Depends On: Mosaic Interpolate (or Mosaic Reinterpolate)
Important Notes: There are several coaddition schemes available, and all but one are set in this module. The final one, Create Absolute Minimum Mosaic, is set in the Mosaic Settings module.
PURPOSE
The interpolated images are co-added to create one mosaic image. The pixel values of the mosaic image are equal to the averaged interpolated pixel values, with the option to weight them with the integration time if needed. (Generally one should not weight by uncertainties when sources are present.) The interpolated uncertainty images are also co-added into one uncertainty mosaic image following the standard assumption of additive variances. The coverage maps are co-added into a single mosaic coverage map.
INPUT
Tile Max X, Y: (int) The X (Y)-size of the tile for performing the co-addition. This should be reduced if computer memory is an issue (see §8.1).
Integration Time Weighted Coadd: Check this box to weight the coaddition by exposure time. MOPEX searches for the header keyword indicated, the default is EXPTIME.
Mosaic Co-Adder output subdirectory: The subdirectory of <output_dir> that you wish to use for the output files. Default is Coadd-mosaic.
Keep coadded Tiles: If checked, the coadded tiles will be saved, even if Delete Intermediate Files was turned on in the Mosaic Settings module. Recommended if you are planning on subsequently running APEX.
Sigma Weighted Coadd: Check this box to use the uncertainty-weighted coaddition option (see below for details). Not Recommended.
Create UNC Mosaic: If checked, the mosaic of uncertainties is created, propagating from the individual input (or S/N Estimator) uncertainties.
Create Standard Deviation Mosaic: If checked, a different flavor of mosaic uncertainty image is created. The pixel values of this image will be equal to the standard deviation of the interpolated image pixels in the stack.
Create Outlier Mosaic: If checked, a mosaic is created of the temporal outliers detected by the Mosaic Outlier module.
Create Dual Outlier Mosaic: If checked, a mosaic is created of the dual outliers detected by the Mosaic Dual Outlier module.
Create Median Mosaic: If checked, a quicklook mosaic is created using the median of each interpolated pixel in the stack. Flux is not conserved.
COMMAND LINE INPUT
&MOSAICCOADDIN
TILEMAX_X = 1000,
TILEMAX_Y = 1000,
INTEG_TIME_KWD = ‘EXPTIME’,
&END
In Global Parameters:
COADDER_DIR = Coadd-mosaic
keep_coadded_tiles = 1
sigma_weighted_coadd = 0
create_unc_mosaic = 1
create_std_mosaic = 0
create_outlier_mosaic = 1
create_dual_outlier_mosaic = 1
run_median_mosaic = 0
INTEG_TIME_KWD: This is the equivalent of checking the Integration Time Weighted Coadd box in the GUI. If INTEG_TIME_KWD = ‘EXPTIME’ is set in the parameter block, intergration time weighted coadd will be used.
OUTPUT
Tile BCD file (coadd_tile_bcd.txt): This text file lists the input images which contribute to each output tile. The format of the file is the tile ID, the number of contributing images, and the image index within the stack.
Tile table (coadd_tiles.tbl): A table listing the tiles, their sizes, and their offsets in the x- and y-directions with respect to the FIF.
Tile uncertainty list (coadd_Tile_*_Unc.fits): The stack of tiled uncertainty images.
Mosaic uncertainty file (coadd_Tile_*_Std_Unc.fits): The stack of tiled standard deviation images.
The tile specification files corresponding to the other optional mosaics requested in the Global Parameters section of the namelist (e.g. the outlier mosaic) are created by the next module, Mosaic Combine, but are written to the output mosaic directories (e.g. Outlier-mosaic/).
DISCUSSION
The co-addition can be performed on Tiles (see §8.1), to address computer memory considerations. The Mosaic CoAdder module performs the co-addition on the individual tiles, and the Mosaic Combine module then combines the tiles into a single mosaic. Even if only a single tile is used, Mosaic Combine still needs to be run, in order to produce the expected products.
There are several weighting schemes available. The interpolated images are normally combined using straight averaging. Exposure time weighting is also available. The co-added uncertainty image is computed regardless of whether interpolated uncertainty images are used for weighting, as long as the uncertainty images are given as an input to the program.
The coverage mosaic is always the number of good pixels in the final stack. The total exposure time for a pixel (when time-weighting) is not available as a mosaic, but can be found per tile in the output_dir/Coadd-mosaic directory. (If there's only one tile, it is like the final mosaic.)
Straight Average: (default) If none of the weighting alternatives are selected, then the co-added pixel value O is given by the following expression:
Equation 5.5
Here Ij is the value of the interpolated pixel in image j, and cj is the pixel value of the corresponding coverage map. Co-added uncertainty U = W-1 is given by the following expression:
Equation 5.6
where σj is the pixel value of the corresponding uncertainty image, and co-added coverage C is simply the sum of the individual input coverages:
Equation 5.7
Exposure Time Weighted Average: If Integration Time Weighted Coadd is selected then MOPEX weights the co-added pixels by the exposure time given in the FITS header keyword specified. For Spitzer data, the integration time is given by the keyword EXPTIME.
Sigma Weighting: This option is not recommended when sources are present. If Sigma Weighted Coadd is selected, then a co-added pixel value O is given by the following expression:
Equation 5.8
Median Mosaic: If the Create Median Mosaic box is checked then the pixels are co-added by taking the median value of the stack. Warning: Flux is not conserved. This option is useful for a quick-look mosaic, but should never be used for science.
Absolute Minimum Mosaic: If the Create Absolute Minimum Mosaic box is checked in the Mosaic Settings module then the output mosaic pixel value is set to the minimum input pixel value in the stack. The corresponding uncertainty pixel is set to the value of the pixel in the interpolated uncertainty image corresponding to the interpolated image with the smallest absolute value pixel. This option is useful for outlier rejection
Standard Deviation Mosaic: If the Create Standard Deviation Mosaic box is checked then a different flavor of mosaic uncertainty image is created. The pixel values S of this image are equal to the standard deviation of the interpolated image pixels in the stack:
