IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
turned on can remove the “patchiness" often seen in mosaics due to bias fluctuations in the array (due
both to the first-frame effect and bright source effects).
8.1.3 Mosaicking of IRAC Data
8.1.3.1 Creating a Common Fiducial Frame
Note that in the following we name the settings needed for the command line version of MOPEX.
Corresponding values need to be set if using the MOPEX GUI.
As a first step in creating a mosaic, run the mosaicker with all the modules turned off except for the
fiducia l frame module (i.e., run fiducial image frame = 1 in the namelist), and include all the files you
wish to mosaic from all four channels in the input list. This will generate the boundaries of the mosaic and
will allow all channels to be mosaicked onto the exact same grid. Rename this file to, e.g., FIF_all.tbl to
prevent it being accidently overwritten. Then, when running the mosaicker, set the fiducial frame to the
file created by the fiducial frame module in this initial run using FIF_FILE_NAME = (path)/FIF_all.tbl,
and turn off the fiducial frame module (i.e., set run_fiducial_image_frame = 0 in the namelist). The use of
the common fiducial frame will ensure that the mosaics from all four channels will be accurately coaligned.
The pixel scale is controlled by the MOSAIC_PIXEL_RATIO X/Y parameters. Set Edge
Padding = 100 to get a good border around the image. You can also specify CROTA2 for the output
mosaic if you wish, or set CROTA2 = 'A' to get the smallest possible mosaic. The pixel size in the mosaic
produced by the final pipeline is exactly 0.6 arcsec x 0.6 arcsec (CDELT1, CDELT2 =
±0.000166666667).
8.1.3.2 Outlier Rejection
The mosaicker has four outlier rejection strategies: single-frame outlier rejection, dual-outlier rejection,
multi-frame outlier rejection and box outlier rejection. For IRAC, the most useful are the dual outlier and
multi-frame rejections. Be sure to set THRESH_OPTION = 1 in the namelist in the multi-frame
&MOSAICOUTLIERIN section. Setting the thresholds too low in the outlier modules can result in
unwanted rejection of pixels in the cores of real objects. Users of these modules should carefully check
the coverage maps produced by the mosaicker to ensure that the centers of real objects are not being
masked out. The outlier rejection modules set bits in the rmasks. Bit 0 is set by the single-frame outlier
rejection, bit 1 by the temporal (multi-frame) rejection, bit 2 by the dual outlier rejection and bit 3 by the
box outlier detection. Which rmask bits are used by the mosaicker is controlled by the
RMask_Fatal_BitPattern. The mosaicker takes the rmasks and makes an rmask mosaic. The switches
USE_OUTLIER_FOR_RMASK, USE_DUAL_OUTLIER_FOR_RMASK and
USE_BOX_OUTLIER_FOR_RMASK control which of the outlier detection modules are used. An
important parameter is RM_THRESH in &MOSAICRMASKIN. When the rmask is made it is projected
back onto the input images to determine which pixels will be masked in the final mosaic. An rmask pixe l
is divided amongst the overlapping pixels in the input image. Input image pixels with projected values of
the rmask mosaic above RM_THRESH have the multi-frame outlier bit (1) set in their rmask. For
channels 1 and 2 a fairly high value (e.g., 0.5) can be used. The more diffuse radhits in channels 3 and 4
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