NIRCam has three imaging categories: survey mode, small source mode, and coronagraphic mode. In addition, the medium-resolution grisms in the long-wave channel can be used for spectroscopy. Survey and small source modes differ only in the area of sky to be observed and hence many operations will be common between these two modes. Grism spectroscopy is very similar to small-source mode, while coronagraphy differs in many respects.
NIRCam Imaging Properties:
|Wavelength range (µm)||0.6 to 2.3|
|2.4 to 5|
|Nyquist λ (µm)||2/4|
|Pixel Format||40962 (short λ)|
|Pixel Scale||0.032″ (short λ)|
|0.065″ (long λ)|
|Field (arc min)||2.2 x 2.2 (one module)|
Small Source Imaging
Sources small enough to fit within the FOV of a single NIRCam chip should be imaged using the Small-Source Imaging Activity Descriptor (AD). A single SW chip will be used for all point-source imaging, and the target image will be placed near its center. It will be possible to simultaneously image the target in the LW channel. Parallel observations can be obtained in the other imaging module, but only at the same wavelength(s) as for the primary module and using the same multiaccum parameters.
In survey-mode both modules and all FPAs will be used to provide simultaneous coverage of a 2.2’x4.4’ FOV at two wavelengths. Both modules must be configured with the same filters, one short- and one long- wave, and the same multiaccum parameters must be used in both channels.
- Coronagraphic capability in both short and long wavelength channels in both modules.
- A fixed, transparent optical mount is located at the focus after the pick-off mirror, with the coronagraphic elements (masks and target-acquisition neutral-density spots) on one surface of this mount.
- Normally the coronagraph is outside the field of view of the detectors, but by inserting an optical wedge (mounted in the pupil wheel) into the beam, the coronagraph is brought into view.
- The five occulters are located on a transparent plate with clear areas of 20” x 20” each.
- Between the occulter fields are 5” x 5” neutral density squares to allow for the acquisition of bright stars.
- There are three radially-symmetric occulters with “sombrero” profiles scaled to match the given wavelengths for disk and extragalactic imaging. Two wedge-shaped occulters with sinc2 profiles provide user-selectable widths, allowing for optimal imaging depending on wavelength.
- Use of any of the coronagraphic masks requires that the target be placed accurately (to within 30 mas) behind the mask. Because the observatory can only provide pointing with an accuracy of 0.7″ (1-σ per-axis), a target acquisition process precedes each coronagraphic visit. Once the target has been acquired and placed behind the chosen mask, coronagraphic science imaging can begin.
To attain the ultimate high-contrast imaging, Lyot stops (apodizing masks) specially tailored for coronagraphic observations are also used. The Lyot stop masks are deposited onto the OTA facing surface of the optical wedge mounted in the pupil wheel. Each pupil wheel has two Lyot spots which are different for the SW and LW channels, The Lyot stops have a clear aperture with approximately 19% of the throughput of the regular imaging pupil. Same spot-specific Lyot stop is used for all three circular occulters and same wedge-specific Lyot stop is used for both wedges
The mechanical mount for each coronagraph includes LED point-source emitters attached at each end that can send a signal back into the imager to check the internal alignment and confocality of NIRCam during ground test and on-orbit calibration.
NIRCam has a grism in the long wavelength channel, the primary purpose of which is to assist with the coarse phasing of JWST. But the grism can also be used for scientific observations. The grism offers the capability to carry out slitless spectroscopy in the wavelength range 2.4 to 5 µm, with spectroscopic resolution R ~ 2000.
NIRCam will also be used for wave front sensing to assure perfect alignment and shape of the different primary mirror segments so that their wavefronts match properly, creating a diffraction-limited 6.6 m telescope, rather than overlapping images from 18 individual 1.3m telescopes. Each imaging module has a pupil wheel with extra optics and pupil analyzers for wave front sensing. The wave front sensing capability is provided fully redundant in both imaging modules because the mission depends critically on its functionality.
The wavefront sensing (WFS) activities occur in two distinct phases of the mission.
During commisioning NIRCam will be used to align all 18 primary mirror segments so that the primary mirror would function as though it were a monolithic mirror. The mirror segments must be positioned so their wavefronts are phased.
Steps in the wave-front sensing and control process during JWST commissioning are shown in the figure below. NIRCam-specific activities are highlighted in yellow. Optional or possible execution paths are shown as dashed red lines.
Periodic Observations To Monitor JWST Image Quality
- WFS observations will be executed every two days to measure the relative alignment of the components of the Optical Telescope Element (OTE), especially the segments of the primary and the six degrees of freedom of the secondary.
- Weak lenses will be used to image a bright target star using several defocus settings.
- Following downlink to the ground, these images will be analyzed at STScI using specialized software under development by Ball Aerospace. The software will perform focus-diverse phase retrieval to determine the optical path difference (OPD) map over the telescope exit pupil.
- In turn, this will be used to determine the mirror actuator moves necessary to optimize the JWST image quality. Such wavefront control is expected to happen no more frequently than once every two weeks under normal operations.
Wavefront Sensing Optics
- The pupil and filter wheels contain the bulk of the elements required for wavefront sensing and control (WFS&C) functions for JWST.
- They are predominantly in the SW channel and include two arrays of grisms for Dispersed Hartman Sensing (DHS), used during the coarse phasing of the primary mirror segments (and in conjunction with the F150W2 WFS passband filter in the filter wheel), and weak lenses to be used during fine phasing of the mirror segments and in their routine adjustment.
- Two other long-wavelength grisms (dispersion directions perpendicular to each other) are included for dispersed fringe sensing to increase the coarse-phasing capture range beyond what can be accommodated by the short wavelength grisms, and may be employed during commissioning if necessary.
- The two DHS devices in each SW channel are rotated 60° relative to one each other, and each samples 10 inter-segment edges. This allows coarse-phasing of segments in a pair-wise fashion, while ultimately achieving phasing of the entire segment array.
The pupil wheels include a pupil alignment pinhole projector assembly (PAPPA). The SW PAPPA has on both sides (inward and outward) a pattern of 18 pinholes associated with the primary mirror segments. The PAPPA will be used in conjunction with the pupil imaging lens (PIL) located in the short wavelength channel. Images taken through the PAPPA and the PIL will accurately map the internal NIRCam pupil onto the SW FPA. Images of a bright star taken using the PIL will map the OTE pupil, and comparisons of the two types of image will show whether NIRCam is properly aligned with the OTE pupil, and allow adjustments to be made (if necessary) using the FAM. A second purpose of PIL is to measure the OTE illumination pattern of the NIRCam pupil, needed as an input to the phase-diversity calculations used during fine phasing of the mirror segments.