NIRISS Optics and Focal Plane
NIRISS has an all-reflective optical design that consists of:
- a pick-off mirror (POM),
- a three mirror collimator assembly (TMA),
- a dual pupil/filter wheel assembly,
- a three mirror camera assembly, and
- a single Hawaii 2RG detector
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NIRISS optical assembly
The optical path is illustrated schematically in Figure 1. The optical assembly is attached to an aluminum optical bench, which is shared with the JWST Fine Guidance Sensor. Three kinematic mounts, made of titanium, attach the optical assembly to the structure of the Integrated Science Instrument Module (ISIM).
Figure 1. Optical path of NIRISS
A schematic plot showing the optical layout of NIRISS. © Honeywell.
Figure 2. Solid body rendering of NIRISS
Solid body rendering of NIRISS. Key components of the optical path are labelled. The kinematic mounts fasten NIRISS to the ISIM structure. © Honeywell.
Figure 3. NIRISS flight hardware
A photograph of the NIRISS flight hardware, at the Goddard Space Flight Center in December 2014, taken between the 2nd and 3rd cryo-vacuum test campaigns. Compare it with Figure 2, where key components are identified. The path between the POM (which is not visible in this image) and the collimator three mirror assembly (TMA) is shielded by internal baffling. © Honeywell.
Pick-off mirror
The pick-off mirror (POM) is a flat mirror composed of an aluminum substrate coated with nickel plating. Light from the fine steering mirror of the JWST Optical Telescope Element (OTE) is focused onto the NIRISS POM and directed into the instrument.
The POM is mounted on a movable stage that serves as the coarse focus mechanism (CFM) for NIRISS.
It has 4 coronagraphic occulters engraved in its surface. These deep, cone-shaped holes in the nickel overcoat are remnants of the original tunable filter imager (TFI) configuration of the instrument. Although NIRISS does not have a coronagraphic mode, these occulters will nevertheless leave their imprint on all images of externally-illuminated sources. When projected onto the detector, the occulters appear as circular spots with diameters of 0.58", 0.75", 1.5", and 2.0" (approximately 9, 11, 23, and 31 pixels, respectively), with positions that depend slightly on the focus.
Figure 4. Image from NIRISS observation, highlighting occulting spots
The imprint of the 4 coronagraphic occulters from the pick-off mirror are visible as 4 circular regions along the top left side of the detector in this image taken with the F090W filter. Image was taken during JWST commissioning, with APT program ID number 1063, observation number 184 (jw01063184003_02201_00001_nis_rate.fits).
Detector
The NIRISS detector is a single Hawaii 2RG sensor chip array with 2048 × 2048 pixels. It provides a field of view of 2.2' × 2.2' with a plate scale of approximately 0.066"/pixel.