The time taken to collect photons on faint objects is related to the inverse of the square of the focal ratio. So a faint fuzzy with 30mins of data at F/4 you would have to image for a bit over an hour to get the same number of photons on your camera at F/6. Just how important this is in practice is however debatable given the power of modern CCD/CMOS detectors and the options in terms of software we now have.
Astrophotography is like building a very tall house of cards. There's so much to learn and get right in terms of the basic foundations, that is:
- Polar alignment
- Guiding/tracking/balance/camera mounting
- Focus & collimation
- Image processing
If you can't get those things right reliably and learn to troubleshoot them, then you'll never get a nice image regardless of the telescope.
I'd strongly suggest you run up at least 12months practice with your F/6 which should be about the same weight. If you still enjoy hitting your head against a brick wall for hours every night getting those items above to work reliably and sweetly, then is the time to worry about changing scopes - whether you go F/4 Newt or Tak FSQ106 on an Astrophysics mount....
