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 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 inNIRCam time-series observations at 1.3–2.2 µm to:

  • avoid saturation of bright sources,
  • provide robustness against telescope jitter, and
  • mitigate flat field uncertainties.

There are three weak lens elements available in the short wavelength channel (0.6–2.3 µm) on thepupil and filter wheels in each NIRCam module.


Table 1. NIRCam weak lenses

Weak lensWaves of defocus
(at 2.12 µm) 
Location
WLP8+8Pupil wheel
WLM8-8Pupil wheel
WLP4+F212N2+4Filter 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 three 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 five 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 wheelFilter wheelWaves of defocusAvailability for science
WLM8
-8Not available for science
WLM8WLP4+F212N2-4Not available for science

WLP4+F212N2+4NIRCam grism time series
WLP8
+8

NIRCam time series imaging
NIRCam grism time series

WLP8WLP4+F212N2+12Not available for science

Figure 1. NIRCam weak lens images

NIRCam weak lens images

Images formed in the NIRCam short wavelength channel for the various defocus values achievable using the weak lenses. Ghost image artifacts to the left of center and below were caused by reflections in ground testing and should not appear in flight. Adapted from Greene et al. 2010, Figure 3.


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 four times the average flux within these diameters. Thus, brighter sources may be imaged without saturating. Expected saturation limits for WLP8 images with the 160 × 160 pixel subarray are given in NIRCam time-series imaging. Table 3 shows the 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) 
4665.0
81326.5
12198 8.0

Figure 2. Weak lens +8 image

The WLP8 defocused image of a point source fits well within a 160 × 160 pixel subarray (white box). A 132-pixel diameter aperture (red circle) contains >97% of the total flux.


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. In time-series observations, the weak lenses may only be paired with filters redward of 1.3 µm.

Figure 3. Weak lens throughputs

Weak lens throughputs

Transmissions of the WLP8 and WLP4+F212N2 weak lens anti-reflective coatings alone (excluding other NIRCam or JWST throughputs) based on laboratory measurements of witness samples (test pieces) (from filters version 4.0: April 2016). WLP8 is in the pupil wheel and must be used in conjunction with a filter in the filter wheel. WLP4+F212N2 includes a narrowband filter with 2.3% bandpass (wider than the 1% bandpass filter F212N). WLP4+F212N2 is in the filter wheel and is used in conjunction with the CLEAR pupil element for science. Engineering modes may use WLP4+F212N2 in combination with other pupil wheel elements, including WLP8, yielding a total of +12 waves of defocus at 2.12 µm.


References

Greene, T., Beichman, C., Gully-Santiago, M., et al., 2010, SPIE, 7731
NIRCam: development and testing of the JWST near-infrared camera




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