There are three types of known or potential optical ghosts visible in IRAC images. The brightest and most common ghosts are produced by internal reflections within the filters. The first-order filter ghosts (one pair of internal reflections) in channels 1 and 2 are triangular, and in the BCD images they appear above and/or to the left of the star in channel 1, and above and/or to the right of the star in channel 2. The channel 1 first order filter ghost contains ≈ 0.5% of the flux of the main PSF in channel 1, and the channel 2 ghost ≈ 0.8% of the flux of the channel 2 PSF. 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., (ΔY, ΔZ) = (Ayy+ By, Azz+ Bz) where (y,z) are normalized coordinates in which the Focal Plane Arrays (FPAs) span the range [0,1] with the axes increasing away from the Spitzer optical axis, and the coefficients are as listed in Table 7.5 below. The +Y direction is in the IRAC (C)BCD +x direction and the +Z direction is in the IRAC (C)BCD –y direction. The peak intensity of the ghost is roughly 0.05% of the (unsaturated) peak intensity of the star. The second-order filter ghosts (two pairs of internal reflections) are much fainter (≈ 25% of the flux and ≈ 6% of the surface brightness of the first order ghosts), rounder, larger, and about twice as far away from the star. The separation between the star and its ghosts increases with distance from the optical axis of the telescope. The channel 3 and 4 filter ghosts appear as small crosses at a larger distance, mostly to the left or right of the star, respectively. 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 channel 3 and 4 filter ghosts contain < 0.2% of the flux of the main PSF in these channels. The separation and orientation are different from channels 1 and 2 because of the different orientations of the filters. Examples of filter ghosts are shown in Figure 7.19.
Table 7.5: Coefficients for channel 1 and 2 ghost locations.
Figure 7.19: Filter and beamsplitter ghosts in IRAC. Top: Channel 1 and 2 mosaics of saturated images of Fomalhaut near the center of each array from PID=90, AORKEY 4875776. The panels are 180 pixels × 140 pixels of 0.6 arcseconds; the greyscale is logarithmic. Bottom: channel 3 and 4 single, highly saturated images of Sirius near the center of each array from PID=1156, AORKEY 16412416. The panels are 90 raw pixels × 70 raw pixels; the greyscale stretch is square-root. The channel 3 and 4 filter ghosts are nearly saturated and the bandwidth effect is saturated. Even the optical banding (light diffracted and internally reflected within the arrays) in the column containing the star center suffers the bandwidth effect, which appears as bright vertical spikes 4 and 8 pixels to the right of the center of the star.
Similar ghosts are created by internal reflections within the beamsplitters. These only affect channels 3 and 4 which are transmitted through the beamsplitters. They appear as a very faint, short, horizontal bar at Δy ≈ –36 pixels relative to a bright star (below the bright star in Figure 7.19), but are often obscured by brighter “banding” artifacts. They are slightly fainter than the filter ghosts.
Figure 7.20: Pupil ghost in channel 2 from V416 Lac.
The faintest identified ghosts appear as images of the Spitzer entrance pupil, i.e., the primary mirror shadowed by the secondary and supports. These pupil ghosts are only found in channels 2 and 4, and require an extremely bright source (e.g., a first magnitude star in a 12-second frame) to be seen due to their low surface brightness. The pupil image is at a fixed location on the array (but in different locations in different channels). However, the pupil image is only partially illuminated by a single source, and the portion of the image that is illuminated depends on the source position on the array. An example of a pupil ghost is shown in Figure 7.20. This figure also shows some fringes in between the pupil ghost and the star. It is not clear if the fringes are directly related to the ghost. The total flux in these ghosts is ≈ 0.05% - 0.5% of the total flux in the PSF.
We flag the ghosts in the pipeline with imask bit 2 (see Section 7.1.1). However, because the relative locations of the ghosts do vary with position on the array, sufficiently large dithering can help reduce or eliminate their effects. The stray light masking software also will flag the filter ghosts. The PRFs that we provide on our web pages include all the ghosts, and the apertures used in calibrating channels 1 and 2 include the filter ghosts. In performing photometry for channels 1 and 2, the filter ghosts should be included