Hi Mike
More of a list of things you should consider, rather than a specific recommendation.
You need to distinguish between:
- Accurate tracking, and...
- Accurate auto guiding.
To keep the planetary disc in the central region of the FOV may not require an expensive, premium mount with a low periodic error that is designed specifically for auto guiding hour long deep sky images.
Auto guiding at focal lengths greater than 2m is difficult, regardless of mount, e.g. Losmandy G-11, Takahashi EM200, etc.
Your “long” Newtonian will be more vulnerable to the wobbles compared to an SCT which has a short, stumpy tube. Likewise larger refractors have a longer moment arm, due to their longer tube lengths.
Having the option to select Sidereal, Lunar and Solar tracking rates can help minimise corrections you make to keep the image centred at high magnifications.
The Moon moves in Dec as well as RA compared to the sidereal rate, so expect to be making constant (but small and consistent) corrections to keep highly magnified lunar features centred. I use the circular red reticules in K3CCDTools to help keep a lunar feature reasonably stationery whilst I capture a 3 or 4 minute avi.
When trying to image the moons of Uranus with my C9.25 and x2.5 Powermate, I am working at F25 which is an effective focal length of 5.875m. More than 50% of the images show elongated trails with exposures of 3 or 4 seconds. At these magnifications, you cannot auto guide and the periodic error of the mount is magnified due to the long effective focal length.
Cheers
Dennis