sjk-
the script is very simple:
do shell script "hdiutil compact /Volumes/Drobo/'backup 1'.sparseimage'"
do shell script "hdiutil compact /Volumes/Drobo/'backup 2'.sparseimage'"
do shell script "hdiutil compact /Volumes/Drobo/'backup 3'.sparseimage'"
...etc
I run it on a schedule using
Cronnix, an aqua frontend for the Unix command "cron."
does the job! good luck.