I've got a 12 inch dob and mainly use powers between 130 and 200x (dependent on the seeing conditions, of course). Which filter would be best for my situation? I'd like to pull as much detail as I can out of the gas giants but seeing isn't always on my side here in suburbia.
It all comes down to how much brightness you can stand, eg. your mirror has an area factor of 12x12=144. The filters will give you reduced brightness equivalent to the following telescope sizes for a given magnification:
50% = 144x0.5=72. SQRT(72)=8.5. A 50% filter gives a view in brightness similar to a 8.5" telescope for a given magnification.
25% = 144x0.25=36. SQRT(36)=6. A 25% filter gives a view in brightness similar to a 6" telescope for a given magnification.
13% = 144x0.13=19. SQRT(72)=4.3. A 13% filter gives a view in brightness similar to a 4.3" telescope for a given magnification.
A polarising filter is available but you will notice a significant loss in resolution. I have used one and I can see small craters on the Moon disappear when using the polarising filter with a C9.25. Planetary detail too suffers greatly.
You may find yourself with all three filters depending on the objects you observe and the magnification you select.
