a flash without the flash part?

Dear Lazyweb,

I shoot with a Canon EOS 5D. I shoot in the dark a lot. It's a great camera, but it doesn't auto-focus very well in low light, so I usually have a flash attached (Speedlite 580EX) and have the flash set to "emit, no fire". This means that when the camera tries to get a focus lock, the flash unit flashes a dim red light, but when I take the picture, the flash itself doesn't fire. (I don't understand how the on-camera, flashless autofocus works, but it doesn't involve emitting visible light.)

This works reasonably well. But, the flash is bulky and heavy.

Can I get a device that is, like, just the bottom inch of the flash unit? The "focus-light" part, but not the "flash" part? (Will an ST-E2 do this?)

Alternately, what's the smallest/lowest-profile flash I can use for this instead? I'd like something I can just leave attached to the camera without making it be twice as big.

Update: I guessed right, the correct answer is "ST-E2".

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