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The pupil wheel and filter wheel house the filters, low- and high-resolution grisms, and aperture mask that enable the four observing modes of JWST NIRISS.


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Introduction

Parent page: NIRISS Instrumentation
See also: 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 nine 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 two wheels can be rotated independently to select the specific combinations of optical elements that define the NIRISS observing modes.

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Figure 1. NIRISS dual wheel assembly, pupil wheel side

NIRISS Dual-Wheel Assembly from the Pupil Wheel side

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The NIRISS dual wheel assembly, shown from the pupil wheel side. Source: Honeywell.



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Figure 2. NIRISS dual wheel assembly, filter wheel side

NIRISS Dual-Wheel Assembly from the Filter Wheel side

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The NIRISS dual wheel assembly, shown from the filter wheel side. Source: Honeywell.



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Figure 3. Layout of optical elements in the pupil filter and filter wheel

Layout of the NIRISS Pupil Wheel and Filter Wheel

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A schematic diagram showing the layout of optical elements in the pupil wheel (left) and filter wheel (right).




Contents of the pupil wheel

Main articles: NIRISS Filters, NIRISS GR700XD Grism, NIRISS Non-Redundant Mask
See also: NIRISS Wide Field Slitless Spectroscopy, NIRISS Single Object Slitless Spectroscopy, NIRISS Aperture Masking Interferometry, NIRISS Imaging


Table 1. Contents of the pupil wheel

 PositionElement nameDescriptionObserving modeRemarks
1F090WBroadband filter centered at 0.90 μmWFSS, Imaging 
2CLEARPEmpty slotImagingIncludes 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%.
3GR700XD

Grism with resolving power ~700 at 1.4 μm.

SOSS

Includes a cylindrical lens to broaden spectral orders in the spatial direction

4NRMAperture maskAMINet transmission of ~15%
5F200WBroadband filter centered at 2.00 μmWFSS, Imaging 
6F150WBroadband filter centered at 1.50 μmWFSS, Imaging 
7F140MMedium-band filter centered at 1.40 μmWFSS, Imaging 
8F158MMedium-band filter centered at 1.58 μmWFSS, Imaging 
9F115WBroadband filter centered at 1.15 μmWFSS, Imaging 

 


Contents of the filter wheel

Main articles: NIRISS FiltersNIRISS GR150 Grisms
See also: NIRISS Wide Field Slitless SpectroscopyNIRISS Single Object Slitless SpectroscopyNIRISS Aperture Masking InterferometryNIRISS Imaging


Table 2. Contents of the filter wheel

 PositionElement nameDescriptionObserving modeRemarks
1GR150RGrism with resolving power ~150 at 1.4 μmWFSSDisperses along the direction of "fast" detector readout.
2GR150CGrism with resolving power ~150 at 1.4 μmWFSSDisperses alongthe direction of "slow" detector readout.
3F277W

Broadband filter centered at 2.77 μm

AMI, Imaging

 

4F444WBroadband filter centered at 4.44 μmImaging 
5F356WBroadband filter centered at 3.56 μmImaging 
6F430MMedium-band filter centered at 4.30 μmAMI, Imaging 
7F380MMedium-band filter centered at 3.80 μmAMI, Imaging 
8F480MMedium-band filter centered at 4.80 μmAMI, Imaging 
9CLEAREmpty slot

SOSS, Imaging

 

 


Operation and overheads

Main article: 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 four positions or fewer. Table 3 lists the overheads (including commanding overheads and backlash corrections) associated with querying the current position of a wheel (Δposition = 0) or moving it by 1, 2, 3, or 4 positions. Due to power constraints, the pupil wheel and filter wheel must be configured sequentially.

 

Table 3. Overheads for motion of the pupil or filter wheel

ΔpositionOverhead
012.12 s
118.12 s
222.12 s
326.12 s
433.12 s

 

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, which may be used to refine knowledge of the location or tilt of spectral traces on the detector, for example.

 


 


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References

 Martel, A. 2016  JWST-STScI-004964
Analysis of NIRISS CV3 Data: The Repeatability of the Pupil Wheel and the Filter Wheel

JWST technical documents


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Last updated

Published July 11, 2017


 

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Published March 02, 2017