Thanks Bob!
This information is invaluable.
The reason I asked is that
I just had a hunch from playing around with some synthetic gaussian spatial ACF images that the 3dFWHMx' s ACF 'a' parameter's upper limit is 0.9940 and hence the -acf option will overestimate the spatial correlation. I guess one should employ the "classic" method FWHM-estimate when the spatial ACF is gaussian.
Regards
Gopi