A.

Technical Issues

The optical assembly of the FGS consists of two modules: the Guider and until mid 2011 the Tunable Filter Imager (TFI). The TFI module allowed narrow–band imaging with spectral resolution R=100 in one of two wavelengths ranges: 1.6 to 2.5 μm and 3.2 to 4.9 μm [2]. The motivation for the TFI replacement with NIRISS came from the numerous technical and scientific factors concerning TIF’s Fabry-Perot etalon. Many of the technical challenges encountered were successfully resolved but the remaining few proved to be very difficult to overcome within the project schedule. At the end of the TFI development, the main identified obstacles were:

In retrospective, the root causes of the etalon development failure can be summarized as:

B.

New Science Goals

The original scientific objectives of the TFI were explanatory observation and first light detection. The latter capability was diminished already in the past when the TFI short wave channel, starting at about 1 μm, was eliminated due to the observatory mass constraints. Still, the TFI 1.5 to 1.6 μm short wavelength range was considered to be useful. However, by 2011 the epoch of re-ionization had become better defined as z=10.6±1.2 with resulting shift of Ly-a emissions to shorter wavelengths and smaller expected range of re-ionization [4, 5]. That would diminish even more the TFI first light detecting capabilities. The NIRISS instrument will bring it back by providing four observing modes: broad-band imaging between 0.9 and 5 μm, R=150 wide-field spectroscopy between 1.0 and 2.5 μm, single-object cross-dispersed slitless spectroscopy optimized for exoplanet transit spectroscopy between 0.6 and 2.5 μm at R=700, and sparse-aperture interferometric high contrast imaging between 3.8 and 4.8 μm [6].

A.

Broad Band Imaging (BBI)

As illustrated in Fig.4, due to mostly reflective optics, NIRISS sensitivity is very similar to NIRCam (another JWST instrument), allowing complementary parallel mode of operation of both instruments.

Fig.4.

NIRISS sensitivity in BBI mode (10σ, 10,000σ, BOL)

Fig.5.

NIRISS GR150 grism

B.

Wide-Field Slitless Spectroscopy (WFSS)

The WFSS mode of NIRISS operation is optimized for Lya emitters (1-2.5 μm) and makes use of a pair of grisms GR150V and GR150H. In order to break wavelength-position degeneracy two prisms are at 90 deg angle to each other and are used in two separate imaging sessions. In this scheme, the intersection of the two perpendicular dispersion lines indicates undeviated wavelength and true sky position of the source. Both GR150 are made of infrasil 301 with resin replicated gratings. Other specifications are: resolution=150, aperture=43 mm, groove density=11.21 lines/mm, blaze angle=1.5 deg, blaze wavelength=1.3 μm and undeviated wavelength=1.05 μm. In Fig.6 GR150 blaze curve is shown as measured and compared to numerically modeled values. The modeling included groove shape imperfections deduced from the Atomic Force Microscopy (AFM) sampling.

Fig.6.

NIRISS GR150 grism efficiency curves, measured vs. modeled.

Fig.7.

NIRISS sensitivity in WFSS mode.

Fig.8.

NIRISS GR700 grism-prism assembly

Fig.9.

NIRISS GR700XD grism

Fig.10.

NIRISS G700XD efficiency, measured vs. modeled

C.

Single-Object Slitless Spectroscopy (SOSS)

This mode of NIRISS operation is optimized for relatively bright stars (e.g. exoplanet transiting systems) in 0.6-2.5 μm spectral range in the first order of dispersion. It is based on a GR700XD grism made of the directly ruled ZnSe. A ZnS cross-dispersion prism is placed in front of the grism for an optimal separation of the first and second order spectra. A weak cylindrical lens, built into ZnS prism induces a defocus in spatial direction and together with 2 deg rotation about Z axis mitigates PSF spectral undersampling. Other specifications are: resolution=700, aperture=33x33mm, groove density=54.4 lines/mm, apex angle=1.9 deg, blaze angle=2.63 deg and blaze wavelength=1.25 μm. As for GR150, efficiency curves were modeled numerically to account for grooves imperfections sampled by AFM.

Simulation of NIRISS SOSS spectra is shown in Fig. 11. The dispersion is placed along the detector fast axis and close to its edge. Reading it out via four parallel amplifiers in 256 x 2048 pixels subarray mode, will allow short integration times to increase detection dynamic range with stars brightness up to J=5.

Fig.11.

NIRISS GR700XD spectra simulation

Fig.12

NIRISS NRM mask in the pupil wheel

Fig.13.

Fully assembled NIRISS (bottom) and FGS-Guider

D.

Sparse-Aperture Interferometric Imaging (SAII)

The SAII is based on the use of a Non-Redundant Mask for high-contrast imaging of the exoplanets very close to their parent stars. The major change, compared to the TFI, is the removal of the coronagraphic mode (due to absence of the pupil apodization masks). The SAII spectral range extends from 3.8 to 4.8 μm covered by three medium bandwidth filters listed in Table. I. This will allow more distant (fainter) stars detection than in TFI.

Table II summarizes AMS performance simulation results.

TABLE II.