James Webb Space Telescope
NIRISS Single-Object Slitless Spectroscopy

• Wavelength range: 0.6 – 2.5 μm
• Spectral resolving power: 700 at 1.25 μm
• Saturation limit: J=7 (with a goal of J=5 or 6)
• Optimized for spectroscopy of transiting exoplanets

GR700XD Design and Spectrum

The figures below show the components and the spectrum for Single-Object Slitless Spectroscopy. The components consist of

• a weak cylindrical lens of the "front" side of the prism to spread (defocus) the spectra by about 20 pixels in the spatial direction
• a weak prism to separate (cross disperse) the orders
• a grism to disperse the light

The direction of defocus is rotated by 2 degrees relative to the detector pixels to mitigate undersampling of the PSF in the spectral direction.

See the sensitivity of the NIRISS in its SOSS mode.

Dual Wheel Configuration for Single-Object Slitless Spectroscopy

In SOSS mode the GR700XD grism (highlighted in green) in the Pupil Wheel may be used only with the Filter Wheel in the CLEAR position (highlighted in green).