JWST Noise Floor I: Random Error Sources in JWST NIRCam Time Series. (arXiv:2010.03564v1 [astro-ph.IM])
<a href="http://arxiv.org/find/astro-ph/1/au:+Schlawin_E/0/1/0/all/0/1">Everett Schlawin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Leisenring_J/0/1/0/all/0/1">Jarron Leisenring</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Misselt_K/0/1/0/all/0/1">Karl Misselt</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Greene_T/0/1/0/all/0/1">Thomas P. Greene</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+McElwain_M/0/1/0/all/0/1">Michael W. McElwain</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Beatty_T/0/1/0/all/0/1">Thomas Beatty</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rieke_M/0/1/0/all/0/1">Marcia Rieke</a>
JWST transmission and emission spectra will provide invaluable glimpses of
transiting exoplanet atmospheres, including possible biosignatures. This
promising science from JWST, however, will require exquisite precision and
understanding of systematic errors that can impact the time series of planets
crossing in front of and behind their host stars. Here, we provide estimates of
the random noise sources affecting JWST NIRCam time-series data on the
integration-to-integration level. We find that 1/f noise can limit the
precision of grism time series for 2 groups (230 ppm to 1000 ppm depending on
the extraction method and extraction parameters), but will average down like
the square root of N frames/reads. The current NIRCam grism time series mode is
especially affected by 1/f noise because its GRISMR dispersion direction is
parallel to the detector fast-read direction, but could be alleviated in the
GRISMC direction. Care should be taken to include as many frames as possible
per visit to reduce this 1/f noise source: thus, we recommend the smallest
detector subarray sizes one can tolerate, 4 output channels and readout modes
that minimize the number of skipped frames (RAPID or BRIGHT2). We also describe
a covariance weighting scheme that can significantly lower the contributions
from 1/f noise as compared to sum extraction. We evaluate the noise introduced
by pre-amplifier offsets, random telegraph noise, and high dark current RC
pixels and find that these are correctable below 10 ppm once background
subtraction and pixel masking are performed. We explore systematic error
sources in a companion paper.
JWST transmission and emission spectra will provide invaluable glimpses of
transiting exoplanet atmospheres, including possible biosignatures. This
promising science from JWST, however, will require exquisite precision and
understanding of systematic errors that can impact the time series of planets
crossing in front of and behind their host stars. Here, we provide estimates of
the random noise sources affecting JWST NIRCam time-series data on the
integration-to-integration level. We find that 1/f noise can limit the
precision of grism time series for 2 groups (230 ppm to 1000 ppm depending on
the extraction method and extraction parameters), but will average down like
the square root of N frames/reads. The current NIRCam grism time series mode is
especially affected by 1/f noise because its GRISMR dispersion direction is
parallel to the detector fast-read direction, but could be alleviated in the
GRISMC direction. Care should be taken to include as many frames as possible
per visit to reduce this 1/f noise source: thus, we recommend the smallest
detector subarray sizes one can tolerate, 4 output channels and readout modes
that minimize the number of skipped frames (RAPID or BRIGHT2). We also describe
a covariance weighting scheme that can significantly lower the contributions
from 1/f noise as compared to sum extraction. We evaluate the noise introduced
by pre-amplifier offsets, random telegraph noise, and high dark current RC
pixels and find that these are correctable below 10 ppm once background
subtraction and pixel masking are performed. We explore systematic error
sources in a companion paper.
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