How to get past the turrets at the beginning of test chamber 15, chapter 3

You need to drop the cube while standing on the platform. The platform will be down while the cube is blocking the beam.

1. block beam with the cube (on top of hard light)
2. step on the platform
3. make cube drop by redirecting the hard light
4. beam will activate the platform, which will get you to the exit, cube will open exit

Here's what I did for the chamber it sounds like you're describing:

1. Obtain a cube
   • Place a hard light wall over the turret pit so when you call for a cube it's not immediately emancipated
   • Hit the button and go catch it (or if you're really lucky, it will bounce back enough to stay up)
2. Go up near the exit with the cube
   • Use the light wall to prevent overshooting the catwalk
3. Once there, place a portal to block off the turrets
   • this is before walking around to the ledge above the button
4. Go put the cube on the button
5. Portal out, using the one you blocked the turrets with (orange in above picture would go on the surface to the left on this side of the emancipation field)
6. Go back up to the exit

If you get it down (or don't mind getting more cubes), you can just throw the cube off the ledge onto the button and skip 3-6 entirely.