NIRCam Weak Lenses
NIRCam's weak lenses defocus incoming light. They will be used for wavefront sensing in aligning JWST's mirrors, and for science in time-series observations.
The NIRCam weak lenses defocus incoming light, spreading it over a larger number of detector pixels. They will be used during commissioning to complete the fine phasing alignment of the JWST primary mirror. After commissioning, they will be used for routine monitoring and adjustment of the mirror phasing.
The weak lenses may also be used for science in NIRCam time-series observations to:
- avoid saturation of bright sources,
- mitigate flat field uncertainties.
Table 1. NIRCam weak lenses
|Weak lens||Waves of defocus|
(at 2.12 µm)
WLP8 and WLM8 provide very similar performance. WLP4 is coupled to narrowband filter F212N2 with a 2.3% bandpass (wider than the 1% bandpass F212N filter in the filter wheel).
All 3 weak lenses will be used for wavefront sensing, but only WLP8 and WLP4 will be available for science in NIRCam time-series observations.
Combining weak lenses with other filter elements
In engineering mode for mirror alignment, the 8-wave weak lenses in the pupil wheel (WLP8 or WLM8) may be combined with various filter wheel elements: short wavelength filters and the weak lens WLP4+F212N2. Various combinations enable images with 5 levels of defocus: -8, -4, +4, +8, and +12 waves.
For science observations, the weak lenses may not be combined with one another. For NIRCam time-series imaging, only WLP8 is available; it may be combined with one of several short wavelength filters. In NIRCam grism time-series mode, both WLP4+F212N2 and WLP8 are available for use in the short wavelength channel while grism observations are obtained at long wavelengths (2.4–5.0 µm).
Table 2. NIRCam weak lens combinations
|Pupil wheel||Filter wheel||Waves of defocus||Availability for science|
|WLM8||-8||Not available for science|
|WLM8||WLP4+F212N2||-4||Not available for science|
|WLP4+F212N2||+4||NIRCam grism time series|
|WLP8||WLP4+F212N2||+12||Not available for science|
Spatial extent and saturation limits
The defocused image diameters are roughly 66, 132, and 198 pixels for 4, 8, and 12 waves of defocus, respectively. The peak intensity is about 4 times the average flux within these diameters. Thus, brighter sources may be imaged without saturation. Expected saturation limits for WLP8 images with the 160 × 160 pixel subarray are given in NIRCam time-series imaging. Table 3 shows the approximate saturation improvements (in magnitudes) gained by using weak lenses versus not using them.
Table 3. Weak lens defocusing and saturation limit improvements
|Waves of defocus||Image diameter |
|Saturation limit change |
The weak lenses deliver their highest transmission (90%–96%) between 1.6–2.2 µm and lowest transmissions (~50%) near ~0.6 and ~1.1 µm.
Greene, T., Beichman, C., Gully-Santiago, M., et al., 2010, SPIE, 7731
NIRCam: development and testing of the JWST near-infrared camera