# JWST NIRCam Defocused Imaging: Photometric Stability Performance and How it Can Sense Mirror Tilts

**Authors:** Everett Schlawin, Thomas Beatty, Brian Brooks, Nikolay K. Nikolov, Thomas P. Greene, Néstor Espinoza, Kayli Glidic, Keith Baka, Eiichi Egami, John Stansberry, Martha Boyer, Mario Gennaro, Jarron Leisenring, Bryan Hilbert, Karl Misselt, Doug Kelly, Alicia Canipe, Charles Beichman, Matteo Correnti, J. Scott Knight  
**arXiv:** [2211.16727](https://arxiv.org/abs/2211.16727)  
**Date:** 2022-11-30  
**Summary page:** https://www.summarizepaper.com/en/paper/2211.16727v1/

## Key points

- The study analyzes the performance of the James Webb Space Telescope (JWST) Near Infrared Camera (NIRCam) during commissioning.
- NIRCam achieved short wavelength precision of 152 ppm per 27 second integration, which is within 50% of theoretical photon noise and read noise.
- Charge trapping ramp and persistence effects were well fit by an exponential function with short characteristic timescales.
- Defocused photometry obtained with NIRCam is uniquely suited to measure the real-time wavefront error of JWST.
- Two different hexagonal primary mirror segments exhibited "tilt events" that can be sensed by simultaneous longer-wavelength NIRCam grism spectral images or using difference images from the Fine Guidance Sensor instrument.
- Systematic effects in data include observed non-linearity and minor 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.
- The study provides valuable insights into JWST's performance during commissioning and demonstrates its potential for high precision time series measurements for exoplanet science.

## Summary

This study presents an analysis of the performance of the James Webb Space Telescope (JWST) Near Infrared Camera (NIRCam) during commissioning, using observations of the exoplanet HAT-P-14 b. The short wavelength precision achieved with NIRCam is found to be 152 ppm per 27 second integration, which is within 50% of the theoretical photon noise and read noise. The charge trapping ramp and 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 defocused photometry obtained with NIRCam is also uniquely well suited to measure the real-time 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. 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 or by using difference images from the Fine Guidance Sensor instrument. Furthermore, this study highlights systematic effects in data including observed non-linearity and minor deviations from linear behavior in the first few groups of each integration which could potentially impact absolute fluxes and transit depths on bright targets where only a handful of groups are possible. Overall, this study provides valuable insights into JWST's performance during commissioning and demonstrates its potential for high precision time series measurements for exoplanet science.