There is no precise limit to the size of a mosaic that MOPEX will produce. However, large datasets may encounter computer memory limitations, so most MOPEX modules employ Tiles to reduce the number of pixels that need to be considered at one time.
Tiles are subsections of the final mosaic image. The size of tiles is set by the user, based on the computer memory considerations. It should be small enough so that all input images overlapping one tile can be held in memory without swapping. MOPEX first produces the tiles for each final mosaic image with the tile sizes specified by the user. Then all the tiles are glued together into a single mosaic image by the Mosaic Combine module.
The first step is estimation of the tile geometry. The interpolated and co-added images are defined on the same Fiducial Image Frame (FIF; see e.g. §4.3.4). The suggested tile sizes in the x- and y- directions Tile Max X and Tile Max Y are parameters in a number of modules. If both FIF dimensions NAXIS1 and NAXIS2 are smaller than Tile Max X and Tile Max Y, correspondingly, then there is only one output co-added image.
Figure 8.1: An example of tiling a FIF. Suppose NAXIS1 = 4450 and NAXIS2 = 1024. The suggested sizes Tile Max X = Tile Max Y = 500. In this case there are 2 tiles in the y-directions of the same size Tile Y = 512 and 9 tiles in the x-direction, 8 of the size Tile X = 494 and the last tile of the size Last Tile X = 498.
If the FIF dimensions are greater than Tile Max X or Tile Max Y, then the co-added image is tiled. Given the dimensions of the FIF (NAXIS1 and NAXIS2) the program finds the number of the tiles, their sizes and relative positions (offsets) with respect to the FIF. The simple algorithm below assures that all the tiles are of almost the same size regardless of what the FIF size is (see Figure 8.1).
N_X = NAXIS1 / (Tile Max X);
If (NAXIS1 % Tile Max X > Tile Max X/2) N_X = N_X +1;
Tile X = NAXIS1 / N_X;
Last Tile X = Tile X + (NAXIS1%N_X).
The % (modulo) operator yields the remainder from the division of the first argument by the second. The calculations in the y-direction are identical. The tile sizes and offsets from the origin of the FIF along with their names are written in output IPAC table coadd_tiles.tbl.
