Bingowings said:

At the beginning of ROTJ Vader turns up to give the Moff the bad news that he can't have more men until he gets the second Death Star working.

So it couldn't have blown up anything around the time of ESB because it wouldn't be operational yet.

I know that already, but just because it wasn't around yet that didn't mean that it was totally irrelevant to the plot yet..... The rebellion alliance were trying to stop the bad guys from blowing up planets with it-yet, when Luke Skywalker refused to join Darth Vader, he never said "I'll never destroy planets with you"......

Yet, in episode 5, they never mentioned saving any planets; and that's is my point here........