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.
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 six short wavelength filters with central wavelengths between 0.9 and 2.0 μm, while the filter wheel has six 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 aperture masking interferometry (AMI), wide field slitless spectroscopy (WFSS), and imaging modes.
Except for F158M, the NIRISS filters originated as NIRCam "flight spares." Long wavelength filters that populate the NIRISS filter wheel were transferred directly. However, 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. Properties of the low pass filters are included in all estimates of the throughput of the short wavelength "double-stack" NIRISS filters.
As a result of their common heritage, the properties of the NIRISS filters are very similar to their counterparts in NIRCam, even when allowing for the addition of the "low pass" element for the short wavelength filters.
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.
Table 1. Properties of NIRISS filters
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 (OTE), the internal optical properties 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.
NIRISS transmission and net throughput curves may be downloaded. Please see the accompanying "ReadMe" file for further information about this distribution.