p30t mirror lock?

I notice my p30t has a ML button. I guess this is a mirror lock and I tried to use it tonight for some moon pics, but it doesnt seem to do anything when i press it : I am using my 80-200mm hoya lens with a 2x teleconverter, when I tried taking pictures of the moon before with this, they were slightly out of focus. My guess is that the mirror was to blame as I was using a tripod with a cable release (ISO 100 film with f8 & 1/60-1/125 second)

The P30t has not a mirror lock-up feature but a Memory Lock button.

I've never used a P30T, but I imagine it's pretty standard - ML will lock the exposure settings while you recompose the image and you'll need the B setting to lock the mirror for as long as the shutter button is pressed.
A locking cable release is particularly useful for loooooooong exposures!