Well... I'd recommend your next purchase be a Televue 5x Powermate.
That will give you an effective focal length of 6000mm and you'll be working in the general planetary imaging sweet zone of f30
If you later add an extension tube you can get even more focal length. However, you'd need quite good seeing to push your 8" to those lengths.
Bintel has a bit of a special on at the moment on Televue product.
Also, keep a regular eye on astromart.com
Televue Powermates come up quite regularly
