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.
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:
- 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.
- 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.
Table 1. Physical properties of the ZnSe grism
|Groove Density||53.1 grooves/mm|
|Blaze Wavelength||1.23 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.
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.
The theoretical throughput of the GR700XD grism is shown in Figure 5, and is based on the physical properties listed in Table 1.
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.
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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.