Choosing between NIS and NISRAPID readout patterns

See also: NIRISS Detector Readout Patterns

Dithering recommendations

See also: NIRISS Dithers, NIRISS Ghosts

Dithering is required for the NIRISS wide field slitless spectroscopy (WFSS) mode and imaging mode, is recommended for the aperture masking interferometry (AMI) mode, and is not allowed for the single object slitless spectroscopy (SOSS) mode. For the WFSS and imaging observing modes, the choice of dither step size and number of steps can improve removal of artifacts from ghosts and cosmic rays. For the AMI mode, the 5-point SUBPIXEL dither pattern is recommended.

If the target field is expected to have a large number of bright sources, ghosts from the sources could potentially impact data quality. In this case, a LARGE dither step size may be preferable to help remove ghosts in the final combined spectral product. Four is the minimum number of recommended dither steps to mitigate detector artifacts, cosmic ray hits, and improve PSF sampling, though fewer steps could be considered if 4 dithers increases the exposure time too much or results in integration ramps with N_Groups ≤ 3. 

Additional information for consideration when choosing a dither pattern for WFSS observations is provided in the NIRISS WFSS Recommended Strategies article. More detailed advice and metrics for selecting the most appropriate dither pattern for a particular science goal is given in the report NIRISS Dither Patterns for the WFSS and Imaging Observing modes.

Beware the imprint of occulting spots 

The NIRISS pick-off mirror has 4 coronagraphic occulters engraved on its surface which projects 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. The projection of the detector on the observing field, and thus the exact location of the spots, depends on the PA_V3 orientation of the spacecraft at the time of observation. 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.

Though the risk of a source overlapping one of these spots is relatively small, if there is a particular target of interest in the field-of-view when using full frame readout, it may be desirable to position the field center so that the target(s) do not fall within ~11" of the detector edges.

Minimizing the effect of snowballs on image quality

See also: Shower and Snowball Artifacts

Snowballs are cosmic ray impacts that can affect up to hundreds of pixels on a near-infrared detector. Though these features are generally removed by the JWST data reduction pipeline during the calwebb_detector1 stage, they can be imprinted in the "_*rate.fits" or "_*rateints.fits" images when using the NIS readout pattern, but not when using the NISRAPID readout pattern. The snowballs are removed in the files processed through the calwebb_image2 or calwebb_spec2d stages of the pipeline when 4 or more dithers are used.

To mitigate the impact of snowballs on data quality, it is recommended to use at least 4 dither positions when using the imaging or WFSS observing modes with the NIS readout pattern. For modes where dithering is not allowed (SOSS), the NISRAPID readout pattern is recommended to minimize the effect of snowballs, though there is a limit of 30 groups in an integration when using NISRAPID. Longer exposures can be achieved by increasing the number of integrations.

Mitigating the light saber in WFSS and imaging observations via aperture angle constraints

See also: NIRISS Known Issues, APT Aladin Viewer

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 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 impact both imaging and wide field slitless spectroscopy (WFSS) observations.

The NIRISS susceptibility region can be visualized with the Astronomer's Proposal Tool (APT) Aladin Viewer when the APT observation template is NIRISS Imaging or NIRISS Wide Field Slitless Spectroscopy. To visualize whether a spoiler star capable of causing the light saber falls within the susceptibility region, visualize the target field in Aladin and set the background image to 2MASS. After zooming out several times, an overlay similar to the one shown in Figure 1 will be visible. The white rectangle shows the NIRISS susceptibility region. The annulus shows the position angles of the observatory, where the green regions are permitted position angles and the red regions are position angles not accessible for this observation. The permitted position angles reflect either the allowed position angles for JWST's field of regard for the target or the user-imposed position angle constraint from the Special Requirements tab in APT.

To see whether a bright star overlaps the susceptibility region for permitted position angles, upload a bright star catalog in Aladin. For convenience, a bright star catalog containing 2MASS sources that are brighter than K < 2 (Vega) is provided, but a user may wish to upload a catalog defined using other criteria. Sources from the catalog are marked in the Aladin overlay (see Figure 1). When using the Select option in Aladin, hover your mouse over the reported position angle in Aladin until the cursor has a horseshoe shape. Use the cursor to rotate around position angles to identify if there are permitted position angles where a bright star falls within the susceptibility region. To get more information on specific stars, click on a star so that the photometry will be reported in the bottom of the Aladin pane. The brighter a star is, the brighter the light saber will be. Inflight performance shows that stars as faint as K ~ 2 (Vega) can produce a faint light saber.

Based on this investigation, users may wish to impose a position angle constraint special requirement to avoid having a bright star in the susceptibility region during an observation. Note: the reported position angles in Aladin are V3 position angles. Any position angle constraints that force an observation into the micrometeoroid avoidance zone (MAZ) require strong justification. It is currently not recommended to constrain an observation into the MAZ for the purposes of avoiding the light saber.

Updates to the susceptibility region coordinates and limiting magnitude at which a star causes a noticeable light saber may be updated as more is learned about this scattered light feature from inflight performance.

Designing an efficient science program with multiple filters 

See also: NIRISS Pupil and Filter Wheels

When designing a program that uses multiple filters, users should be aware that overheads are associated with each mechanism move. To design an efficient observing program, it is recommended to select filters in sequential order in the pupil wheel or filter wheel, as illustrated in Figure 2.