MOPEX Online Manual

APEX Modules: Mosaic Coadder

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, weighted, when available, with the interpolated uncertainty images. 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.

The co-addition can be performed on Tiles, to address computer memory considerations. The Mosaic Coadder module perfoms the coaddition 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, the Combiner still needs to be run, in order to produce the expected products.

The co-addition process can be performed using several options: Straight average; uncertainty-weighted average; quick-look median (flux not conserved); and absolute minimum. The last of these options is selected in the Mosaic namelist with the trigger create_absolute_minimum_mosaic. A fifth option, exposure time weighting, is available in the command-line version of MOPEX, but is not yet implemented by the GUI.

PARAMETER BLOCK

&MOSAICCOADDIN
 TILEMAX_X = 1000,
 TILEMAX_Y = 1000,
&END

INPUTS

TILEMAX_X, Y: (int) the X (Y)-size of the tile for making the coadd.

OUTPUTS

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 are created by the next module, Mosaic Combiner, but are written to the output mosaic directories.

DISCUSSION

Note that APEX does not carry out any outlier rejection. If you wish to use an outlier rejection scheme to clean up your data, you must run mosaic.pl with the trigger keep_coadded_tiles = 1 in the Mosaic namelist, and use the output coadded tiles as input into APEX. To do this, copy the output coadded tiles from Mosaic into the APEX output directory before running APEX. APEX will then automatically take the coadded tiles as input.

There are several weighting schemes available. The interpolated images can be combined using the straight averaging or they can be weighted with the interpolated uncertainty images. In command-line MOPEX, exposure time weighting is also available, but it is not implemented yet for the GUI. The coadded 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.

Straight Average: (default) If weighting is not used, then the co-added pixel value O is given by the following expression:

Here I_j is the value of the interpolated pixel in image j, and c_j is the pixel value of the corresponding coverage map. Co-added uncertainty U=W^-1 is given by the following expression:

where σ_j is the pixel value of the corresponding uncertainty image, and co-added coverage C is simply the sum of the individual input coverages

Sigma Weighting: (sigma_weighted_coadd = 1) NOT RECOMMENDED. If sigma weighting is selected, then a co-added pixel value O is given by the following expression:

Exposure Time Weighted Average: (INTEG_TIME_KWD = 'EXPTIME') If this keyword is specified then MOPEX weights the coadded pixels by the exposure time given in the fits header keyword specified. For Spitzer data, the integration time is given by the keyword EXPTIME

Median Mosaic: If the median mosaic is requested (by setting run_median_mosaic = 1 in the namelist) 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 absolute minimum mosaic is requested (by setting run_absolute_minimum_mosaic = 1 in the namelist) 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 Standard Deviation mosaic is requested (by setting create_std_mosaic = 1 in the namelist) 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:

The variable T=1 in the product as produced by the GUI. It is the exposure time which can be used in the command-line version of MOPEX.