Macintosh version of Rip-Off or Star Thief

A few years ago, I bought a shareware Macintosh game similar to the old
arcade game, Rip-Off, as well as the Apple II game, Star Thief. The
idea of defending a limited number of objects with a shooting space
fighter ship was the same, but the Mac version allowed players to
choose different ships and weapons. It's been a while, and I've not
only forgotten the name of the game but also lost it on my computer.
The closest program I've found on the net now is Starbase Defender, but
it's not as similar. Does anyone know the name of the game I'm thinking
of, as well as whether it's available anywhere? Thanks.