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 7.21). 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 Multiple Instrument Chamber (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 peak surface brightness 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 7.21: 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. The dimensions of the image are about 0.7 degrees (x-axis) by 1.1 degrees (y-axis).
The “splotches” (see Figure 7.22) are areas of more concentrated stray light that appear when a bright source is about 20 – 32 arcminutes off the center of the FOV, to the left or right in array coordinates. The splotches were seen in the SWIRE field near Mira and in the Beta Gru tests in channels 1, 2, and 4. The presence or absence of a splotch is very sensitive to the position relative to the bright object - a bright splotch can be present in one image and absent in an image with the telescope pointed a few pixels away. Even fainter splotches appeared in channel 2 about 1 degree away from Beta Gru, along the same directions. The rings and splotches are not corrected in the pipeline.
Figure 7.22: Channel 2 images from the SWIRE map showing stray light splotches from Mira, which was about 30 arcminutes away. Successive pairs of images were slightly dithered. The last pair is about 5 arcminutes from the first pair, but has a similar splotch. Note the absence of any stray light in the second image, though it was centered only a few pixels away from the first image. The images are from PID=181, AORKEY 5838336; EXPID 187 – 192, 199, and 200.