The energy shield can only be opened for a short time, so you'll have to stay very close to your transports.
8 seconds sound clip from the Star Wars: Episode V - The Empire Strikes Back movie soundboard.
You can hear this line at 00:24:05 in the Blu-ray version of the movie.
Quote context
[...]
- You are in command now, Admiral Piett.
- All troop carriers will assemble at the north entrance.
- The heavy transport ships will leave as soon as they're loaded.
- Only two fighter escorts per ship.
- Two fighters against a Stardestroyer?
- When you've gotten past the energy shield, proceed directly to the rendezvous point.
- OK Everybody to your stations. Let's go.
[...]