IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
concentrate the scattered light along the rows and columns, causing the optical banding. The pattern is
due to interference that depends on wavelength and the spatial extent of the source at each wavelength.
The banding/scattering pattern does not vary much for point sources with a continuous spectrum, but a
narrow-band source has a complex banding/scattering pattern.
Users should be aware of the uncertainties resulting from banding, specifically when attempting
measurements of faint sources near the affected rows or columns. For bright sources with significant
banding, aperture photometry may not be successful, and it would be better to measure these sources
using frames of shorter exposure times. Users are encouraged to experiment with image restoration
techniques of their choice. Algorithms similar to the pull-down corrector may have some effectiveness in
mitigating banding.
7.3.3 Optical Ghosts
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., (DeltaY,
DeltaZ) = (Ayy+ By, Azz+ Bz) 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
Table 7.4 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 7.16.
Table 7.4. Coefficients for channel 1 & 2 ghost locations.
Channel Ay By Az Bz
1 0.04351 0.00288 0.04761 0.00211
2 0.04956 0.00105 0.04964 0.00387
Data Features and Artifacts 126 Optical Artifacts