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.
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The NIRCam weak lenses defocus incoming light, spreading it over a larger number of detector pixels. They were used during commissioning to complete the fine phasing alignment of the JWST primary mirror. Since commissioning, they are being 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.
There are 3 weak lens elements available in the short wavelength channel (0.6–2.3 µm) on the pupil and filter wheels in each NIRCam module.
Table 1. NIRCam weak lenses
Weak lens | Waves of defocus (at 2.12 µm) | Location |
---|---|---|
WLP8 | +8 | Pupil wheel |
WLM8 | -8 | Pupil wheel |
WLP4+F212N2 | +4 | Filter wheel |
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 are used for wavefront sensing, but only WLP8 and WLP4 are 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 | +8 | ||
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 (pixels; approx.) | Saturation limit change (magnitudes) |
---|---|---|
4 | 66 | 5.0 |
8 | 132 | 6.5 |
12 | 198 | 8.0 |
Transmission
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.
References
Greene, T., Beichman, C., Gully-Santiago, M., et al., 2010, SPIE, 7731
NIRCam: development and testing of the JWST near-infrared camera