Simon, I have had a lot of viewing nights with bad seeing. I use to think my Meade 9mm ep was a dud until one night of good seeing, I viewed Saturn at 414x with that 9mm barlowed in my lx90. I've only had two good viewing nights like that in the last 5 years; normally I can only use up to 150x. The sky conditions can be very deceiving and cause this kind of problem.
