IRAC Instrument Handbook - IRSA - California Institute of Technology

7.3.4 Large Stray Light Ring and Splotches
IRAC Instrument Handbook
There is a faint ring of scattered light with a mean radius of about 23.7 arcminutes in channel 1 that is
visible around bright objects. There is also a slightly larger, fainter ring in channel 2. They were first
noticed in mosaics of a SWIRE field that was adjacent to the bright star Mira in PID 181 (see Figure
7.18). The ring was visible in channel 1 and 2 mosaics. We verified that the ring was an artifact by
observing the field near Beta Gru where we observed pieces of the ring in the same places relative to the
star. Starlight that is specularly reflected off the telescope mirrors cannot enter the MIC directly when the
star is more than 16 arcminutes off the telescope boresight. IRAC's pupil stop is a little oversized, so the
ring is probably light that is once or twice diffusely scattered at areas outside the secondary and/or near
the top of the primary conical baffle. The mean surface brightness of the ring in channel 1 is 4.5 x 10 -10
(±30%) times the mean surface brightness of the center pixel of a pixel-centered point source. Presumably
there are stray light rings in channels 3 and 4, but they are too faint to see.
Figure 7.18: Part of the channel 1 mosaic (from observations in PID 181; AORKEYs 5838336, 5838592,
5839872 and 5840128) of the SWIRE field near Mira showing the 24 arcminute radius ring of stray light from
the telescope.
Data Features and Artifacts 129 Optical Artifacts