Introducing NIRISS: A New Science Instrument for Webb

René Doyon, doyon@astro.umontreal.ca and Alex Fullerton, fullerton@stsci.edu

Last July, the Canadian Space Agency (CSA) reluctantly decided to discontinue work on the Tunable Filter Imager (TFI) for Webb. This decision, which was made in consultation with the science team for the Webb Fine Guidance Sensor (FGS), came after it became clear that various technical issues associated with operating the Fabry-Perot etalon at cryogenic temperatures were unlikely to be resolved in time to meet the instrument’s delivery schedule.

In its place, CSA is providing a new science instrument that enhances the capabilities of the Webb observatory while also being simpler to build and operate. The reconfigured hardware has been named the Near InfraRed Imager and Slitless Spectrograph (NIRISS). The NIRISS configuration eliminates the etalon and re-populates the dual wheel of the TFI with spare broad-band filters from the Near Infrared Camera (NIRCam), several carefully defined new filters, and three dispersing elements (grisms). As with the TFI, NIRISS shares its optical bench with the Fine Guidance Sensor (FGS), but is functionally separate from it.

NIRISS is designed to provide observations that will advance each of the four Webb science themes: (1) the end of the “dark ages”—“first light” and reionization; (2) the assembly of galaxies; (3) the birth of stars and protoplanetary systems; and (4) planetary systems and the origin of life. However, as a reflection of the interests of the science team, it has been optimized for investigations of first light and the detection and characterization of exoplanets. NIRISS will complement and extend the existing near-infrared capabilities of Webb by providing alternate and, in some cases, enhanced techniques for carrying out specific observational programs.

 

Hardware configuration

Figure 1 shows the internal layout of NIRISS. Light from the Webb optical telescope element (OTE) is intercepted by the pick-off mirror and collimated before passing through optical elements in the pupil wheel and filter wheel assembly. The camera focuses light onto the detector, which is a 2048 × 2048 mercury-cadmium-telluride array manufactured by Teledyne Imaging Sensors. The field of view of NIRISS is 2.2 × 2.2 arcmin, which provides a plate scale of ~65 milliarcsec per pixel.

Figure 2 provides a schematic illustration of the nine optical elements in the pupil wheel and filter wheel.

The pupil wheel carries:

  • an open element (OPENP). When it is selected, only the component in the filter wheel conditions the light. The Pupil Alignment Reference (PAR) mirror and support struts produce a small central obscuration. The PAR is only used during ground testing.
  • a grism (G700XD), which provides cross-dispersed spectra with resolving power R ~ 700 in first order, near the blaze wavelength (1.23 microns).
  • a seven-hole aperture mask with non-redundant baselines (NRM).
  • two medium-band ‟blue” filters, which are centered at 1.40 and 1.58 microns.
  • four broad-band ‟blue” filters, which are centered at 0.90, 1.15, 1.50, and 2.00 microns. These filters are NIRCam flight spares combined with long-wavelength (>2.5 microns) blockers.

All the elements in the pupil wheel have Lyot stops, which either outline the OTE (a 30-sided polygon known as a tricontagon or triacontagon), or consist of a square (G700XD) or the NRM.

The filter wheel carries:

  • an open position (OPENF), which allows light to be conditioned only by a component in the pupil wheel.
  • two grisms (G150H, G150V), which provide spectra with R =150 near the blaze maximum (1.30 microns) in first order. The grisms are identical, but are mounted so that spectra are dispersed in orthogonal directions on the detector. These directions are indicated schematically by H and V, which stand for ‟horizontal” and ‟vertical,” respectively.
  • three medium-band filters, with central wavelengths of 3.8, 4.3, and 4.8 microns. These wavelengths constrain the spectral energy distributions of brown dwarfs and exoplanets, and were chosen specifically for use with the NRM.
  • three broad-band ‟red” filters, which are centered at 2.77, 3.56, and 4.44 microns. These filters are also flight spares from NIRCam.