NIRISS Pupil and Filter Wheels
The pupil wheel and filter wheel house the filters, low- and high-resolution grisms, and aperture mask that enable the 4 observing modes of JWST NIRISS.
On this page
See also: NIRISS Instrumentation, NIRISS Filters, NIRISS GR700XD Grism, NIRISS GR150 Grisms, NIRISS Non-Redundant Mask
NIRISS has a dual-wheel assembly that consists of the pupil wheel (PW) and filter wheel (FW) that are placed back-to-back as shown in Figure 1. Each wheel has 9 positions. Light from the collimator first passes through the "active" element in the pupil wheel and then through the "active" element of the filter wheel. The 2 wheels can be rotated independently to select the specific combinations of optical elements that define the NIRISS observing modes.
Figure 1. NIRISS dual wheel assembly, pupil wheel side
The NIRISS dual wheel assembly, shown from the pupil wheel side. © Honeywell.
Figure 2. NIRISS dual wheel assembly, filter wheel side
The NIRISS dual wheel assembly, shown from the filter wheel side. © Honeywell.
Figure 3. Layout of optical elements in the pupil filter and filter wheel
A schematic diagram showing the layout of optical elements in the pupil wheel (left) and filter wheel (right).
Contents of the pupil wheel
See also: NIRISS Filters, NIRISS GR700XD Grism, NIRISS Non-Redundant Mask, NIRISS Wide Field Slitless Spectroscopy, NIRISS Single Object Slitless Spectroscopy, NIRISS Aperture Masking Interferometry, NIRISS Imaging
Table 1. Contents of the pupil wheel
| Position | Element name | Description | Observing mode | Remarks |
|---|---|---|---|---|
| 1 | F090W | Broadband filter centered at 0.90 μm | WFSS, Imaging | |
| 2 | CLEARP | Empty slot | Imaging | Includes the permanently mounted pupil alignment reference (PAR), which was used for optical alignment during assembly and ground-based testing. It reduces transmission through the slot by ~16%. |
| 3 | GR700XD | Grism with resolving power ~700 at 1.4 μm. | SOSS | Includes a cylindrical lens to broaden spectral orders in the spatial direction |
| 4 | NRM | Aperture mask | AMI | Net transmission of ~15% |
| 5 | F200W | Broadband filter centered at 2.00 μm | WFSS, Imaging | |
| 6 | F150W | Broadband filter centered at 1.50 μm | WFSS, Imaging | |
| 7 | F140M | Medium-band filter centered at 1.40 μm | WFSS, Imaging | |
| 8 | F158M | Medium-band filter centered at 1.58 μm | WFSS, Imaging | |
| 9 | F115W | Broadband filter centered at 1.15 μm | WFSS, Imaging |
Contents of the filter wheel
See also: NIRISS Filters, NIRISS GR150 Grisms, NIRISS Wide Field Slitless Spectroscopy, NIRISS Single Object Slitless Spectroscopy, NIRISS Aperture Masking Interferometry, NIRISS Imaging
Table 2. Contents of the filter wheel
| Position | Element name | Description | Observing mode | Remarks |
|---|---|---|---|---|
| 1 | GR150R | Grism with resolving power ~150 at 1.4 μm | WFSS | Disperses along the direction of "fast" detector readout. |
| 2 | GR150C | Grism with resolving power ~150 at 1.4 μm | WFSS | Disperses along the direction of "slow" detector readout. |
| 3 | F277W | Broadband filter centered at 2.77 μm | AMI, Imaging | |
| 4 | F444W | Broadband filter centered at 4.44 μm | Imaging | |
| 5 | F356W | Broadband filter centered at 3.56 μm | Imaging | |
| 6 | F430M | Medium-band filter centered at 4.30 μm | AMI, Imaging | |
| 7 | F380M | Medium-band filter centered at 3.80 μm | AMI, Imaging | |
| 8 | F480M | Medium-band filter centered at 4.80 μm | AMI, Imaging | |
| 9 | CLEAR | Empty slot | SOSS, Imaging |
Operation and overheads
See also: JWST Observing Overheads and Time Accounting Overview
The pupil and filter wheels can rotate in either direction. Consequently, any optical element can be placed into the "active" position by a move of 4 positions or fewer. The wheel mechanisms have angular repeatability of ±0.1651° for the pupil wheel and ±0.1585° for the filter wheel. The exact angular position of each wheel is recorded in a FITS header keyword (PWCPOS, FWCPOS), which may be used to refine knowledge of the location or tilt of spectral traces on the detector, for example.
References
Martel, A. 2022 JWST-STScI-008298
The Early Behavior of the NIRISS Pupil Wheel and Filter Wheel
Martel, A. 2016 JWST-STScI-004964
Analysis of NIRISS CV3 Data: The Repeatability of the Pupil Wheel and the Filter Wheel