Tadpoles are scattered light artifacts that may be mistaken for emission line sources in NIRCam WFSS data. They appear most prominently and ubiquitously in Grism C module B data.

Shells are fainter diffuse features seen occasionally due to scattered light from very bright sources.

Both tadpoles and shells are seen primarily in module B (Figure 1). The module B grism has anti-reflective coating on only one side. The module A grism has the coating on both sides, significantly suppressing these artifacts.

Tadpoles

Each tadpole consists of either one bright knot or multiple bright knots in a line parallel to the spectral traces in Grism C data (Figure 2) or at an angle in Grism R module B.

Tadpole brightness is correlated with source continuum brightness. A source with F322W2 continuum AB mag 17 (19.5) yields a tadpole head with brightness ~10 (1) DN/s.

Tadpole locations

The tadpoles are observed near the spectral trace, far (~1.5 arcmin) from the direct-imaging position (the position of the source if the pupil where were set clear). The tadpole head is offset from the direct-imaging position by roughly (-40, 1385) pixels in (x, y) in F322W data and (36, 1350) pixels in F444W data. This position varies by up to 10 pixels depending primarily on the x position. Note the tadpole is offset to the left in F322W2 and the right in F444W.

Weaker tadpoles observed in Grism R modules A and B

While tadpoles are strongest in Grism C module B, they are also observed more weakly in Grism R and in module A (Figure 4).

Grism R module B tadpoles are not parallel to the spectral traces, but rather at an angle. They appear to be isolated to the upper few hundred pixels in y on the detector. Unlike the Grism C tadpoles, these move with respect to the spectra when dithering.

Tadpoles may also appear in module A data, though significantly suppressed by the anti-reflective coating. One possible example is shown in Figure 4. It is parallel to the spectral traces.