There are currently 2 observation decks. One at each terminal. The old and definetly the best one on the old T1 is closed but you can take pretty decent shots from the 2 new teraces as well. Opening hours are stated on HAM
's website. The T1 terace is free of charge, for acces to the T2
terace you need to eat something at the connected restaurant.
But there are far better spots around the airport, so if you've got more time then just an hour or 2 between 2 flights you should consider to visit one of these.
A very good website for spotting in HAM
is Frank's www.fuairliner.de
where you will find a pretty good list of spotting points @ HAM
. Nevertheless things have changed in the past. Spotting point #2 isn't available any more, and point 3 "T4" is now known as T2
If you want more info don't hesitate to ask me!