Hi Ian.
Yep that should definitely be possible. I'll run through how I'd do it so you can see if there is anything you're missing.
Compose image, sort focus and exposure for the result that you want. Leave camera on manual focus and manual exposure so that the camera doesn't change anything between shots. If you're not shooting RAW, I would also use manual white balance. If shooting raw, it is less important as you can sort that later if need be.
You mentioned that you want 30 second exposures. I realise you can set this as a shutter speed but I would set the camera to bulb mode as I think it will be more reliable to set this from the timer so it is in full control.
Before you go any further at this point, this one is really important, make sure long exposure noise reduction is turned off and that shutter delay mode is also off and no mirror up option etc is selected.
Turning attention to the timer.
The 'delay' option allows you to set a time delay for the first of your shots to be taken. Let's leave that on 00:00'00" for now though.
Next 'long' option dictates your shutter speed in seconds. In your case, this will be 00:00'30".
The 'intvl' dictates the interval time between consecutive shots. If you're doing something like start trails I would just set this to 1 second.
And lastly 'N' dictates the number of shots so set that to 25.
Once you've done all that, just hit timer start/stop button and sit back for 12.5 minutes. It should just rattle through the shots one by one.
Hope that does the trick.
