Used SCT focusing question

I recently picked up a used Meade MTS-SCT 8". So far, I've been really happy with it. I'm having fun and getting a chance to learn a lot about the sky. I do have a focusing question, however. At higher powers, esp. with the 4.7mm Nagler eye piece that came with it, I'm finding it impossible to obtain sharp focus. I wear glasses, but look through the scope without them. Is this a limitation of the scope or eyepiece? Or is it a problem with my eyes? Overall, the scope is pretty clean, just a little dust on the mirror (inside the OTA!) and on the corrector. I've also adjusted the collimation, and that seems spot on.

Oh, one more question: this scope came with the Kendrick dew heater system. It seems that I can see some heat distortion (hot air rising) when I use higher powers. Is this normal? Should I just expect the best results when dew isn't an issue? Would a dew shield be better than the large corrector lens heater?

Thanks a bunch!
-Van

The short focus nagler is probably too much power for the scope - at over 400x, it's certainly pushing it. And your local seeing conditions probably won't allow that high of a magnification except on the best of nights, with any scope. On many nights, I'm limited to about 200x as my maximum useful magnification by the atmosphere.

Can't help you with the dew heater.

480X is pritty hight power the view will be dropping off by that power
SV

That's good news, actually. That means nothing's defective or borken!

Thanks!