The following three spectra show what the teardrop looks like in the extracted spectra.
Figure 7.8 Illustration of 14 micron teardrop in the extracted spectra of HR 7341. The top two panels show the results for Regular Point Source extraction and the bottom two panels show the results for Full Width extraction. The teardrop is most clearly seen in panels 2 and 4, where it appears as an upturn kicking in at around 13.5 microns.
Figure 7.9 Same as Figure 7.8, but for HR 2194.
Figure 7.10 Similar to Figure 7.8, but for 3C 454.3, a red source. Comparisons to models have been omitted due to unavailability of models.
The teardrop is due to light leakage, optics, or detector response. It has not been introduced by pipeline processing. To illustrate this, the figures below show the various products from the pipeline for one observation of the standard star HR 7341. The teardrop is present in all of the pipeline products beginning as early as the Linearize cube.
Figure 7.11 LNZ Cube: Output of linearize – Frames 1-4.
After collapsing the cube above, the pipeline produces the following products:
Figure 7.12 Various pipeline products which all show the 14 micron teardrop: After the various droop corrections (top left); after stray light correction but before flat fielding (top right); after flat fielding (lower left); flat fielded but NOT stray light corrected (lower right).
As illustrated the next three figure, the teardrop shape changes slightly with slit position, indicating that it is most likely correlated with the angle at which the light enters the slit, consistent with an internal reflection.
Figure 7.13 Nine mapping positions (2, 12, 22, 27, 32, 37, 42, 52, and 62) of the flat fielding data for eta1 Dor, zoomed in on the teardrop. The teardrop shape changes slightly with slit position, consistent with an internal reflection origin.
Figure 7.14 Two dimensional SL2 spectral image showing an observation of HR 7341. The wavelength region spanning 6.6-7.5 microns is highlighted with a box. We do not see any excess emission similar to the teardrop between 6.6 and 7.5 microns in SL2. This suggests the feature in SL2 at 6.5-7.6 microns is different in nature from the SL1 teardrop, and likely does not depend on extraction aperture.
In the figure below, we examine the strength of the teardrop versus the source flux density. We find that 1) the strength of the teardrop does not correlate with source flux density; and 2) for point-source extractions, the teardrop tends to be stronger for nod 2 compared to nod 1.
Figure 7.15 The strength of the teardop (quantified as the ratio of flux densities at 14.0 and 13.5 microns) versus the source flux density (measured at 12 microns) for 8 sources. For each source, black symbols represent full-slit extractions and red symbols represent point-source extractions. Plus signs represent nod 1 and asterisks represent nod 2. There is no correlation between the strength of the teardrop and the flux of the source. However, the teardrop is almost always stronger at nod 1 than nod 1 for point source extraction.
Mitigation: The behavior of the teardrop excess depends in a complicated way on source position in the slit and extraction aperture. There is no good way to correct for this feature. Users should use extreme caution before interpreting features between 13.2-15.0 microns.