  We observed a full-orbit phase curve of the hot Jupiter WASP-43b with
MIRI/LRS as part of the Transiting Exoplanet Community Early Release Science
Program. Here we report preliminary findings for the instrument performance
from the team's MIRI Working Group. Overall we find that MIRI's performance for
phase curve observations is excellent, with a few minor caveats. The key
takeaways for Cycle 2 planning with MIRI/LRS are: (1) long-duration
observations (> 24 hours) have now been successfully executed; (2) for phase
curves, we recommend including a one-hour burn-in period prior to taking
science data to mitigate the effects of the ramp systematic; and (3) we do not
yet recommend partial phase curve observations. In addition, we also find that:
the position of the spectrum on the detector is stable to within 0.03 pixels
over the full 26.5-hour observation; the light curves typically show a
systematic downward ramp that is strongest for the first 30 minutes, but
continues to decay for hours; from 10.6-11.8 microns, the ramp effect has
remarkably different behavior, possibly due to a different illumination history
for the affected region of the detector; after trimming the integrations most
affected by the initial ramps and correcting the remaining systematics with
analytic models, we obtain residuals to the light-curve fits that are typically
within 25% of the photon noise limit for 0.5-micron spectroscopic bins;
non-linearity correction is not a significant source of additional noise for
WASP-43, though it may be an issue for brighter targets; the gain value of 5.5
electrons/DN currently on CRDS and JDox is known to be incorrect, and the
current best estimate for the gain is approximately 3.1 electrons/DN; new
reference files for the JWST calibration pipeline reflecting these findings are
under development at STScI.
