3.5.3 Diffraction Pattern of Telescope near Bright Sources

Observers should remember that the interaction of the light beam with the telescope surfaces will result in the appearance of characteristic diffraction patterns at the telescope focal plane. As in other telescopes, these patterns consist primarily of Airy rings (due to the finite telescope aperture) and radial spikes (due to the secondary mirror supports). For bright sources, the power on these features may be significant. For example, experiments with the simulation software STINYTIM indicate that for a source with a brightness of 1 Jy at 8 microns, the extent of the PSF will be detectable at the 1 mJy level up to 10 arcsec away from the source. For a source that is 1 Jy at 25 microns, the 1 mJy level is reached at distances of 30". Observers that are interested in detecting faint structures next to bright sources should take into account the shape of the Spitzer's PSF when designing their AORs. As a rule of thumb, it will only be possible to detect flux smaller than 1/100 of the source flux at distances larger than 1.5 to 2 slit widths from the source (for the low-res modules), without careful source mapping and sky subtraction. This rule of thumb is provided as a general guideline only, and should not replace a detailed PSF analysis using Stinytim.

As a result, full slit extraction picks up the features in both nod position. Observers are urged to check their full slit extraction for each nod position with narrower, fixed width 2 pixel wide extractions for the nod positions. Real emission features should be present in the spectrum even in the narrower extractions at both nod positions.