The new topic started when we came to the point where we wondered what the Imperial forces really planned for their assault on Hoth.
It seemed that Ozzel's plan wasn't that bad, contrary to what Vader thought, but it required the immediate bombardment of targets of choice, upon exiting hyperspace, while Vader, complaining that they emerged too close to the planet, actually planned to pop out fuch farther, possibly outside the system. I'm not sure how that could have helped much, though.
Whatever Ozzel's plan was, nothing happened, no ships fired onto the structures to destroy at least the most important defenses.
At that point, it was too late, because the shield generator was already working and casting a protective theater shield.
The rebels pumped up power into the shield generators after the ships exited hyperspace, not when they destroyed the imperial probe.
Kazeite suggests that Ozzen didn't know the rebels had shields generators.CONTROLLER: General, there's a fleet of Star Destroyers coming out of
hyperspace in sector four.
RIEEKAN: Reroute all power to the energy shield. We've got to hold
them till all transports are away. Prepare for ground assault.
Which Socar replies to as saying that it "wouldn't make any sense given that the shield generator is exactly what the probe droid showed them earlier in the movie. Even if he was THAT incompetent and failed to realize this, you would think someone else would point that out to him."
The assumed shield generator:

That is nothing more than a brainbug, one that the EU entertains since a good while now, notably in video games.
Kazeite posted the following extract from the script:
The version, I got from there, comes under this version:"Through the cockpit window, Veers and his pilot can see the Rebel power generators in the distance."
"VEERS: Yes, Lord Vader. I've reached the main power generator."
"Inside his walker, General Veers prepares to fire on the Rebel power generators.
"VEERS: Distance to power generators?"
Which is what is exactly said in the film.INT. IMPERIAL SNOW WALKER - COCKPIT
Through the cockpit window, Veers and his pilot can see the Rebel power
generators in the distance.
A hologram of Darth Vader appears on a control panel screen.
VEERS
Yes, Lord Vader. I've reached the
main power generator. The shield
will be down in moments. You may
start your landing.
Destroying that power generator, that main one, will deprive the shield generator of any energy.
At no point do they say that they're targetting the shield generators.
VEERS
All troops will debark for ground
assault. Prepare to target the
main generator.
And we know the end of this. The main generator is destroyed.EXT. HOTH - BATTLEFIELD
Veers's walker continues to advance toward the Rebel base. The
smoldering walker that Luke exploded stands smoking just to the right
of Veers's path.
INT. IMPERIAL SNOW WALKER - COCKPIT
Inside his walker, General Veers prepares to fire on the Rebel power
generators.
VEERS
Distance to power generators?
PILOT
One-seven, decimal two-eight.
Veers reaches for the electrorangefinder and lines up the main
generator.
VEERS
Target. Maximum fire power.
So, I've been looking around to see what the EU thinks of this.
I found the following picture:
Most interesting is the size of the ion cannon's power core.
In the film, all the rest of the power was said to be rerouted towards the shield, so the cannon could only tap into that generator.
Yet, this generator provided enough energy for the cannon to take down a whole ISD in one to three shots.
Which just begs the question, if we follow the EU: why the hell spacebattles between kilometer long spaceships last for a while, when what you need is just one ion cannon, a small generator such as this one and another bigger one for the shields?
Amusing, is the tunnel which, according to the Star Wars: Complete Locations, on the left hand inferior corner of the picture, is described thusly:
Wookieepedia, hardly known for going for the most reasonable interpretations of facts, provides the following information about this battlecruiser:"Droid access tunnels to base main reactors: stolen parts from a derelict Praetor-class Star Battlecruiser."
So basically, there's these unidentified generators recovered from an unknwown type of ship, called Praetor-class, which can provide enough power to deflect a whole fleet's fire.Characteristics
Little is known for sure about the Praetor design, except that it was designated a Star Battlecruiser.[1]
Given that Echo Base's shields, which were powered by a Praetor's reactors, were able to resist a bombardment from Executor and her escorting Star Destroyers, the reactor systems of the Praetor-class were more powerful than those of the Imperial-class.[2][1]
History
During the Galactic Civil War, reactors from a decommissioned Praetor were acquired by the Alliance to Restore the Republic and used as the main power sources for Echo Base, deep underground on the frozen planet Hoth. These reactors also powered the defenses protecting the base, such as the DSS-02 shield generator, and could also feed power to the v-150 ion cannon, powering the generator system directly attached to the cannon.[1]
The defenses of Hoth give some indication of the power of a Praetor-class ship. The shield alone was strong enough to hold off an attack by the massive Executor and a flotilla of Imperial-class Star Destroyers, and the ion cannon was capable of disabling multiple Imperial-class vessels. It had a firing rate of one shot per six seconds.[1]
By way of comparison, reactor equipment from an Imperial-class Star Destroyer provided power for the previous main Rebel base, Massassi Station on Yavin 4, and its defenses. These defenses were said to be capable of holding off a single large battleship.[2] In fact, the Imperial military proved incapable of mounting a successful assault for several months, awaiting the completion and arrival of Executor.[3]
Sources
* Star Wars Episode V: The Empire Strikes Back
* Inside the Worlds of Star Wars Trilogy
* Star Wars: Complete Locations
[edit] Notes and references
1. 1.0 1.1 1.2 1.3 1.4 Star Wars: Complete Locations, p. 142-143 + fold-out
2. 2.0 2.1 Star Wars: Complete Locations, p. 138
3. Classic Star Wars Volume 2: The Rebel Storm
Can you feel the wank?VEERS
My lord, the fleet has moved out
of light-speed. Com-Scan has
detected an energy field protecting
an area around the sixth planet of
the Hoth system. The field is
strong enough to deflect any
bombardment.
Instead of having a huge planetary reactor specifically designed to provide power for planetary defenses, it is now a bunch of scrapped cores from a spaceship no one has seen, but which was many times more powerful than an ISD.
And by many times, I really mean it. Veers didn't seem to consider that they could bring that shield down with time.
These main generators, which remains unseen, are likely located as deep as the ion cannon's one (that is, more than 1 km under ice and rock).
However, this flies in the face of the film, which show that the main generator, with no "s", is located outside, and looks like just one big piece of hardware.
But again, what's puzzling is the claim about the size of the ion cannon generator. It is stupid.