NIRCam WFSS Tadpoles and Shells

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.

On this page

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.

Figure 1. NIRCam WFSS tadpole and shell

Example of tadpole and shell artifacts observed in F322W2 Grism C module B data from NIRCam commissioning program 1076: jw01076126001_02101_00003_nrcblong (see JWST file naming conventions).


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.

Figure 2. Tadpole examples

Tadpole artifacts observed in F322W2 Grism C module B data from NIRCam commissioning program 1076. Green circles on most panels indicate sources identified by DAOFIND that could be mistaken for bright emission lines with faint or no continuum visible in its spectrum.


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.

Figure 3. Tadpoles observed in F322W2 and F444W

  

Tadpoles observed in F322W2 (top) and F444W (bottom) Grism C module B data. Red stars mark the direct-imaging positions to the right of the 1st-order spectrum at 3.95 µm. Red squares mark the tadpole positions to the left of the 2nd-order spectrum in F332W2 and to the right of the spectral trace in F444W. Data are from NIRCam commissioning program 1076: jw01076126002_02101_00004_nrcblong and  jw01076119001_02101_00004_nrcblong.


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.

Figure 4. Weaker tadpoles observed in Grism R

  

Weaker tadpoles are observed in Grism R data from module B (top) and perhaps module A (bottom). Note the module B Grism R tadpoles are not parallel to the spectral traces, as they are in Grism C data. The tadpole in module A was tentatively identified by visual inspection in NIRCam commissioning program 1076, observation jw01076113001_02101_00001_nrcalong.



Latest updates
Originally published