Reflection from bright sources in the field of view can cause optical ghosts in NIRISS imaging and WFSS observations.

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See also: NIRISS GR150 Grisms

For the NIRISS imaging and wide field slitless spectroscopy (WFSS) modes, there is a chance that optical ghosts may appear in the field of view (FOV) and significantly affect some sources. These optical ghosts are from internal reflections of bright sources between the detector and the filter in the light path

Ghosts in NIRISS are characterized by extended morphology. The exact shape may vary over the FOV and the optical element (e.g., a filter or grism) in the light path. The integrated intensity of ghosts is proportional to the original source, ranging ~0.2%–4%, also depending on the optical element and the position of the source. The characterization of ghosts originating from extended sources is currently unknown. The intersection of lines between bright sources and ghosts, as shown in Figure 1, is the ghost axis point (GAP). Knowing the coordinates where these intersections occur can help identify the locations of ghosts.  

The current knowledge of GAPs and other features of ghosts has been acquired from the testing campaigns on the ground and is limited to specific cases. More features will be evaluated based on on-orbit data.

Figure 1. Example of ghosts in NIRISS direct image

NIRISS F150W direct image taken in the Optical Telescope Element/Integrated Science Module campaign. The position of each ghost can be related to the position of the original bright source through the GAP position that is specific to the optical element. A zoom-in image of one of the ghosts is shown, revealing the extended morphology.

Mitigation strategies

The impact of ghosts on observations can be assessed by simulating the observing scene with MirageDithering may help to suppress some ghosts in the final image product from the pipeline IMAGE3 step.

Figure 2 shows examples comparing 3 dither sizes that are available for the NIRISS primary observing mode (SMALL/MEDIUM/LARGE*). In these examples, a cutout image of a ghost identified in one of the ground testing campaigns (the one shown in the right panel of Figure 1) was used. While the result may depend on the actual morphology of ghosts (i.e., positions of the source and the choice of filter), the example here suggests that a SMALL dither may not be large enough, and may let outer parts of the ghost overlap, which results in a significant residual (~3.6% of the total flux of the input ghost) in the final stacked image. Therefore, a dither size of MEDIUM or LARGE is generally recommended.

Figure 2. Comparison of ghosts after the final stack

Comparison of simulated ghosts with SMALL (top panel), MEDIUM (middle), and LARGE (bottom) dither sizes, with the F150W filter. Each panel shows data products from the pipeline IMAGE2 step at the same reference sky coordinates (red cross) so the movement of the ghost can be monitored, and the final product of the IMAGE3 step.
A caveat is that having a large dither size does not necessarily clean the footprint of ghosts completely. As seen in the examples above (the middle and bottom panels of Figure 2), there are some positive pixels left in the final combined images, possibly because the sigma clipping could not exclude faint parts of the original ghosts. For more complete suppression of the ghost footprint, extra steps to identify and then mask such ghosts (for example, as documented in this repository) would be required before the final stack. 

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Martel, A. 2019 JWST-STScI-004877
The Ghosts of NIRISS: Imaging

Originally Published


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