NIRISS Features and Caveats
This article alerts observers to instrumental artifacts that may affect observation planning or the interpretation of data obtained with NIRISS.
Internal reflections within the optical path of NIRISS can result in optical ghosts in NIRISS imaging and wide field slitless spectroscopy (WFSS) observations (Martel 2019). See the NIRISS Ghosts article for more information about ghosts in imaging observations, and how to identify ghosts when analyzing data. An example of ghosts observed from early inflight observations is shown in Figure 1. Dispersed ghosts are discussed in the NIRISS GR150 grisms article.
Imprint of occulting spots
The NIRISS pick-off mirror has 4 coronagraphic occulters engraved on its surface that project circular spots with diameters of 0.58", 0.75", 1.5", and 2.0" (approximately 9, 11, 23, and 31 pixels, respectively) located about 11" from the top of the detector (Figure 2). The occulting spots were used as part of the coronagraphic mode of the tunable filter imager (Doyon et al. 2010), NIRISS’s former instrument configuration. These features leave imprints on imaging and WFSS observations, so careful planning of WFSS observations may be needed if there are particular objects of interest near that region of the detector. The "medium" and "large" dither step sizes are sufficient to move a source over the smaller 2 spots but not enough to fully cover the larger 2 spots. There will be a very small region that is not covered by the "large" 4-point dither pattern.
Overlapping spectral orders in SOSS mode
The single object slitless spectroscopy mode produces spectra in 3 orders that cover the wavelength range from 0.6 to 2.8 µm. The 1st order covers wavelengths between 0.9 and 2.8 µm, the 2nd order covers wavelengths between 0.6 and 1.4 µm, and the 3rd order is very weak and is very weak and will generally not be useful. The traces of the spectral orders overlap at wavelengths between 2.4 and 2.8 µm in the 1st order (>1.2 µm 2nd order) when using the standard GR700XD/CLEARP filter combination. Adding an exposure using the GR700XD/F277W combination to a SOSS observation can help isolate data in the 1st order between 2.4 and 2.8 µm. The spectra in this overlap region are disentangled by the code Algorithm to Treat Order ContAmination (ATOCA, Darveau-Bernier et al., in prep) in the extract_1d step of the calwebb_spec3 stage of the JWST Science Calibration Pipeline in versions 1.8 and later. Note that photometric calibration is applied after the spectral extraction step for the SOSS observing mode.
Inflight observations revealed that light from a "susceptibility region" far away from the NIRISS field-of-view (2.0º < V2 < 5.0º, 12.4º < V3 < 12.8º) can scatter into the detector via a rogue path (i.e., light can enter the telescope aperture without reflecting off the primary or secondary mirror). In many cases, the scattered emission is from zodiacal light that causes a horizontal band about 25–30 pixels high across the full width of the detector, affecting 1%–2% of the detector, and is about 1% brighter than the background. If, however, there is a bright star with a near-infrared magnitude (J, H, K) brighter than ~2 (Vega mag) in the susceptibility region, this feature becomes sharper and brighter. This light saber, an example of which is shown in Figure 3, produces a local background that is about 10% higher than the global background, but it is a factor of about 10-7 fainter than the star which causes it.
The light saber can affect both NIRISS imaging and WFSS observations. Spectra from the GR150C grism are dispersed along the light saber axis while spectra from the GR150R grism are dispersed vertically, which would increase the extent of the light saber in these exposures by another ~50–100 pixels (affecting ~5%–7% of all pixels).
A tool will be developed to assist with the identification of bright targets in the susceptibility region, which will help users asses the effect of the light saber on a specific observation.
Anomalous spikes are observed in bright stars in the short wavelength NIRISS filters (F090W, F115W, F140M, F150W, F158M, and F200W). These spikes are more diffuse than diffraction spikes, and come in 2 flavors (Figure 4): 11 and 5 o'clock spikes and 1 and 7 o'clock spikes. The angle of the anomalous spikes is a strong function of detector X position: θ1 has a maximum value of 25º at X = 0 and θ2 has a maximum value of -12º at X = 2048 (with θ1 defined as positive and θ2 defined as negative). The transition from the 11/5 o'clock spikes to the 1/7 o'clock spikes occurs at X ~ 1,400. The intensity of the anomalous spikes is strongest in F090W, and decreases with filter wavelength. The 11/5 o'clock spikes were also seen in ground testing (Martel 2019).
Light from stars outside the NIRISS field of view can scatter off the focal plane array (FPA) baffle into the NIRISS detector and affect both imaging and WFSS observation. The region of the detector affected by the baffle is 172 pixels wide and extends from 45 to 217 pixels outside of the detector. Early inflight observations revealed that when a star is placed between 45 to 175 pixels away from the detector, scattered light is sometimes visible at the edges of the detector (Figure 5). When planning observations, users should be aware that a bright star within ~200 pixels (~13") of the edge of the NIRISS detector could potentially cause scattered light. So far, stray light has been observed when a star of brightness K ~ 11.5 (Vega) or brighter has been within 200 pixels of the detector.
Diffraction spikes from stars outside the field of view
Sufficiently bright stars outside of the NIRISS field of view can have diffraction spikes that impinge on the detector. An example of this is shown in Figure 7, where the likely stars causing these spikes have magnitudes of J ~ 4.5 (Vega). Users can consider imposing a position angle constraint when planning observations, should there be bright stars nearby, depending on whether low background across the whole detector is crucial for the science goals of an observing program.
Doyon, R., Hutchings, J., Rowlands, N., et al. 2010, SPIE, 7331, 7331E
The JWST tunable filter imager (TFI)
Martel, A. 2019 JWST-STScI-004877
The Ghosts of NIRISS: Imaging