Home
LibraryD. Gasman (KU Leuven), I. Argyriou (KU Leuven) J.E. Morrison (Steward Obs.), D.R. Law (STScI), A. Glasse (Royal Obs. Edinburgh), K.D. Gordon (STScI), P.J. Kavanagh (Maynooth Univ.), C. Lage (UC Davis), P. Patapis (ETH Zürich), G.C. Sloan (STScI, UNC)
2024, A&A, 689, A626
Full manuscript available from the arXiv (2406.10835).
Context. The James Webb Space Telescope (JWST) has been collecting
scientific data for over two years now. The Medium Resolution Spectrometer
(MRS) of the Mid-InfraRed Instrument (MIRI) has been one of the telescope's
most popular modes, and has already produced ground-breaking results.
Scientists are now looking deeper into the data for new exciting
discoveries, which introduces the need to characterise and correct known
systematic effects to reach the photon noise limit. Five important limiting
factors for the MRS are the pointing accuracy, non-linearity, detector
charge migration, detector scattering - resulting in both spatial broadening
and spectral interferometric fringing - the accuracy of the point-spread
function (PSF) model, and the complex interplay between these.
Aims: The Cycle 2 calibration programme 3779, entitled The MIRI/MRS
Library', proposed a 72-point intra-pixel dither raster of the calibration
star 10-Lac, which provides a unique dataset tailored for the purpose of
addressing the limiting factors on the MRS data accuracy. In this first
work of the paper series, we aim to address the degeneracy between the
non-linearity and charge migration (brighter-fatter effect) that affect the
pixel voltage integration ramps of the MRS. Due to the low flux in the
longer wavelengths, we only do this in the 4.9-11.7 micron region (spectral
channels 1 and 2).
Methods: We fitted the ramps individually per pixel and dither, in order to
fold in the deviations from classical non-linearity that are caused by
charge migration. The ramp shapes should be repeatable depending on the part
of the PSF that is sampled. By doing so, we defined both a grid-based
linearity correction, and an interpolated linearity correction.
Results: Including the change in ramp shape due to charge migration yields
significant improvements compared to the uniform illumination assumption
that is currently used by the standard JWST calibration pipeline. The
standard deviation on the pixel ramp residual non-linearity is between 70
and 90% smaller than the current standard pipeline when self-calibrating
with the grid. We are able to interpolate these coefficients to apply to any
unresolved source not on the grid points, resulting in an up to 70% smaller
standard deviation on the residual deviation from linearity. After applying
the correction, the full-width at half maximum is up to 20% narrower for
sources that cover the full pixel dynamic range. Furthermore, the depth of
the fringes is now consistent up the ramp, improving the standard deviation
on the difference in fringe depth between the start and ends of integrations
by ~60%.
Conclusions: Pointing-specific linearity corrections allow us to accurately
model the pixel ramps across the PSF, and for the first time, fix the
systematic deviation in the slopes. In this work we demonstrated this for
unresolved sources. The discovered trends with PSF sampling suggest that, in
the future, we may be able to model ramps for spatially extended and
resolved illumination as well.
Last modified 30 September, 2024. © Gregory C. Sloan and others.