  We use JWST NIRCam short wavelength photometry to capture a transit
lightcurve of the exoplanet HAT-P-14 b to assess performance as part of
instrument commissioning. The short wavelength precision is 152 ppm per 27
second integration as measured over the full time series compared to a
theoretical limit of 107 ppm, after corrections to spatially correlated 1/f
noise. Persistence effects from charge trapping are well fit by an exponential
function with short characteristic timescales, settling on the order of 5-15
minutes. The short wavelength defocused photometry is also uniquely well suited
to measure the realtime wavefront error of JWST. Analysis of the images and
reconstructed wavefront maps indicate that two different hexagonal primary
mirror segments exhibited "tilt events" where they changed orientation rapidly
in less than ~1.4 seconds. In some cases, the magnitude and timing of the flux
jumps caused by tilt events can be accurately predicted with a telescope model.
These tilt events can be sensed by simultaneous longer-wavelength NIRCam grism
spectral images alone in the form of changes to the point spread function,
diagnosed from the FWHM. They can also be sensed with the FGS instrument from
difference images. Tilt events possibly from sudden releases of stress in the
backplane structure behind the mirrors were expected during the commissioning
period because they were found in ground-based testing. Tilt events have shown
signs of decreasing in frequency but have not disappeared completely. The
detectors exhibit some minor (less than 1%) deviations from linear behavior in
the first few groups of each integration, potentially impacting absolute fluxes
and transit depths on bright targets where only a handful of groups are
possible. Overall, the noise is within 50% of the theoretical photon noise and
read noise. This bodes well for high precision time series measurements.
