Hi Orestis...
There are numerous threads on the same topic, try searching through some of them for some suggestions...
Personally, i just upgraded my set of GSO plossls to the Baader Hyperion series (got 5, 8, 13, and 24mm's), I bought them online for $115 each and highly recommend them. For you, I would recommend the 13mm lens, it has a nice flat 68 degree AFOV, good eye relief, fits both 1.25 and 2 inch focusers, and will give you approx 70x magnification in your scope (plenty for most DSO's and planetary viewing). As an added bonus, it also has several built-in T-threads to fit various DSLR cameras. Well worth looking into...
...lol
Davin
