely
well on-orbit. Designed to complement the other instruments on-board the Hubble Space Telescope (HST) and enhance
the overall science performance of the observatory, WFC3 is effectively two instruments in one. The UVIS channel,
with its pair of e2v 4Kx2K CCD chips provides coverage from 200 to 1000 nm while the IR channel, with a Teledyne
HgCdTe focal plane array (FPA) on a Hawaii-1R multiplexer, covers the 800-1700 nm range. This report summarizes
the performance of the WFC3 detectors, including primary characteristics such as quantum efficiency, read noise, dark
current levels, and cosmetics, as well as hysteresis prevention and the impact of radiation damage in the CCDs. In
addition, we discuss effects in the IR detector such as persistence, count rate non-linearity, 'snowballs', and 'negative'
cosmic rays.
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