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JWST NIRISS has 12 medium- and broadband filters that cover the wavelength range between 0.8 and 5.0 μm in support of applications involving aperture masking interferometry, wide field slitless spectroscopy, and imaging.

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Introduction

NIRISS has a total of 12 filters, which are located in the NIRISS pupil and filter wheels as shown in Figure 1. The pupil wheel contains 6 short wavelength filters with central wavelengths between 0.9 and 2.0 μm, while the filter wheel has 6 long wavelength filters with central wavelengths between 2.8 and 4.8 μm. When used in combination with other optical elements, these filters support the NIRISS modes of aperture masking interferometry (AMI), wide-field slitless spectroscopy (WFSS), and imaging

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Figure title

Figure 1. Distribution of filters in the pupil wheel and filter wheel

Allocation of NIRISS filters

Figure caption

Schematic diagram showing the location of bandpass filters in the pupil (left) and filter (right) wheel of NIRISS. In the pupil wheel, short wavelength broad- and medium-band filters are indicated in blue and cyan, respectively. In the filter wheel, long wavelength broad- and medium-band filters are indicated in red and orange, respectively.


Historical context

Except for F158M, the NIRISS filters originated as NIRCam "flight spares." The long wavelength filters that populate the NIRISS filter wheel were transferred directly. However, the filters with central wavelengths shorter than 2.5 μm required the addition of a "low pass" filter to eliminate "red leaks" inherent in the design of their optical coatings. 

These red leaks (from radiation at wavelengths > 2.5 μm) did not affect NIRCam: the short-wavelength filters were designed to be used with HgCdTe detectors with a 2.5 μm cut-off, which preclude the detection of photons at longer wavelengths. Since these photons were still detectable by the NIRISS detector (5.2 μm cutoff), the "double-stack" filter design shown in Figure 2 was implemented for all the filters in pupil wheel except F158M. The properties of the low-pass filters are included in all estimates of the throughput of the short wavelength, "double-stack" filters of NIRISS.

As a result of their common heritage, the properties of the NIRISS filters are very similar to their counterparts in NIRCam, even allowing for the addition of the "low-pass" element for the short-wavelength filters. 

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Figure 2. Double-stack design of NIRISS short wavelength filters

NIRISS Double-Stack Filters (exploded view)

Figure caption

Exploded view of the structure of a NIRISS "double stack" filter. Origin: Honeywell.


Properties of the NIRISS filters

Figure 3 illustrates the transmission curves for the entire suite of NIRISS filters. Key properties of the filters are quantified in Table 1. 

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Figure 3. Transmission curves for the NIRISS filters

NIRISS filter transmission curves

Figure caption

Transmission curves for the entire suite of NIRISS filters, based on measurements at cryogenic temperatures by the manufacturer.

 

Table 1. Properties of NIRISS filters

Filter

LaTeX Math Inline
body\lambda_{pivot}~(\mu\rm m)\,^1

BW (μm)2Effective response3

LaTeX Math Inline
body\lambda_{-} (\mu \rm m)\,^4

LaTeX Math Inline
body\lambda_{+} (\mu \rm m)^4

Observing mode
Broad-band filters
F090W0.9000.1940.8800.7961.004WFSS, Imaging
F115W1.1500.2410.8391.0131.282WFSS, Imaging
F150W1.4980.3330.9361.3311.670WFSS, Imaging
F200W1.9840.4610.9451.7512.225WFSS, Imaging
F277W2.7760.7150.8752.4133.142AMI, Imaging
F356W3.5950.9210.9023.1414.068Imaging
F444W4.4351.1210.8583.8805.023Imaging
Medium-band filters
F140M1.4050.1470.8971.3321.480WFSS, Imaging
F158M1.5870.1960.8761.4881.688WFSS, Imaging
F380M3.8280.2050.8673.7263.931AMI, Imaging
F430M4.2860.2020.7944.1824.395AMI, Imaging
F480M4.8170.2980.6904.6694.971AMI, Imaging
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Notes:

