JWST NIRCam Defocused Imaging: Photometric Stability Performance and How it Can Sense Mirror Tilts by Everett Schlawin et al. on Wednesday 30 November

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.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.16727v1