5.2.1 Making an image from a snapshot

The simplest way to grid a snapshot into an image is snapccd, but this program has been superseded by the much more powerful tool snapgrid, which is based on snapplot. An example:

    19% snapgrid snap_in image_out xrange=-10:10 yrange=-10:10 \
        nx=128 ny=128 xvar=x yvar=z evar="m*m"

grids the $X$ and $Z$ coordinates of the snapshot snap_in to an $128 \ast 128$ image image_out. The range in gridded coordinates is from -10 to 10 in both $X$ and $Z$, with pixel coordinates defined in the center of a cell. Note that the emissivity (evar=m*m) is given as the square of the mass, which could be applicable for ionized hydrogen gas when the mass in the snapshot would have been indicative of the gas density. In the case of stars or neutral hydrogen gas evar=m would have been more appropriate (which is actually the default).

Often it is then desirable to smooth the image to improve the signal to noise ratio; although the degree of smoothing depends on the average number of 'objects' per pixel. Example:

    20% ccdsmooth image_in image_out gauss=0.3

smoothes the image to a circular beam with FWHM (Full Width Half Maximum) of 0.3 physical units. In the above gridding example this amounts to about 2 pixels.