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IRAC: Scattered and stray light |
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Stray light from outside the IRAC fields of view is scattered into the
active region of the IRAC detectors in all four channels. The problem is
significantly worse in channels 1 and 2 than in channels 3 and 4. Stray
light has two implications for observers. First, patches of stray light
can show up as spurious sources in the images. Second, background light,
when scattered into the arrays, is manifest as additions to the flat
fields when they are derived from observations of the sky. Stars which fall into those regions which scatter light into the detectors produce distinctive patterns of scattered light on the array. We have identified scattered light avoidance zones in each channel where observers should avoid placing bright stars if their observations are sensitive to scattered light. The zones for channels 1 and 2 are shown in the Spot overlays (Figure 1 below), as these produce the largest amounts of scattered light. Zones 1A, 1B, 2A, and 2B (which produce the strongest scattered light) typically scatter about 2% of the light from a star into a scattered light "splatter pattern" which has a peak of about 0.2% of the peak of the star (Figure 2 below). They arise due to scattering of light from the edges of the holes in the covers of the focal plane arrays. The A and B zones join up to form a continuous region of scattering which is widest at the upper left and right corners of the array, and narrows to a waist in the center. Zone 1C produces diffuse scattered light across the array, but the light is only noticeable if zone 1C is illuminated by very bright stars (Figure 3 below). The zones for channels 3 and 4 are a narrow strip about 3 pixels wide, 16 pixels outside of the array and surrounding it (Figures 4, 5). They are produced by a different mechanism involving scattering within the detectors themselves. Scattered light is well-rejected by the post-BCD pipeline if dithering and mapping offsets are large enough to ensure stars are moved out of the scattering zones, and the redundancy is high enough to allow for effective outlier rejection. The medium and large dither pattern scales with 4 or more dithers per position are adequate for this. Most cases of scattered light are identified and flagged as part of the pipeline processing.
 Figure 1: Locations of IRAC stray light avoidance zones as seen in Spot visualizations.
 Figure 2: Example of a star in zone 2A scattering light into a channel 2 image (superposed onto a larger mosaic to show the position of the scattering star).
 Figure 3: Example of a star in zone 1C scattering light into channel 1.
 Figure 4: Example of a star in the strip surrounding channel 3 scattering light.
 Figure 5: A star in the strip surrounding channel 4 producing two scattered light spots. The peak of the first spot in from the edge of the array corresponds to a position on the "lattice" structure seen in the flat field. The second, more extended spot is only seen for the brightest scatterers in channels 3 and 4 and probably corresponds to a second reflection of the light causing the first spot. The effects of the scattered light on the flats are illustrated in Figures 6-9 below. The SSC has implemented a correction in the pipeline to remove diffuse stray light from the BCDs, flats, and skydarks. This correction scales a template of the diffuse scattered light pattern by the expected zodiacal background level and subtracts it from the data. The behavior of the scattered light is different in channels 1 and 2 compared with channels 3 and 4. In channels 1 and 2, the pattern is in the form of a "butterfly wing" at the bottom of the array in raw data coordinates (i.e., at the top of the array in BCD coordinates). The scattered light avoidance zones 1A, 1B, 2A, and 2B are larger than the regions the scattered light falls into. This results in the "butterfly wings" having amplitudes ~5% of the background intensity, even though only about 2% of the light from a given point in the stray light zones is scattered into them. In channels 3 and 4, the pattern consists of narrow strips about 30 pixels from the edges of the array (the "lattice" or "tic-tac-toe" pattern). There is no strip in the flat field on the negative x side of the channel 3 frame where the array is vignetted by the pickoff mirror, and the strip at the top of the channel 3 array is brighter and has a relatively bright secondary reflection about 50 pixels from the top of the array (see Figure). In channels 3 and 4, the pattern is only ~1% of the background. The origin of the scattered light is currently not known for sure, but the in-focus nature of the scattered light spots suggests that little or none is contributed by the telescope, and most arises within the MIC. In channels 1 and 2 it may arise from starlight glancing off the inside of the hole cut in the FPA cover. In channel 3 and 4, an area around the edge of the detectors themselves is probably responsible.
click for full imageFigure 6: The left-hand panel shows the positions of the stars causing scattered light in Channel 1 of the Spitzer images, and on the right-hand panels are plotted the approximate positions (centroids for channel 1) of the scattered light spots produced by these stars. The greyscales represent the flat fields, showing that most of the scattered light positions correspond to high regions in the flat fields, indicating that the flat field enhancements are most likely produced by the scattered background light.
click for full imageFigure 7: The left-hand panel shows the positions of the stars causing scattered light in Channel 2 of the Spitzer images, and on the right-hand panels are plotted the approximate positions (centroids for channel 2) of the scattered light spots produced by these stars. The greyscales represent the flat fields, showing that most of the scattered light positions correspond to high regions in the flat fields, indicating that the flat field enhancements are most likely produced by the scattered background light.
click for full imageFigure 8: The left-hand panel shows the positions of the stars causing scattered light in Channel 3 of the Spitzer images, and on the right-hand panels are plotted the approximate positions (peaks for channel 3) of the scattered light spots produced by these stars. The greyscales represent the flat fields, showing that most of the scattered light positions correspond to high regions in the flat fields, indicating that the flat field enhancements are most likely produced by the scattered background light.
click for full imageFigure 9: The left-hand panel shows the positions of the stars causing scattered light in Channel 4 of the Spitzer images, and on the right-hand panels are plotted the approximate positions (peaks for channel 4) of the scattered light spots produced by these stars. The greyscales represent the flat fields, showing that most of the scattered light positions correspond to high regions in the flat fields, indicating that the flat field enhancements are most likely produced by the scattered background light.
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