IRAC: Ghost Images

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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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