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Ghost images are visible near very bright sources in channels 1 and 2.
These ghost images are caused by internal reflections within the tilted
filters. Because of the increase in the optical path length, ghost images
are not in focus. The separation between the main image and its ghost
is roughly proportional to the distance of the
main image from the Spitzer optical axis in both Y and Z directions, i.e.,
(Delta Y, Delta Z) = (A_y y + B_y, A_z z + B_z) where (y,z) are normalized
coordinates in which the FPAs span the range [0,1] with the axes
increasing away from the Spitzer optical axis, and the coefficients are as
listed in the table below. The peak intensity of the ghost is roughly
0.05% of the (unsaturated) peak intensity of the star.
Table: Coefficient for channel 1 & 2 ghost locations
| IRAC
Channel | A_y | B_y | A_z | B_z |
| 1 | 0.04351 | 0.00288 | 0.04761 | 0.00211 |
| 2 | 0.04956 | 0.00105 | 0.04964 | 0.00387 |
Filter ghosts also appear for very bright (saturated) sources in channels
3 and 4. These ghosts are cross-shaped ("+"). They are offset from the
primary image by approximately (+36 pix, +2 pix) and (-36 pix,+2 pix) in
the Spitzer (Y,Z) directions for channels 3 and 4 respectively. The
Z-offset varies slightly with position on the array. The peak intensities
of these ghosts are <0.5% of the (unsaturated) peak intensity of the
source; they are in the process of being characterized. Filter ghosts for
channels 1 and 2 are flagged by the SSC pipeline.
The figure illustrates ghost images for saturated
exposures of the star Fomalhaut. Left-to-right and
top-to-bottom are 0.6-sec frametime images in channels 1 &
2, and 12-sec frametime images in channels 3 & 4.
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