IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
The "splotches" (see Figure 7.19) are areas of more concentrated stray light that appear when a bright
source is about 20−32 arcminutes off the center of the FOV, to the left or right in array coordinates. The
splotches were seen in the SWIRE field near Mira and in the Beta Gru tests in channels 1, 2 and 4. The
presence or absence of a splotch is very sensitive to the position relative to the bright object − a bright
splotch can be present in one image and absent in an image with the telescope pointed a few pixels away.
Even fainter splotches appeared in channel 2 about 1 degree away from Beta Gru, along the same
directions. We thank R. Arendt for providing us with most of the information that was presented in this
section.
Figure 7.19: Channel 2 images from the SWIRE map showing stray light splotches from Mira, which was
about 30 arcminutes away. Successive pairs of images were slightly dithered. The last pair is about 5
arcminutes from the first pair, but has a similar splotch. Note the absence of any stray light in the second
image, though it was centered only a few pi xels away from the first image. The images are from PID 181,
AORKEY 5838336; EXPID 187-192, 199, and 200.
7.4 Cosmic Rays and Solar Protons
The SSC mosaicker, MOPEX, identifies energetic particle hits as follows. All pixels in BCDs that
contribute to a given pixel in the final mosaic are identified, and significant outliers (a user-specified
number of sigmas above or below the filtered mean of all overlapping pixels of overlapping BCDs) are
rejected. This method is very similar to the outlier rejection performed by shifting and adding ground
based images. The rejected pixels can be inspected in the “Rmask" output files (one per input image).
Outlier rejection in MOPEX can be adjusted. The parameters used in the online pipeline-generated
Data Features and Artifacts 130 Cosmic Rays and Solar Protons