NIRISS Ghosts
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 caused by 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 ~0.1%–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 Figures 1 and 2, is the ghost axis point (GAP). Knowing the coordinates where these intersections occur can help predict and identify the locations of ghosts.
The current knowledge of GAPs and other features of ghosts has been acquired from ground testing campaigns and on-sky commissioning data. More features will be evaluated using available on-sky data.
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.
Figure 2. An example of NIRISS ghosts (on-sky)
Click on the figure for a larger view.
Left: An example of bright stars that produce ghosts in a NIRISS image. The magenta X marks the ghost axis point (GAP), the point around which ghosts are reflected from the sources. The stars producing the ghosts are circled in cyan and the objects circled in magenta are the ghosts. The red Xs mark sources that would be expected to produce ghosts, but not are identified indicating that only certain regions in the detector are susceptible to producing ghosts.
Right: A zoom-in of the image to illustrate the blobby morphology of ghosts, and the double ghosts produced in this particular filter (F150W). These images are from observation 116 in APT program 1063 with the F150W filter.
Mitigation strategies
The impact of ghosts on observations can be assessed by simulating the observing scene with Mirage. Dithering may help to suppress some ghosts in the final image product from the pipeline IMAGE3 step.
Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.
Click on the figures for a larger view.
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 calwebb_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 calwebb_image3 step.
References
Martel, A. 2019 JWST-STScI-004877
The Ghosts of NIRISS: Imaging