1 The pivot wavelength,

LaTeX Math Inline
body\lambda_{pivot}
, relates the flux per unit wavelength (
LaTeX Math Inline
bodyF_\lambda
) to the flux per unit frequency (
LaTeX Math Inline
bodyF_\nu
). It is defined as:  
LaTeX Math Inline
body\lambda^2_{pivot} = {\int{ \lambda T(\lambda)\, \rm{d}\lambda}}\,\, / \,\,{\int{ \left(T(\lambda)\,/\,\lambda \right)\, \rm{d}\lambda}}
, where 
LaTeX Math Inline
bodyT(\lambda)
is the filter transmission curve.

2 The bandwidth BW is the integral of the filter transmission curve normalized by the maximum transmission (

LaTeX Math Inline
bodyT_{max}
): 
LaTeX Math Inline
bodyBW = \int T(\lambda) \,{\rm d}\lambda\,\, /\,\, {T_{max}}
.  

3 The effective response is the mean transmission value over the wavelength range of

LaTeX Math Inline
body\lambda_{pivot} \pm BW / 2
.

4 The short (

LaTeX Math Inline
body\lambda_{-}
) and long (
LaTeX Math Inline
body\lambda_{+}
) half power wavelengths of a filter are the wavelengths at which its transmission falls to 50% of its peak value.

 


NIRISS system throughput

Figure 4 shows the net photon-to-electron conversion efficiency (PCE) for each of the NIRISS filters.These values were estimated by combining current understanding of the throughput of the JWST Optical Telescope Element, the internal optical of NIRISS, and the quantum efficiency of the NIRISS detector. At any epoch, these effects are taken into account by the JWST Exposure Time Calculator (ETC).Although Figure 4 is useful for "back-of-the-envelope" calculations, critical applications should use the ETC to derive definitive estimates of performance.

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Figure title

Figure 4. System throughput of the NIRISS filters

NIRISS filter photon-to-electron conversion efficiency curves

Figure caption

Net throughput curves for each of the NIRISS filters.The net throughput represents the estimated performance of the JWST optical telescope element and the internal optics of NIRISS.

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Sections / Comments to add

Leaks

 Ghosts: Mention that the filters are tipped to redirect ghost images off the detector.Talk about the complex ghosting associated with the double-stack filters

Tricontagon masks

 


Ancillary data

The NIRISS transmission and net throughput curves are available (click to download).Please see the ReadMe file for further information about this distribution.

 


 

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Related links

JWST User Documentation Home

NIRISS Observing Modes

NIRISS Wide Field Slitless Spectroscopy
NIRISS Aperture Masking Interferometry
NIRISS Imaging 
 

NIRISS Detector

JWST Detector MULTIACCUM Integration
NIRISS Detector Readout Patterns

NIRISS Operations

NIRISS Operations

NIRISS Performance

NIRISS Bright Limits
NIRISS Sensitivity
NIRISS AMI-Specific Treatment of Limiting Contrast 

Recommended Strategies

NIRISS Recommended Strategies

NIRISS Science Use Cases

NIRISS AMI Observations of Extrasolar Planets Around a Host Star
NIRISS WFSS and NIRCam Imaging of Galaxies Within Lensing Clusters

Observing Methods

JWST Slitless Spectroscopy
JWST High-Contrast Imaging
JWST Parallel Observations

Exposure Time Calculator

JWST Exposure Time Calculator Overview

Astronomer's Proposal Tool

JWST Astronomers Proposal Tool Overview
Creating a NIRISS Observing Program
JWST APT Coordinated Parallel Observations 

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References

JWST technical documents

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Last updated

Published July 11, 2017


 

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Published March 02, 2017


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UpdatedJuly 11, 2017
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TeamNIRISS 
AuthorFullerton, Ravindranath, La Massa 
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UpdatedJuly 11, 2017
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