Spitzer Documentation & Tools
IRAC Instrument Handbook
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8.4.3        Mosaicking of IRAC Data       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 fiducial frame module (i.e., run_fiducial_image _frame = 1 in the namelist), and include all the files you wish to mosaic from all the four channels in the input list. This will generate the boundaries of the mosaic and will allow all the channels to be mosaicked onto the exact same grid. Rename this file to, e.g., FIF_all.tbl, to prevent it from 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 the four channels will be accurately co-aligned. 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 arcseconds x 0.6 arcseconds.       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 pixel 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 can be more effectively rejected by setting RM_THRESH to a lower value, e.g., 0.05, which has the effect of growing the rmask ,and thus it rejects the more diffuse edges of a radhit.


All the outlier rejection modules require uncertainty images. The BCD uncertainty images are adequate for this purpose, and the list of them should be specified with the SIGMALIST_FILE_NAME keyword in the namelist. To use them set have_uncertainties = 1 and compute_uncertainties internally = 0. To compute your own uncertainty images, set compute_uncertainties_internally = 1, have_uncertainties = 0, and set the appropriate values in the &SNESTIMATORIN section of the namelist (see the MOPEX documentation for more details). Outlier rejection creates another set of masks, the rmasks. These indicate the pixels flagged by outlier rejection, and which are used by the mosaicker. A mosaic of the rmasks can be provided by setting run_mosaic_rmask = 1. As a check on the outlier rejection, it is often helpful to examine the coverage maps output by the mosaicker. If the outlier rejection has been over-zealous, then there will be reductions in coverage at the positions of real sources in the mosaic. Blinking the mosaics and coverage maps in, e.g., DS9 can thus be very helpful for determining whether the outlier rejection is set up correctly to reject only genuine outliers.       Mosaicker Output Files

The output directory structure after running the mosaicker looks like: BoxOutlier, Coadd, DualOutlier, Interp, ReInterp, Combine, Medfilter, Rmask, Detect, Sigma, Dmask, Outlier with “-mosaic” appended to these names, and the files in the output directory are FIF.tbl, header_list.tbl, and a namelist file with a date stamp. The directory Combine contains the mosaic, mosaic.fits, a coverage map, mosaic_cov.fits, and an uncertainty map mosaic_unc.fits.       To Drizzle or Not to Drizzle?

The mosaicker has three interpolation options, set by the INTERP_METHOD keyword. The default is a linear interpolation. Drizzling is available as an option, as is a grid interpolation (useful for creating quick mosaics if the PSF quality is not important). Our experience with the drizzle option suggests that it is effective when used on data sets with many dithers per sky position, and it can reduce the point response function (PRF) width by 10% – 20%, though at the expense of an unevenly-weighted image. The coverage map produced by the mosaicker can be used to investigate the pixel-to-pixel variation in the coverage of the drizzled image.       Mosaicking Moving Targets

Although Spitzer does track moving targets to a sub-pixel accuracy, the BCD pipeline only produces mosaics of IRAC data in fixed celestial coordinates. The user may opt to generate his or her own mosaic in a moving coordinate reference frame by setting the appropriate flags in MOPEX. The individual BCDs or CBCDs should be overlap-corrected first and then the mosaicker should be run with the flag MOVING_OBJECT_MOSAIC = 1 set, using outlier rejection. Stars in the frames may be removed by outlier rejection, and the resultant composite of a moving target will be produced.  

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