NIRISS GR700XD Grism

The JWST NIRISS GR700XD grism produces spectra for wavelengths between 0.6 and 2.8 μm with a resolving power at blaze maximum in first order for R ≈ 700. The GR700XD enables single object slitless spectroscopy.

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Main article: NIRISS Single Object Slitless Spectroscopy Mode
See also: NIRISS-Specific Time-Series Observations

The GR700XD grism produces multiple orders of cross-dispersed spectra for a single object between wavelengths 0.6 and 2.8 μm. It provides resolving power of R ≈ 700 at the blaze wavelength in first order. The GR700XD grism enables the single object slitless spectroscopy mode of NIRISS, which is optimized for time-series observation applications that demand a high degree of spectro-photometric stability. 

The GR700XD grism has two components that are butted together. In the order that they are encountered by an incoming beam of light, these are:

  1. A prism made of Zinc Sulfide (ZnS). The wedge of the prism provides cross-dispersion of the spectral orders diffracted by the grism. In addition, the input surface of the prism has a weak cylindrical lens ground into it, which provides modest defocus for the spectral orders produced by the grism. By design, this defocus spreads the dispersed light over more pixels in the spatial dimension, which mitigates the effect of flat field errors and allows brighter sources to be observed without saturating. An anti-reflection coating was applied to both sides of the prism to minimize losses at the interfaces. 

  2. A grism made of Zinc Selenide (ZnSe), with the properties listed in Table 1. An anti-reflection coating was deposited on the side of this element that faces the prism.

Figure 1 shows a cutaway view of these two components, while Figure 2 shows the flight hardware for both the prism (input) side and the grism (output) side.

Figure 1. Cutaway view of the GR700XD grism

NIRISS GR700XD exploded view

This diagram shows the construction of the GR700XD grism and its mounting. In this view, light passing through the optical train of NIRISS moves from right to left, so that the prism is encountered first. Source: Honeywell.
Figure 2. GR700XD grism prior to installation

NIRISS GR700XD Flight Hardware

The NIRISS GR700XD grism before installation in the pupil wheel. Left: View of the ZnS prism from the "light input" side. Right: View of the rule surface of the ZnSe grism from the "light output" side. Source: Honeywell.


Table 1. Physical properties of the ZnSe grism

Property
MaterialZnSe
Prism Angle10.4º
Groove Density53.1 grooves/mm
Blaze Wavelength1.23 μm


Figures 1 and 2 also show that the GR700XD mounting in the pupil wheel includes a square aperture mask. The mask is undersized compared with the pupil, and obscures ~34% of the incoming light. Figure 3 shows a schematic view of the GR700XD grism.

Figure 3. Schematic view of the GR700XD grism



Resolving power

Main article: NIRISS Sensitivity, NIRISS Bright Limits

The PSF of NIRISS is undersampled over the wavelength ranges covered by the GR700XD grism. The resolving power thus varies with wavelength, as illustrated in Figure 4. 

Figure 4. The spectral resolving power for point sources for the GR700XD grism

The NIRISS PSF is undersampled over the wavelength range covered by the GR700XD grism in SOSS mode, and the resolving power increases with wavelength for a constant spectral bandpass Δλ. The solid point corresponds to the R ≈ 700 @1.4 μm.

The theoretical throughput of the GR700XD grism is shown in Figure 5, and is based on the physical properties listed in Table 1.

Figure 5. Theoretical GR700XD throughput curves for the first, second, and third order



Dispersion and spectral orders

Main article: NIRISS Detector Subarrays

The GR700XD grism generates 3 spectral orders of cross-dispersed spectra for a single target. The 1st spectral order covers wavelengths 0.9 to 2.8 μm with R ≈ 700, and the 2nd order covers wavelengths 0.6 to 1.4 μm at R ≈ 1,t400. At longer wavelengths the spectral orders overlap. The 3rd order peaks ~0.6 μm, mostly beyond the sensitivity range of the detector, and will thus likely be too faint to be scientifically useful

The spectra can be obtained in two subarray configurations, with the SUBARRAY 96 × 2048 (SUBSTRIP96 1) used when only the 1st order spectra needs to be recorded or for bright targets that require very fast timing readouts. The nominal subarray is 256 × 2048 (SUBSTRIP256) which captures both the 1st and 2nd orders which are the most useful for science observations. Full frame readout is also supported.

Figure 6. The cross-dispersed spectrum obtained using the GR700XD grism


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Coordinate system and dispersion direction

Main article: JWST Instrument Ideal Coordinate Systems

The geometry of the GR700XD dispersion direction, in the coordinate system used by the JWST calibration pipeline, is shown in Figure 7. The +V1 axis is along the telescope boresight.

Figure 7. Dispersion direction of the GR700XD grism

Schematic diagram that illustrates the dispersion direction for spectra produced by the GR700XD grism in the coordinate system used by the calibration pipeline.



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